Шинжлэх Боловсрол

Шинжлэх Ухааны Яам

ТЕХНОЛОГИЙН И Х Ухаан

Сургууль

Георадарын Хоёр

Болон Гурван

Чанрыг Тодорхойлж

Тайлбарлах Нь

Maksim Bano

ШУТИС Х Цээдулам

Геологи Уул

Уурхайн Сургууль

Motoyuki Sato

Antoine Schlupp

ШУТИС Геологи Гэрэл

Уурхайн Уул

Keywords: GPR, Active faults, Mongolia, paleoseismology

I would like to thank all those who helped and supported me from the beginning to the end of this PhD dissertation work and gave me every opportunity to complete my doctoral dissertation. First of all, Dr. Maksim Bano taught me the techniques and development of GPR methodology and research work from the very beginning, and then who chose this PhD research topic, became my supervisor, and to help got a scholarship to study in France. Thank you very much.

I would also like to thank Dr. Tseedulam, the Mongolian supervisor of my thesis work, and Dr. Antoine Schlupp and Dr. Ulziibat Munkhuu, the co-supervisors, for their support during this time.

I would like to thank my thesis examiner and committee members for working as my dissertation reviewer Dr. Pascal SAILHAC, Dr. Motoyuki SATO, giving me valuable advice and helping me defend it.

I would like to express my deep gratitude to the staff of Campus France and all the staff of the French Embassy in Mongolia for providing me with the opportunity to get acquainted with the life and culture of France by conducting my thesis work at the University of Strasbourg. Please.

We would like to thank Dr. Jean-Rémi DUJARDIN and the Geophysical Team of the University of Strasbourg for their supported and helped of geophysical data processing and imaging technique.

I would like to thank Dr. Jérôme Van Der Woerd, Director of the Doctoral School of the University of Strasbourg, for his support and assistance in everything from the beginning to the end of this my thesis work. I would also like to thank all the faculty members of the Mongolian University of Science and Technology for teaching usefull geological lectures.

I would like to express my gratitude to Battogtokh, a friend of the active fault research team, and to the geophysical research team.

I would like to thank director Dr. S.Demberel, Dr. M.Ulziibat, Dr. Ts.Batsaikhan and the staff, researchers of the Institute of Astronomy and Geophysics, the project leader Dr. Odonbaatar and the project team.

Finally, I would like to express my deep gratitude to my wife, children, and mother for always being by my side and gived me hope, encouragement, and energy in the during of all this hard and endless work.

Résumé Etendu Titre : Caractérisation des failles actives par la technique d'imagerie géoradar en 2D et 3D et interprétation des résultats

Tsend-Ayush NYAMBAYAR La Mongolie est l'un des pays les plus exposés aux tremblements de terre, car ces derniers se produisent non seulement dans la zone de subduction des plaques tectoniques mais aussi entre les microplaques au milieu du continent. La Mongolie fait partie géologiquement de la zone orogénique de l'Asie centrale, et ses montagnes sont sismiquement actives en raison de leur faible taux de glissement. Sur les 44 tremblements de terre majeurs qui se sont produits en Asie depuis le début du siècle dernier, deux se sont produits en Mongolie. Les tremblements de terre de Bulnai (8.4Mw, 1905) et de Gobi-Altai (8.1Mw, 1957) étaient classés en neuvième et 12 ème place, respectivement (voir tableau 1). L'urbanisation en Mongolie est à la hausse, en particulier dans la capitale, Oulan-Bator, qui compte environ 1,5 million d'habitants. Oulan-Bator est entouré de plusieurs failles actives, et l'augmentation cumulée du nombre des séismes depuis 2005 est devenue un réel problème (Figure 1). Tableau 1. Classement de la magnitude du moment sismique de 1900 à 2018. Tableau créé à partir des données de l'USGS sur les séismes de magnitude supérieure ou égale à 8,0 de 1900 à 2018. L'objectif de cette thèse de doctorat est de déterminer et d'expliquer les caractéristiques et les paramètres des failles actives à l'aide de technologies modernes, telles que les techniques d'imagerie 2D et 3D de la méthode géoradar (ground penetrating radar ; GPR). Dans le cadre de ce travail de recherche doctorale les activités suivantes sont prévues :

1. Réaliser des mesures et des études avec plus de 2 antennes de fréquences différentes sur les principaux types de failles actives et faire des corrélations des images qui en résultent.

2. Développer la méthodologie, les principes et la conception de l'acquisition afin d'identifier les structures du sous-sol et d'étudier les principaux types de failles actives.

3. Faire des interprétations paléo-sismologiques et géologiques structurales de la faille de Songino, qui est la principale zone d'étude, en interprétant les résultats de mesure et d'imagerie détaillés de la méthode GPR.

Les méthodes paléo-sismologiques traditionnelles ont été utilisées pour étudier la géomorphologie des escarpements sur les sédiments fracturés par les séismes, ainsi que les mouvements des sédiments à traverser les traces de failles en surface ; l'objectif était d'ouvrir un paléo-tranchées à une profondeur de 1 à 3 mètres, de le comparer avec les résultats d'études GPR et de prélever des échantillons pour déterminer l'âge du dernier séisme. La recherche paléo-sismologique utilisant les mesures GPR permet une collecte plus rapide et plus précise de zones d'informations plus détaillées, plus profondes et plus larges que les études paléosismologiques traditionnelles.

Cette étude fait partie intégrante de l'évaluation de l'aléa sismique à Oulan-Bator.

L'évaluation des risques sismiques est très importante pour le développement futur de la capitale d'Oulan-Bator qui est le seul centre économique, commercial et de services et dans laquelle vivent plus de 50% de la population de la Mongolie.

Déterminer la nature de la faille de Songino est essentiel pour comprendre les interactions entre les failles de Khustai et d'Emeelt, qui ont le plus grand impact sismique à Oulan-Bator. En outre, cela devrait également aider à expliquer les interactions et les modèles entre les différentes failles actives autour d'Oulan-Bator.

Le marqueur paléo-sismologique est toujours situé près de la surface, dans les premiers mètres du sous-sol. Au cours des deux dernières décennies, la méthode GPR est devenu de plus en plus populaire dans les études paléo-sismologiques en raison de ses propriétés non destructives, de sa rapidité de mis en oeuvre et de son faible coût, permettant des mesures précises sur de grandes surfaces. Le GPR est une méthode géophysique idéale afin d'étudier les sédiments quaternaires et les structures du sous-sol urbains, ainsi que pour identifier les zones appropriées d'excavations des tranchées et pour l'interpolation des données entre le paléo chenaux fluviaux. Le premier chapitre, qui inclue la méthode GPR et le traitement des données, présente un aperçu de la théorie, de la méthodologie et des applications de la méthode géoradar. Il détaille également la méthodologie des mesures géoradar 2D et 3D sur le terrain, le traitement de données, les concepts de l'imagerie géoradar, les principes et l'interprétation des résultats.

De longs enregistrements paléo-sismiques peuvent donc résider près de la surface dans les premiers mètres du sol [START_REF] Mccalpin | Paleoseismology[END_REF]. Au cours de la dernière décennie, la méthode GPR est devenu de plus en plus populaire pour les études paléo-sismologiques en raison de son caractère non destructif, de sa rapidité de mise en oeuvre et de son faible coût, qui lui permettent d'être appliqué de manière dense sur de vastes zones. La méthode GPR se caractérise par le fait qu'elle collecte de grandes quantités de données plus rapidement que les autres méthodes géophysiques. La méthode GPR a été particulièrement adaptée afin de localiser les failles actives dans les formations quaternaires et les zones urbaines [START_REF] Smith | Ground penetrating radar : antenna frequencies and maximum probable depths of penetration in Quaternary sediments[END_REF][START_REF] Cai | Application of ground-penetrating radar to investi-gation of near-surface fault properties in the San Francisco Bay region[END_REF][START_REF] Audru | GPR investigations on active faults in urban areas: the Georisc-NZ project in Wellington, New Zealand[END_REF][START_REF] Demanet | The use of geophysical prospecting for imaging active faults in the Roer Graben, Belgium[END_REF][START_REF] Gross | Shallow geometry and displacements on the San Andreas Fault near Point Arena based on trenching and 3D georadar surveying[END_REF], d'identifier des sites potentiels pour de futures excavations [START_REF] Salvi | Investigation of the active Celano-L'Aquila fault system, Abruzzi (central Apennines, Italy) with combined ground-penetrating radar and palaeoseismic trenching[END_REF][START_REF] Liberty | The Portland Hills Fault : uncovering a hidden fault in Portland, Oregon using high-resolution geophysical methods[END_REF][START_REF] Jewell | GPR studies in the Piano di Pezza area of the Ovindoli-Pezza Fault, Central Apennines, Italy: extending paleoseismic trench investigations with high resolution GPR profiling[END_REF][START_REF] Malik | Paleoseismic evidence from trench investigation along hajipur fault, himalayan frontal thrust, nw himalaya : Implications of the faulting pattern on landscape evolution and seismic hazard[END_REF] et pour interpoler les données entre les tranchées [START_REF] Ferry | Ground-penetrating radar investigations along the North Anatolian fault near Izmit, Turkey[END_REF][START_REF] Slater | Ground-penetrating radar investigation of active faults along the Dead Sea Transform and implications for seismic hazards within the city of Aqaba, Jordan[END_REF]. Dans de nombreux cas, elle est combinée à d'autres méthodes d'imagerie, notamment la électrotomographie [START_REF] Demanet | The use of geophysical prospecting for imaging active faults in the Roer Graben, Belgium[END_REF][START_REF] Lehmann | Paleoseismological investigations at the Rurrand Fault, Lower Rhine Embayment[END_REF][START_REF] Vanneste | Pseudo-3D imaging of a low-slip-rate, active normal fault using shallow geophysical methods : The Geleen fault in the Belgian Maas River valley[END_REF]. En termes de profondeur, de précision et de rapidité de mise en oeuvre, la méthode GPR est la mieux adaptée pour déterminer le niveau cible aux profondeurs des premières dizaines de mètres au-dessus du sol. L'objectif ici est d'utiliser le GPR le long des failles actives pour détecter les déplacements accumulés au cours du temps par des marqueurs morphologiques enfouis.

La méthodologie de cette recherche est basée sur la combinaison de nombreuses technologies de pointe et suit la séquence suivante :

1. Télédétection et analyse d'images satellitaires.

2. Relevé géologique de la zone : traces de failles, escarpement, déplacement, points de contrôle, conditions de relevé GPR.

3. Levé GPR ; conception du levé GPR 2D et 3D, acquisitions de données, mesures de la topographie de la surface du sol.

4. Ouverture des paléo-tranchées, étude paléo-sismologique et échantillonnage afin de déterminer l'âge des failles.

5. Faire une interprétation complète des résultats de toutes les mesures, de la géologie structurale et de la géodynamique.

La méthodologie principale de cette recherche est la technique d'imagerie géoradar 2D et 3D, mais d'autres méthodes supplémentaires (Télédétection, DGPS) sont utilisées afin d'améliorer la qualité et l'efficacité de ce travail de recherche. La méthodologie de prospection géoradar est avant tout une technique qui cherche à expliquer la nature, l'origine, la cause et la régularité des failles.

Afin de vérifier la fiabilité et la précision de l'imagerie géoradar lors de la réalisation de mesures, 3 à 5 profils parallèles ont été réalisés à chaque point de ciblage à une distance de 1 à 2 mètres les uns des autres. Par conséquent, la qualité de l'imagerie du résultat final est élevée et il n'y a aucun doute sur la structure de l'image qui en résulte. En interprétant les résultats des images finales géoradar, nous avons pu déterminer les limites des différentes couches en les comparants à la géologie structurale de la proche surface du sous-sol. Nous avons analysé le décalage des réflexions des ondes radar afin de déterminer les déplacements dus aux failles actives.

Le deuxième chapitre, intitulé Applications et développement de la technique d'imagerie GPR aux structures des failles actives. Cette partie discute efficacité de la technique d'imagerie géoradar dans les études paléo-sismologiques, y compris l'interprétation des résultats de paléo-tranchés dans le cas de tous les types de failles actives. La méthodologie de l'imagerie 3D est également expliquée dans l'exemple d'étude de la faille d'Emeelt.

Au cours du XXe siècle, les connaissances sur les phénomènes sismiques ont beaucoup évolué grâce au développement de la sismologue, de la géodésie et de la paléo-sismologue.

Alors que la sismologie et la géodésie fonctionnent sur des calendriers à court terme par rapport au temps de récurrence des séismes, la paléo-sismologie ouvre un champ d'observation couvrant plusieurs cycles de tremblements de terre. Les observations de répétitions de forts séismes en un ou plusieurs sites le long d'une faille ont permis d'établir des modèles conceptuels de successions de ruptures sismiques. Alors que certaines études décrivent les ruptures de surface avec des longueurs et des glissements différents d'un séisme à l'autre [START_REF] Daëron | 12,000year long record of 10 to 13 paleoearthquakes on the Yammouneh Fault, Levant fault system, Lebanon[END_REF][START_REF] Weldon | Wrightwood and the earthquake cycle: What a long recurrence record tell us about how faults work[END_REF], d'autres décrivent des comportements répétitifs avec une distribution de glissement plus uniforme [START_REF] Sieh | A Review of Geological Evidence for Recurrence Times of Large Earthquakes[END_REF], [START_REF] Schwartz | Fault behavior and characteristic earthquakes: Examples from the Wasatch and San Andreas Fault Zones[END_REF][START_REF] Rubin | Long dormancy, low slip rate, and similar slip-per-event for the Emerson fault, eastern California shear zone[END_REF]Tapponnier, et al., 2001a).

Le gouvernement de la Mongolie a approuvé le « Programme national de réduction des risques de catastrophe sismique » en 2009, et conformément à ce plan, le ministère de la Construction et du Développement urbain a mis en place 12 centres provinciaux d'ingénieriegéologie, d'hydrogéologie, de cartographie sismique des microrégions et de bases de données sismologiques.

L'équipe de recherche de l'Institut d'astronomie et de géophysique a mené ce programme en 2012-2017. Dans le cadre de cette étude, l'équipe de recherche des failles actives a mené non seulement une étude GPR détaillée des failles autour d'Oulan-Bator et de la partie centrale de la Mongolie, mais aussi des études géologiques et paléo-sismologiques afin de combiner et comparer tous les résultats (Figure 3). La structure inverse de la partie sud de la faille de Mogod est nord-sud, tandis que la structure qui résulte de l'imagerie radar (Figure 4) est clairement dans la direction opposée. Au sommet de l'escarpement, il y a une démarcation visible de la faille du nord-est au sud-ouest.

Les stratifications visibles à l'intérieur de la structure chevauchée, montrent clairement la direction du mouvement dans cette section. La figure 4 montre une interprétation des images radar obtenues avec les antennes 250 MHz et 500 MHz du profil-1, les deux images sont cohérentes.

L'étude de la tranchée paléo-sismologique a été menée en collaboration avec des scientifiques français et russes et comprend des photos de paroi de tranchée prises par les scientifiques français Laurent Bollinger (CEA) et Yann Klinger (IPGP). Diagraphie manuelle sur la paroi ouest de la tranchée paléo-sismologique montre clairement la couche jaune qui s'est disloquée et chevauchée (Figure 5).

Les résultats de la méthode des tranches de temps de profondeur du traitement, de l'imagerie et de l'analyse GPR pseudo-3D montrent que la méthode est plus efficace dans les études de failles actives ou les études paléo-sismologiques. Il est clairement montré que le déplacement horizontal de la faille active et la localisation de la trace de faille sont prédominants Le troisième chapitre, intitulé la technique de caractérisation paléo-sismologique de la faille active de Songino, Oulan-Bator, Mongolie, basée sur la méthode géoradar. En utilisant cette technique, l'étude présentée ici fournit des informations détaillées sur les structures des failles actives et explique les résultats de l'imagerie géoradar.

L'activité sismique observée aux alentours d'Oulan-Bator (UB), capitale de la Mongolie, est relativement faible par rapport à l'activité observée dans l'ouest de la Mongolie.

Néanmoins, depuis 2005, l'activité sismique autour de UB a augmenté [START_REF] Dugarmaa | Earthquake hazard assessment of Ulaanbaatar, the capital city of Mongolia. Earthquake micro-zoning map[END_REF], et s'organise à l'ouest de UB selon deux directions perpendiculaires : en 2009 année découverte de la faille Emeelt (environ 25 km de long) située à environ 10 km à l'ouest de UB (Schlupp, et al., 2012 ;[START_REF] Munkhsaikhan | Seismic activity near Ulaanbaatar: Implication for seismic hazard assessment[END_REF] et de la faille Hustai (le segment principal a une longueur d'environ 80 km) avec son plus proche segment à environ 15 km à l'ouest de UB [START_REF] Ferry | Tectonic Morphology of the Hustai Fault (Northern Mongolia), A Source of Seismic Hazard for the city of Ulaanbaatar[END_REF][START_REF] Ferry | Tectonic Morphology of the Hustai Fault (Northern Mongolia)[END_REF].

La faille de Songino (20 km de long, localisée à 20 km à l'ouest d'UB) est située entre les failles d'Emeelt et de Hustai. La longueur et la morphologie de ces failles indiquent qu'elles peuvent produire des séismes de magnitude 6,5 à 7,5 [START_REF] Schlupp | Investigation of active faults near Ulaanbaataar. Implication for seismic hazard assesment[END_REF][START_REF] Dujardin | Imagerie géoradar (GPR) en milieu hétérogène Application aux failles actives en Mongolie et aux dépôts pyroclastiques du Tungurahua (Equateur)[END_REF][START_REF] Munkhsaikhan | Seismic activity near Ulaanbaatar: Implication for seismic hazard assessment[END_REF].

L'équipe mixte franco-mongole de recherche sur les failles actives a d'abord réalisé des levés GPR sur les failles actives d'Emeelt et de Khustai en 2010, puis en 2011-2012 sur les failles acfives d'Avdar, Sharkhai, Gunj et Mogod. Les résultats de ces études ont été intégrés dans plusieurs thèses de maîtrise et de doctorat [START_REF] Dujardin | Imagerie géoradar (GPR) en milieu hétérogène Application aux failles actives en Mongolie et aux dépôts pyroclastiques du Tungurahua (Equateur)[END_REF], [START_REF] Abeer | Tectonique active de la région d'Oulan Bator, Mongolie: Analyse morphotectonique et paléosismologique des failles actives de Sharkhai et Avdar[END_REF], [START_REF] Munkhsaikhan | Seismic activity near Ulaanbaatar: Implication for seismic hazard assessment[END_REF].

Après la reconnaissance géomorphologique de la faille de Songino en 2012, nous avons décidé d'utiliser des mesures GPR 2D pour étudier les dépôts souterrains potentiellement affectés par la faille sur une zone plus large. Les résultats devraient nous permettre de localiser avec précision les futures tranchées.

Les trois premières mesures de profils Ground Penetrating Radar sur la faille de Songino La figure 9a montre le profil-2 du 14 juin 2013, on remarque un événement radar de très forte amplitude qui est décalé par la faille. La zone 5 montre la structure d'érosion du sol sous l'escarpement de la faille, mais la localisation du plan de faille et l'angle de pendage du plan de faille ne sont pas visibles. Le résultat de l'image radar obtenu avec l'antenne blindée de 250

MHz, montre qu'il est possible d'analyser la direction du mouvement de la faille indiquant qu'il s'agit d'une faille normale sous l'escarpement. Cependant, sur la figure 9b la direction du pendage de la faille et la direction du mouvement sont similaires à celles de l'image du profil GPR de la Figure 9a.

La localisation du profil P1 de la zone 3D6 est montrée sur la Figure 10. L'image radar de ce profil (P1) est obtenu avec l'antenne blindée de 250MHz, montre des microplis formés sous le sol et sont indiqués par les flèches noires (Figure 11). De plus, les plis et les caractéristiques de mouvement des sédiments du sol sont clairement observés (marqués par des flèches noires et lignes blanches sur la Figure 11). La hauteur de l'escarpement dans cette zone est d'environ 2,1 mètres. L'angle créé par la hauteur de la surface pliée est de 5,7º Une étude GPR détaillée de la faille Songino a révélé de nombreuses informations sur la structure, la direction du mouvement et l'accumulation de sédiments alluviaux des perturbations du sol causées par la faille. Sur la base de toutes ces données, une interprétation unifiée suggère que le mouvement principal de la faille Songino est une structure de décrochement latéral gauche du sud-est au nord-ouest. L'effet de compression sur la partie ouest, ou la structure sédimentaire d'escarpement formée à la surface du sol, est l'effet de l'excavation à un angle d'environ 40° par rapport à la direction de mouvement de la faille. Le sédiment avec un escarpement de structure normale, à l'est, est l'effet de la tension de la structure qui se courbe dans le sens du mouvement de la faille de Songino et forme un angle d'environ 21°. La géologie régionale de la Mongolie est couramment abordée en utilisant une analyse tectono-stratigraphique des terrains pour aider à interpréter sa structure et son histoire. En ce sens, un terrain est défini comme une entité ou un fragment géologique limité par une faille qui se caractérise par une taille régionale et une histoire géologique distinctive qui diffère sensiblement de celle des blocs crustaux voisins [START_REF] Tomurtogoo | A brief explanatory note to the Tectonic Map of Mongolia at a scale of 1:1,000,000[END_REF]. Puisque la faille de Songino est une structure directement liée à la faille active de Khustai son mouvement de retrait est lié au mouvement de faille de Khustai.

En outre, il a été démontré que l'ensemble de nombreuses mesures radar effectuées le long de la trace de faille active peut déterminer le déplacement principal de toute faille active.

Il est plus raisonnable de déterminer le mouvement principal des failles actives en identifiant avec précision les déplacements et les structures fracturées en plusieurs points cibles le long de la trace de faille, en les analysant.

Le déplacement vertical de l'escarpement de la section à structure normale était

Судалгааны зорилго, зорилт

Орчин үеийн дэвшилтэд технологи болох Георадарын 2 болон 3 хэмжээст зураглалын техникээр идэвхитэй хагарлын шинж чанар, параметрүүдийг тодорхойлж, тайлбарлахад энэхүү докторын судалгааны ажлын зорилго оршино.

Уг судалгааны ажлын хүрээнд дараахи ажлуудыг хийж гүйцэтгэхээр төлөвлөсөн. Үүнд:

1. Идэвхитэй хагарлын үндсэн төрлүүд дээр 2-оос дээш хэд хэдэн өөр давтамжийн антенүүд ашиглан хэмжилт судалгаа гүйцэтгэж үр дүнгийн зураглалын харилцан уялдааг гаргах.

Судалгааны хамрах хүрээ

Уламжлалт палеосейсмологийн судалгааны арга нь газар хөдлөлтөөр үүссэн хагарлын хөрсөн дээр үүсгэсэн мөргөцөгийн геоморфологи, болон хөрсний шилжилт хөдөлгөөнүүдийг газрын гадаргуу дээрхи эвдрэлийн ул мөрөөр дамжуулан судалдаг бөгөөд доош гүний хувьд 1-3 м гүний шруф болон тренчийн судалгааг гүйцэтгэдэг байсан. Тэгвэл георадарын тандан судалгааны тусламжтай палеосейсмологийн судалгааг хийснээр илүү гүний буюу 4-40м гүний мэдээллийг үл эвдэх аргаар хурдан шуурхай өндөр нарийвчлалтайгаар авах боломж бүрдэж байгаа болно. Бидэнд байгаа MALA брендийн 4 өөр давтамжийн антенаар бид 4-40м гүний зураглал хийх боломжтой боловч тухайн газар нутгийн хөрсний онцлог болон ус чийгний агууламж, мөн хүчтэй ойлгогч давхаргууд зэргээс хамаарч георадарын сигналын нэвтрэх гүн нь өөрчлөгддөг гэдгийг хэлэх хэрэгтэй болов уу. Мөн георадар нь биет болон давхаргуудаас буцаж ойсон ойлтын сигналыг бүртгэдэг учир долгионы ойлтын хуулийн хүрээнд л зураглал хийгддэг болно. Тиймээс 45⁰ аас илүү унлын өнцөг бүхий налуу бүтцийн хувьд радио долгионыг хажуу тийш сарниулах тул ийм бүтцийг тухайн орчний хэвтээ давхаргуудын шилжилт, өөрчлөлтөөр нь анализ хийдэг.

Судалгааны ач холбогдол

Энэхүү судалгааны ажил нь нийслэл Улаанбаатар хотын газар хөдлөлтийн аюулын үнэлгээний ажлын хүрээнд хийгдсэн тус ажлын нэг салшгүй хэсэг юм. Газар хөдлөлтийн аюулын үнэлгээний ажил нь Монгол орны хүн амын 50% аас илүү хувь нь суурьшсан, эдийн засаг, худалдаа үйлчилгээ дэд бүтцийн цор ганц төв болоод байгаа нийслэл хотын цаашдын төлөвлөлтөд маш чухал ач холбогдолтой ажил юм.

Сонгино хагарлын шинж чанрыг нарийвчлан тогтоосноор Улаанбаатар хотын газар хөдлөлтийн аюулын хамгийн том нөлөө бүхий Хустай болон Эмээлт хагарлуудын зүй тогтол хөдөлгөөнийг зөвөөр үнэлэх, харилцан уялдааг ойлгоход нэн чухал ач холбогдолтой юм. Цаашлаад Улаанбаатар хотын эргэн тойрны идэвхитэй хагарлуудын хоорондын харилцан хамаарал, зүй тогтлуудыг тайлбарлахад мөн адил дөхөм болох учиртай юм.

Нэр томьёоны тодорхойлолт

GPR-Ground Penetrating Radar гэдэг газрын гүний хөрс шороо чулуулаг болон барилга байшин, авто зам, уул уурхайн далан хучилт зэрэг хатуу бие доторх бүтцийг судалдаг арга зүй бөгөөд монголоор Георадар гэж нэршиж байгаа болно.

GPS-Global

Position System гэдэг байрлал тогтоох хиймэл дагуулын төхөөрөмж бөгөөд дифференциал горимын хос төхөөрөмж ашиглаж RTK-Real Time Kinematic горимд хэмжилт хийснээр 1 сантиметрийн өндөр нарийвчлалтайгаар байрлалыг тодорхойлж өгдөг учраас геологи, геофизикийн хээрийн судалгааны ажилд өргөн ашиглагддаг.

2D гэдэг нь геологи, геофизикийн зүсэлтүүдийг хоёр хэмжээст зураглалын аргаар дүрслэн харуулах хэлбэр бөгөөд харин 3D гэдэг нь геологи, геофизикийн зүсэлтүүдийг гурван хэмжээст зураглалын аргаар дүрслэн харуулах хэлбэр юм.

GPR-Slice гэдэг нь гурван хэмжээст хэмжилт, зураглалын үед өргөн ашиглагддаг анализын нэг төрөл бөгөөд өндөр нарийвчлал бүхий гурван хэмжээст мэдээллийн кубыг үүсгэсний дараа газрын гадаргуутай паралелаар дурын гүнд хийсэн хэвтээ зүэлтүүдийг хэлдэг.

Палеосейсмологи-Палеосейсмологи нь эртний газар хөдлөлтийн шинж тэмдгүүдийг илрүүлж, газар хөдлөлтийн аюулыг тооцоолох зорилгоор геологийн хурдас, чулуулгийг судалдаг. Палеосейсмологи нь ихэвчлэн намаг, нуур, голын эрэг зэрэг сүүлийн хэдэн мянган жилийн турш хурдас тасралтгүй бий болсон геологийн дэглэмээр хязгаарлагддаг.

Геоморфологи-Геоморфологи нь дэлхийн гадаргуу дээр эсвэл түүний ойролцоо үйл ажиллагаа явуулж буй физик, хими, биологийн процессоос үүссэн топографи ба батиметрийн шинж чанаруудын үүсэл, хувьслын талаархи шинжлэх ухааны судалгаа юм.

Структур геологи-Структур геологи нь геологийн хурдас, чулуулгийн нэгжийн хэв гажилтын түүхийг харгалзан гурван хэмжээст тархалтыг судалдаг.

Судалгааны ажлын бүтэц

Георадарын хоёр болон гурван хэмжээст дүрслэлийн арга техникээр идэвхтэй хагарлын шинж чанрыг тодорхойлж тайлбарлах нь сэдэвт докторын нэг сэдэвт судалгааны бүтээл нь удиртгал, 5 бүлэг, ном зүй, гэсэн үндсэн бүтэцтэй ба зураг-110, хүснэгт-18 багтаасан нийт 199 хуудас бүхий материал болсон байна.

Удиртгал хэсэгт судалгааны ажлын үндэслэл, тулгамдаж буй асуудлууд, зорилго, зорилтыг тусгасан. Мөн уг судалгааны ажлын үр өгөөж, ач холбогдол, судлагдсан байдал зэргийг тусгаж өгөхийг зорьсон болно.

СУДЛАГДСАН БАЙДАЛ

Палеосейсмологийн талаархи ул мөрүүд нь газрын гадаргуугийн ойролцоо, газрын эхний метрт байрладаг [START_REF] Mccalpin | Paleoseismology[END_REF]. Сүүлийн хорь гаруй жилийн хугацаанд Георадар нь хөнөөлгүй шинж чанар, хэмжилтийн хурд, өртөг багатай учраас өргөн уудам газар нутагт нарийвчилсан хэмжилт хийх боломжийг олгодог тул палеосейсмологийн судалгаанд улам бүр түгээмэл хэрэглэгдэх болж байна. Георадар нь одоо байгаа бусад геофизикийн аргуудаас өндөр нарийвчлалтай байдаг. Георадар нь дөрөвдөгчийн хурдсын тогтоц, хот суурин газрын хөрсний бүтцийн судалгаа, төлөвлөлтийн ажилд нэн тохиромжтой байсан [START_REF] Smith | Ground penetrating radar : antenna frequencies and maximum probable depths of penetration in Quaternary sediments[END_REF][START_REF] Cai | Application of ground-penetrating radar to investi-gation of near-surface fault properties in the San Francisco Bay region[END_REF][START_REF] Audru | GPR investigations on active faults in urban areas: the Georisc-NZ project in Wellington, New Zealand[END_REF][START_REF] Demanet | The use of geophysical prospecting for imaging active faults in the Roer Graben, Belgium[END_REF][START_REF] Gross | Shallow geometry and displacements on the San Andreas Fault near Point Arena based on trenching and 3D georadar surveying[END_REF], мөн судалгааны палеотренч малтлага хийх өндөр магадлалтай газрыг тодорхойлох зорилгоор [START_REF] Salvi | Investigation of the active Celano-L'Aquila fault system, Abruzzi (central Apennines, Italy) with combined ground-penetrating radar and palaeoseismic trenching[END_REF][START_REF] Liberty | The Portland Hills Fault : uncovering a hidden fault in Portland, Oregon using high-resolution geophysical methods[END_REF][START_REF] Jewell | GPR studies in the Piano di Pezza area of the Ovindoli-Pezza Fault, Central Apennines, Italy: extending paleoseismic trench investigations with high resolution GPR profiling[END_REF][START_REF] Malik | Paleoseismic evidence from trench investigation along hajipur fault, himalayan frontal thrust, nw himalaya : Implications of the faulting pattern on landscape evolution and seismic hazard[END_REF], болон суваг хоорондын өгөгдлийг интерполяцлахад нэн тохиромжтой [START_REF] Ferry | Ground-penetrating radar investigations along the North Anatolian fault near Izmit, Turkey[END_REF][START_REF] Slater | Ground-penetrating radar investigation of active faults along the Dead Sea Transform and implications for seismic hazards within the city of Aqaba, Jordan[END_REF]. Ихэнх тохиолдолд үүнийг бусад геофизикийн дүрслэх аргуудтай хослуулдаг ба ялангуяа цахилгаан томографи аргатай хослуулах нь өндөр үр дүнтэй [START_REF] Demanet | The use of geophysical prospecting for imaging active faults in the Roer Graben, Belgium[END_REF][START_REF] Lehmann | Paleoseismological investigations at the Rurrand Fault, Lower Rhine Embayment[END_REF][START_REF] Vanneste | Pseudo-3D imaging of a low-slip-rate, active normal fault using shallow geophysical methods : The Geleen fault in the Belgian Maas River valley[END_REF]. Гүний тандан судалгааны гүн, нарийвчлал, хэмжилтийн хурдны хувьд Георадар нь өргөн цар хүрээг хамарсан талбайн хэдэн арван метрийн хэмжээтэй бүтцийг тодорхойлоход хамгийн тохиромжтой арга юм.

Георадарын судалгааны арга, техник нь геофизикийн судалгааны салбар дахь хамгийн залуу бөгөөд хамгийн хурдацтай хөгжиж буй арга, техник бөгөөд Монгол орны хувьд өргөн дэлгэр ашиглагдаагүй байсан. 2010 оноос өмнө Монгол улсад хийгдсэн Георадарын судалгааны ажлын жишээнүүд бараг байхгүй байсан бөгөөд идэвхитэй хагарлын судалгааны ганц жишээ байсан. Энэ нь Дундговь аймгийн Дэрэнгийн идэвхтэй хагарал дээр Швед улсын MALA Geoscience компаний 100 МГц-ийн антентай RAMAC GPR системээр CMP (Common Mid-Point) аргачлалаар зөвхөн нэг профайл дээр хийж, палео-тренчийн судалгааны үр дүнтэй хослуулан хийсэн байсан [START_REF] Lu | Characterization of the shallow structures of the Deren Fault by ground penetrating radar[END_REF].

Хустай, Эмээлт хагарлуудыг олон орны эрдэмтэн судлаачид судалсан байдаг бөгөөд өөр өөрсдийн зүгээс тайлбарласан байдаг бол харин Сонгино хагарлыг 2012 онд Франц улсын Страсбургийн Их Сургуулийн палеосейсмологийн судалгааны баг илрүүлж Шинжлэх Ухааны Академийн Одон Орон Геофизикийн Хүрээлэнгийн судалгааны багтай хамтран нарийвчилсан судалгааг эхлүүлсэн. Сонгино хагарлын талаар өөр ямар нэгэн судалгаа байхгүй болно.

СУДАЛГААНЫ АРГА ЗҮЙ

Судалгааны ажлын арга зүй нь орчин үеийн дэвшилтэд технологиудын олон арга зүйг хослуулан ашиглаж хийгддэг ба доорхи дарааллийн дагуу явагддаг. Уг судалгааны ажлын гол арга зүй нь Георадарын 2 болон 3 хэмжээст зураглалын техник боловч судалгааны ажлын чанрыг дээшлүүлж өндөр үр дүнтэй болгохын тулд бусад нэмэлт арга зүйг хослуулан хэрэглэдэг болно. Георадарын тандан судалгааны арга зүй нь үндсэндээ тайлбар судалгаа бөгөөд хагарлын шинж чанар, үүсэл хөгжил, шалтгаан болон зүй тогтлыг тайлбарлахыг эрэлхийлж байгаа юм.

Өгөгдөл цуглуулах

Георадарын хэмжилтийг гүйцэтгэхдээ зураглалын найдвартай, үнэн зөв байдлыг нягтлан шалгах зорилгоор, хэмжилт хийсэн цэг бүр дээр хоорондоо 1-2 метрийн зайтай 3-5 паралел зүсэлтүүдийг хийж байсан болно. Тийм учраас эцсийн үр дүнгийн зураглалын чанар өндөр байгаа бөгөөд гарч буй бүтцийн хувьд эргэлзээ байхгүй юм.

ӨГӨГДЛИЙН БОЛОВСРУУЛАЛТ, ҮР ДҮН

Бид Георадарын хэмжилтийн хөндлөн зүсэлтүүдийн боловсруулалтыг MATLAB ийн хэсэгчилсэн программын хэсгүүдийг ашиглаж мэдээлэл боловсруулалтад зориулж бичигдсэн RadLab гэдэг программ хангамжаар гүйцэтгэсэн. Уг программ хангамжийг Франц улсын Страсбургын Их Сургуулийн (EOST) хамт олон боловсруулсан бөгөөд бид хамтын ажиллагааныхаа хүрээнд уг программ хангамжийг ашиглаж, хөгжүүлэлт хийж байгаа болно.

Өгөгдөлд шинжилгээ, анализ хийх

Георадарын зураглалын эцсийн үр дүнд тайлал хийхдээ ялгаатай үе давхаргуудын хил зааг тэдгээрийн шилжилт хөдөлгөөн болон долгионы ойлт, хугарал, замхрал, долгионы фазын гажилт өгч байгаа хэсгүүдэд анализ хийх замаар тухайн хөрс, хурдас чулуулгын геолегийн бүтэцтэй харьцуулан эцсийн дүгнэлт хийдэг.

ДҮГНЭЛТ

Георадарын 2 болон 3 хэмжээст зураглалын техникээр идэвхитэй хагарлын шинж чанар, параметрүүдийг тодорхойлж, тайлбарлах докторын судалгааны ажлын тавьсан зорилгын хүрээнд гүйцэтгэхээр төлөвлөсөн ажлуудыг амжилттай хийж гүйцэтгэсэн ба Идэвхитэй хагарлын үндсэн төрлүүд дээр 3 өөр давтамжийн антенүүдийг тухайн судалгааны даалгавартаа тохируулан ашиглаж, хэмжилт судалгаа гүйцэтгэж үр дүнгийн зураглалын харилцан уялдаа хамаарлыг гаргаж чадсан.

Судалгааны үр дүнгийн шинжлэх ухааны шинэлэг байдал;

Олон улсын хэмжээнд хийгдсэн палеосейсмологийн судалгааны ажлууд дээр зөвхөн тухайн хагарлынхаа георадарын зүсэлтүүдийг палеотренчийн үр дүнтэй харьцуулсан байдаг бол миний судалгааны ажлын хувьд хагарлын төрлийг тодорхойлох боломжтой гэдгийг жишээ судалгаанууд дээр бодит байдлаар үзүүлэн гаргаж ирсэн.

Хагарлын сунлын дагуу хийгдсэн олон хэмжилтийн үр дүнгүүдийн нийлбэрээр тухайн хагарлын үндсэн шилжилт хөдөлгөөнийг тогтоох боломжтой гэдгийг харуулсан.

Идэвхитэй хагарлын үндсэн хөдөлгөөнийг хагарлын сунлын дагуу олон цэг дээр шилжилт хөдөлгөөн, эвдэрлийн бүтцүүдийг нарийн тогтоож, тэдгээрт нэгдсэн анализ хийж тогтоох нь илүү үндэслэлтэй юм. [START_REF] Sieh | A Review of Geological Evidence for Recurrence Times of Large Earthquakes[END_REF] describes earthquakes repeating periodically on the same segment of the fault. Large earthquakes that can break several segments and moderate earthquakes accommodating slip deficits, c) the characteristic model describes earthquakes repeating periodically over the same rupture length and with similar magnitudes, after [START_REF] Schwartz | Fault behavior and characteristic earthquakes: Examples from the Wasatch and San Andreas Fault Zones[END_REF] 2.4 for location). The main surface fault ruptures (b and c) are shown as heavy lines and are taken from [START_REF] Baljinnyam | Ruptures of major Earthquakes and active deformation in Mongolia and its surroundings[END_REF]. The three main shock sub events (marked 1, 2 and 3) and the mechanism of the principal aftershock (670120) are taken from our analysis of the P and S waveforms. .5 m away from the trench wall. Three main different units were identified in the trench highlighted here by brown, yellow and orange colors. They are superimposed on the trench and the profile for comparison. The brown is a thin brownish unit filling the channel, the yellow is a yellowish coarse deposits unit (centimetric sized) and the orange is a yellowish unit with fine material. Within the brown unit, a gravel lens is observed and highlighted by the red lines. A good consistency is observed between the GPR data and the trench. The gravel lens (red lines) is almost perfectly aligned with a reflection in the GPR profile. The left flank of the channel (limit between the yellow and the orange unit) is as well matching a reflection (arrows) and the yellow unit corresponds to an attenuated signal. On the east side, the unit's interfaces seem to match with dipping reflections but with a lateral offset that can be explained by the distance between the trench and the GPR profile (2.5 m) after [START_REF] Dujardin | Imagerie géoradar (GPR) en milieu hétérogène Application aux failles actives en Mongolie et aux dépôts pyroclastiques du Tungurahua (Equateur)[END_REF] .17 Geometry and growth of a wedge structure and bulldozer effect (Burg, 2017). 172

List of figures

Introduction

Several active faults have been identified around Ulaanbaatar (UB) city, and in this doctoral dissertation we present the results ground penetrating radar (GPR) obtained on these faults. We will first need to answer the following questions:

Is Mongolia a seismically active and dangerous country? If so, how dangerous is the country? Why is Mongolia so seismically active? And so many questions will arise. Therefore,

in order to answer the above questions, it is necessary to first compare the historical earthquakes that took place in the world with the historical earthquakes that took place in Mongolia.

Most people believe that earthquakes often occur on the plate boundary of the world's continents, and that Mongolia is safe because it is in the center of the Central Asia. Mongolia is an earthquake risk country because earthquakes occur not only within the subduction zones of the plates, but also between the micro-plates in the middle of the continent. Mongolia is in the orogeny's region of the Central Asia in terms of geology, and these mountains are seismically active due to their ongoing development by low slip rate. One century of seismicity in Mongolia (1900Mongolia ( -2000) ) show where and how much land are active in the whole territory (Figure 0.5). Since the beginning of the last century, there have been about 100 earthquakes with a magnitude of more than 8.0 on the Earth, almost 50% of which occurred in Asia. From (1957) is the 12th ranked of the earthquake magnitude from 1900 to 2018 year in the world (Table 01). However, these earthquakes are less catastrophic and less damaging, due to the low population density and nomadic lifestyle of this time. However, the urbanization in Mongolia is continually increasing, especially in the capital city of Ulaanbaatar where a population of around 1.5 million is concentrated. Hence, the study of seismic hazard is of first importance for the UB capital which is surrounded by several active faults and showing an increasing of seismic activity since 2005 (See [START_REF] Dugarmaa | Seismic map of Mongolia and site effect microzoning at the capital, Ulaanbaatar[END_REF][START_REF] Schlupp | Investigation of active faults near Ulaanbaataar. Implication for seismic hazard assesment[END_REF][START_REF] Ferry | Tectonic Morphology of the Hustai Fault (Northern Mongolia), A Source of Seismic Hazard for the city of Ulaanbaatar[END_REF][START_REF] Munkhsaikhan | Seismic activity near Ulaanbaatar: Implication for seismic hazard assessment[END_REF].

From the beginning of the last century to the present, data on the magnitude and damage caused by earthquakes and catastrophes around the world clearly show that damages after earthquakes are directly related to the concentration of population and urbanization (Table 012 The current deformation of Central Asia due to the Indo-Asian collision is estimated by the Global Positioning System (GPS) at about 4 cm/year [START_REF] Calais | Continental deformation in Asia from a combined GPS solution[END_REF], while the South China block is compressed at 1 cm/year (Figure 0.4). The first GPS observations in Mongolia began in 1994 and showed that all deformations were much smaller than in the south [START_REF] Calais | New constraints on current deformation in Asia from continuous GPS measurements at UlanBaatar, Mongolia[END_REF][START_REF] Calais | GPS measurements of crustal deformation in the Baïkal-Mongolia area (1994-2002): Implication for current kinematics of Asia[END_REF][START_REF] Calais | Continental deformation in Asia from a combined GPS solution[END_REF].

Figure 0.2 Cumulative number of earthquakes between 2000 and 2021 (area of 140x140 km).

Green line is starting time of new station installation, in 2000 installation of "CTBT Songino array" and in 2008 installation of "UB-mobile", MNDC data.

It is well known that strong earthquakes (Mw ≥ 6) are relatively rare on the human time scale, but they cause significant damage and often many victims. This is the reason why we seek to better understand and anticipate earthquakes. The major questions we would like to be able to answer are: where and when will the next strong earthquake occur on a given fault?

What will be its magnitude and its effects in terms of strong movements of the ground? While most major 'active' faults are now known (Tapponnier, et al., 2001a;[START_REF] Bozkurt | Neotectonics of Turkey-a synthesys[END_REF][START_REF] Prentice | Northern California LiDAR data; Atool for mapping the San Andreas fault and Pleistocene marine terraces in heavily vegetated terrain[END_REF] and with them the location of future great earthquakes, the anticipation of the datemagnitude effects parameters of the strong events to come remains a challenge not attained to date despite a very large number of studies. One of the difficulties of difficulties often are these strong events often long (several hundred and thousands of years), especially than the person's observation period. It is then impossible to know whether the strong earthquakes on a fault are similar or dissimilar, whether they repeat regularly or irregularly, whether they produce the same effects, etc. Only the Park field segment on the San Andreas fault broke during 5

'successive' events of Mw ≈ 6 (1881Mw ≈ 6 ( , 1901Mw ≈ 6 ( , 1922Mw ≈ 6 ( , 1934Mw ≈ 6 ( , 1966Mw ≈ 6 ( , 2004) ) which could have been observed during the last two centuries. Although earthquakes have something in common, they are not similar in terms of location of epicenter and direction of propagation of the rupture and they are not repeated regularly [START_REF] Bakun | The Parkfield, California, Prediction Experiment[END_REF][START_REF] Jackson | The 2004 Parkfield earthquake, the 1985 prediction, and characteristic earthquakes: Lessons for the future[END_REF]. If such irregularities are the norm, anticipating the parameters of future strong earthquakes could be difficult, if not impossible.

One way to expand our knowledge of strong earthquakes and how they repeat themselves is to investigate the past and determine the characteristics (where, when, what magnitude) of strong prehistoric events. Getting to know these characteristics would be a great help in anticipating those of future events. This field of study of past earthquakes (i.e., prior to the historical period and therefore generally in the last ≥ 2000 years) is that of paleo-seismology [START_REF] Yeats | The Geology of Erthquakes[END_REF][START_REF] Weldon | Wrightwood and the earthquake cycle: What a long recurrence record tell us about how faults work[END_REF][START_REF] Mccalpin | Paleoseismology[END_REF]. Figure 0.5 One century [START_REF] Benedetti | Motion on the Kaparelli fault (Greece) prior to the 1981 earthquake sequence determined from 36CI cosmogenic dating[END_REF]of seismicity in Mongolia (1900Mongolia ( -2000)), RCAG and DASE team.

This is notably the case of paleo-seismological trenches [START_REF] Rockwell | Palaeoseismology: Historical and Prehistorical Records of Earthquake Ground Effects for Seismic Hazard Assessment[END_REF][START_REF] Mccalpin | Paleoseismology[END_REF] samples of escarpments exhumed to measure the content of cosmogenic isotopes [START_REF] Schlagenhauf | Using in situ Chlorine-36 cosmonuclide to recover past earthquake histories on limestone normal fault scarps: a reappraisal of methodology and interpretations[END_REF][START_REF] Schlagenhauf | Earthquake supercycles in Central Italy, inferred from 36CI exposure dating[END_REF][START_REF] Palumbo | Slip history of the Magnola fault (Apennines, Central Italy) from 36CI surface exposure dating: evidence for strong earthquakes over the Holocene[END_REF] samples of corals raised seismically [START_REF] Zachariasen | Submergence and uplift associated with the giant 1833 Sumatran subduction earthquake: Evidence from coral microatolls[END_REF][START_REF] Sieh | Earthquake supercycles inferred from sea-level changes recorded in the corals of west Sumatra[END_REF] etc. Finally, it is rare that a paleo-seismological method makes it possible to determine both the displacements produced by the earthquake and the age of the earthquake. Most paleo-seismological studies carried out to date only document the age of some past earthquakes [START_REF] Weldon | Wrightwood and the earthquake cycle: What a long recurrence record tell us about how faults work[END_REF][START_REF] Daëron | 12,000year long record of 10 to 13 paleoearthquakes on the Yammouneh Fault, Levant fault system, Lebanon[END_REF][START_REF] Marco | Long-term earthquake clustering: a 50,000-year paleoseismic record in the Dead Sea Graben[END_REF] but not their movements.

A good documentation is rarely documented by measurement of the earthquake transitions [START_REF] Sieh | The repetition of large earthquake ruptures[END_REF][START_REF] Yeats | The Geology of Erthquakes[END_REF][START_REF] Weldon | Wrightwood and the earthquake cycle: What a long recurrence record tell us about how faults work[END_REF]. There is a long time that observed to the extended period observed to the extended transition, but they do not have a precision because they are based on indirect evidence [START_REF] Liu-Zeng | Serial ruptures of the San Andreas fault, Carrizo Plain, California, revealed by three dimentional excavations[END_REF].

Table 0-2 List of deadliest 20 th century earthquakes (USGS sources).

However, if the determination of the dates of occurrence of strong past earthquakes is crucial, that of the displacements produced is also very important. As the earthquake displacement varies along the ruptures [START_REF] Manighetti | Evidence for selfsimilar, triangular slip disributions on earthquakes: implications on earthquake and fault mechanics[END_REF], it is a priori necessary to document the slip produced by each of the earthquakes passed through several points of the breaking length. The maximum and average displacements produced at the surface can then be determined, and these quantities are fundamental for estimating the magnitude of past events.

It is important to note that the displacements produced on the ground surface are often only a fraction of the displacements produced in depth by the earthquake [START_REF] Manighetti | Earthquake scaling, fault segmentation and structural maturity[END_REF].

The magnitudes estimated at the surface are therefore always minimum values.

The motivation for this thesis work was to help developing a new methodology and acquire new data to better estimate the seismic hazard relating to active faults. These developments and acquisitions have targeted active faults but will eventually be applied to other active faults around Mongolia.

Therefore, this doctoral thesis work was carried out in 5 chapters: GPR method and data processing, applications of GPR imaging technique for active fault structures, paleo- [START_REF] Lu | Characterization of the shallow structures of the Deren Fault by ground penetrating radar[END_REF].

seismological
Chapter 1 : GPR (Ground Penetrating Radar) method and data processing 1 Introduction

The dropping faults move the morphological markers in an approximately horizontal plane and parallel to the surface of the ground. Long records of paleo seismic can therefore reside close to the surface, in the first meters of the ground [START_REF] Mccalpin | Paleoseismology[END_REF]. Geophysical imagery is a potential means of detecting and imaging these records. The choice of the most appropriate geophysical imaging method to image the first meters of soil and detect possible buried objects or structures depends mainly on the geological context, the depth and size of the target, the resolution required, but also of more practical aspects such as the type of land (reliefs, vegetation), the financial cost and the difficulty of data acquisition and processing. Indeed, many geophysical methods exist [START_REF] Reynolds | An Introduction to Applied and Environmental Geophysics[END_REF][START_REF] Telford | Applied Geophysics second edition[END_REF] and are distinguished by their sensitivity, depth of investigation and resolution. Most have already been applied to active faults in terrestrial and marine environments [START_REF] Mccalpin | Paleoseismology[END_REF]. The most common are seismic reflection and refraction, electrical tomography, electromagnetism, magnetism and gravimetry. The scale of application and the resolution of these methods are of the order of a few centimeters (GPR) to ten kilometers (seismic reflection). They are generally applied in fault studies and in paleo seismology in particular in order to 1) recognize sites before slicing or drilling [START_REF] Salvi | Investigation of the active Celano-L'Aquila fault system, Abruzzi (central Apennines, Italy) with combined ground-penetrating radar and palaeoseismic trenching[END_REF][START_REF] Vanneste | Paleoseismologic investigation of the fault rupture of the 14 April, 1928 Chirpan earthquake (M 6.8), southern Bulgaria[END_REF], 2) define the geometry of the fault plane in depth (up to the crustal scale) [START_REF] Vergne | Seismic evidence for stepwise thickening of the crust across the NE Tibetan plateau[END_REF][START_REF] Wittlinger | Tomographic evidence for localized lithospheric shear along the Altyn Tagh fault[END_REF][START_REF] Nguyen | Slow active fault detection and imaging using multiple geophysical methods : the Trevaresse thrust (Provence, France)[END_REF], 3) detect faults that do not reach the surface [START_REF] Cavinato | Sedimentary and tectonic evolution of Plio-Pleistocene alluvial and lacustrine deposits of Fucino Basin (central Italy)[END_REF][START_REF] Caputo | Late Quaternary activity along the Scorciabuoi Fault (Southern Italy) as inferred from electrical resistivity tomographies[END_REF], 4) measure the vertical displacements of the stratigraphic layers (McClymont, et al., 2009a;Yetton & Nobes, 1998a). Over the past decade, the GPR has become more and more popular in paleo seismological studies due to its non-destructive nature, its speed of implementation and its low cost, which allow it to be densely applied over large areas. GPR also achieves higher resolutions than other existing geophysical methods. The GPR has been particularly suitable for locating fault plans in quaternary formations and urban areas [START_REF] Smith | Ground penetrating radar : antenna frequencies and maximum probable depths of penetration in Quaternary sediments[END_REF][START_REF] Cai | Application of ground-penetrating radar to investi-gation of near-surface fault properties in the San Francisco Bay region[END_REF][START_REF] Audru | GPR investigations on active faults in urban areas: the Georisc-NZ project in Wellington, New Zealand[END_REF][START_REF] Demanet | The use of geophysical prospecting for imaging active faults in the Roer Graben, Belgium[END_REF][START_REF] Gross | Shallow geometry and displacements on the San Andreas Fault near Point Arena based on trenching and 3D georadar surveying[END_REF], to identify sites with high potential for future excavations [START_REF] Salvi | Investigation of the active Celano-L'Aquila fault system, Abruzzi (central Apennines, Italy) with combined ground-penetrating radar and palaeoseismic trenching[END_REF][START_REF] Liberty | The Portland Hills Fault : uncovering a hidden fault in Portland, Oregon using high-resolution geophysical methods[END_REF][START_REF] Jewell | GPR studies in the Piano di Pezza area of the Ovindoli-Pezza Fault, Central Apennines, Italy: extending paleoseismic trench investigations with high resolution GPR profiling[END_REF][START_REF] Malik | Paleoseismic evidence from trench investigation along hajipur fault, himalayan frontal thrust, nw himalaya : Implications of the faulting pattern on landscape evolution and seismic hazard[END_REF] and to interpolate data between trenches [START_REF] Ferry | Ground-penetrating radar investigations along the North Anatolian fault near Izmit, Turkey[END_REF][START_REF] Slater | Ground-penetrating radar investigation of active faults along the Dead Sea Transform and implications for seismic hazards within the city of Aqaba, Jordan[END_REF]. In many cases, it is combined with other imaging methods, notably electric tomography [START_REF] Demanet | The use of geophysical prospecting for imaging active faults in the Roer Graben, Belgium[END_REF][START_REF] Lehmann | Paleoseismological investigations at the Rurrand Fault, Lower Rhine Embayment[END_REF][START_REF] Vanneste | Pseudo-3D imaging of a low-slip-rate, active normal fault using shallow geophysical methods : The Geleen fault in the Belgian Maas River valley[END_REF]. In terms of depth of investigation, resolution, and speed of implementation, the GPR is the most suitable method for detecting metric-sized targets in the first ten meters of the ground over wide areas.

The objective here is to use the GPR along active faults in order to detect the displacements accumulated over time by buried morphological markers. The existence of such markers in the first meters of the ground, mainly abandoned rivers, has been highlighted in some 3D trenches [START_REF] Wesnousky | An offset Holocene stream channel and the rate of slip along the northern reach of the San Jacinto fault zone, San Bernardino Valley, California[END_REF][START_REF] Marco | Late Holocene activity of the Dead Sea Transform revealed in 3D palaeoseismic trenches on the Jordan Gorge segment[END_REF][START_REF] Noriega | Stream channel offset and Late Holocene slip rate of the San Andreas fault at the Van Matre Ranch site, Carrizo Plain, California[END_REF][START_REF] Rockwell | Paleoseismology of the Johnson Valley, Kickapoo, and Homestead Valley faults : clustering of earthquakes in eastern California shear zone[END_REF][START_REF] Rockwell | Palaeoseismology: Historical and Prehistorical Records of Earthquake Ground Effects for Seismic Hazard Assessment[END_REF][START_REF] Liu-Zeng | Serial ruptures of the San Andreas fault, Carrizo Plain, California, revealed by three dimentional excavations[END_REF] or by geophysical imagery [START_REF] Mcneilan | Style and rate of Holocene slip, Palos Verdes fault, southern California[END_REF][START_REF] Ferry | Ground-penetrating radar investigations along the North Anatolian fault near Izmit, Turkey[END_REF]. The low spatial extent of these studies, however, limits the detection of these shifts to only around ten meters, or the displacement produced by the last few strong events.

We describe in this chapter the different steps that we followed during our application of the GPR to paleo seismology. At each step, we describe the necessary data and the methods of processing and analysis. We can thus conclude on the reliability, objectivity, and advantages of this approach, as well that discuss the uncertainties, the drawbacks, and the subjectivity of the results.

Principle of the GPR

Ground Penetrating Radar (GPR) is a high-resolution, near surface, geophysical method based on the propagation, reflection and scattering of high frequency (from 10 MHz to 2 GHz) electromagnetic (EM) waves in the Earth [START_REF] Daniels | Introduction to subsurface radar[END_REF][START_REF] Jol | Ground Penetrating Radar: Theory and Applications[END_REF][START_REF] Van Dam | Landform characterization using geophysics -Recent advances, applications and emerging tools[END_REF].

EM waves sent into the ground are reflected at interfaces of electric and dielectric contrast and the reflected signals are recorded. Those discontinuities can result from changes in the water content, the chemical composition, or the texture of the deposits. In sedimentary deposits, variations in grain size and water saturation are the main factors triggering reflections. GPR has proven its efficiency in many fields of geosciences but seems generally most efficient in sedimentary deposits, marked lithological boundaries, fractures and/or faults [START_REF] Gross | Location and geometry of the Wellington Fault (New Zealand) defined by detailed three-dimensional georadar data[END_REF][START_REF] Deparis | On the potential of ground penetrating radar to help rock fall hazard assessment: a case study of a limestone slab, Gorges de la Bourne (French Alps)[END_REF][START_REF] Mcclymont | Shallow fault segmentation of the Alpine fault zone, New Zealand, revealed from 2and 3-D GPR surveying[END_REF], even for subtle changes in the nature, size, shape, orientation and packing of grains [START_REF] Guillemoteau | Influence of grain size, shape and compaction on georadar waves: examples of aeolian dunes[END_REF]. In the sedimentological context, interpretation generally uses GPR-facies analysis, analogous to their use in seismic studies [START_REF] Jol | GPR in sediments: advice on data collection, basic processing and interpretation, a good practice guide[END_REF][START_REF] Neal | Ground-penetrating radar and its use in sedimentology: principles, problems and progress[END_REF]. Ground Penetrating Radar (GPR) is a geophysical imaging method that detects reflections and diffractions of electromagnetic waves occurring at the dielectric, electrical and magnetic contrasts of the sub-surface within an electrically resistive medium. This is a relatively recent geophysical method which has found a wide field of applications over the last twenty years [START_REF] Annan | GPR-History, Trends, and Future Developments[END_REF] for a background, and [START_REF] Jol | Ground Penetrating Radar: Theory and Applications[END_REF][START_REF] Beres | Application of Ground-Penetrating-Radar Methods in Hydrogeo-logie Studies[END_REF] for a review. description of the main applications).

In the context of actives faults, the ground penetrating radar (GPR) method gives good and useful results. It can help to characterize faults by identifying offsets of radar reflections and buried fluvial channel deposits [START_REF] Beauprêtre | Finding the buried record of past earthquakes with GPR based palaeoseismology: a case study on the Hope fault, New Zealand[END_REF]. The investigation depth depends on the electromagnetic (EM) attenuation of the medium and the frequency of the antenna used.

The lower the frequency, the greater the penetration depth, which varies from a few centimeters in conductive materials up to 50 m for low conductivity (less than 1 mS m -1 ) media [START_REF] Jol | Ground Penetrating Radar: Theory and Applications[END_REF].

The vertical resolution depends on the velocity of EM waves and the frequency used. In general, features such as sedimentary structures, lithological boundaries, fractures and/or faults are clearly visible with GPR [START_REF] Deparis | On the potential of ground penetrating radar to help rock fall hazard assessment: a case study of a limestone slab, Gorges de la Bourne (French Alps)[END_REF][START_REF] Christie | Assessing fault displacement and off-fault deformation in an extensional tectonic setting using 3-D groundpenetrating radar imaging[END_REF][START_REF] Mcclymont | Shallow fault segmentation of the Alpine fault zone, New Zealand, revealed from 2and 3-D GPR surveying[END_REF] even when these features differ only by small changes in the nature, size, shape, orientation and packing of grains [START_REF] Guillemoteau | Influence of grain size, shape and compaction on georadar waves: examples of aeolian dunes[END_REF].

By describing in the following the principle of electromagnetic (EM) wave propagation, we will introduce the concepts and vocabulary inherent in this method, as well as the signal and noise sources that we will try to amplify and to reduce, respectively, during processing.

Propagation of electromagnetic waves

The principle of GPR is based on the ability of materials to polarize under the presence of an electromagnetic field. This electromagnetic (EM) field propagates in the ground in the form of electromagnetic waves. This propagation is expressed by the equation EM waves obtained from Maxwell's equations, eliminating either the field electric, or the magnetic field.

We obtain the following wave equation by eliminating the magnetic field in a homogeneous and isotropic medium:

𝛻𝛻 2 𝐸𝐸 + 𝑘𝑘 2 𝐸𝐸 = 0 1.1a 𝛻𝛻 2 𝐸𝐸 -𝜇𝜇𝜇𝜇 • 𝜕𝜕𝜕𝜕 𝜕𝜕𝜕𝜕 -𝜇𝜇𝜀𝜀 • 𝜕𝜕 2 𝜕𝜕 𝜕𝜕𝜕𝜕 2 = 0 1.1b (1) (2) (3)
with 𝐸𝐸 the electric field, k the wave number, µ the magnetic permeability (H.m -1 ), ε the dielectric permittivity (F.m -1 ), σ the electrical conductivity (S.m -1 ). For the sedimentary (no-magnetic) rocks, the magnetic permeability (µ) is supposed to be equal to µo =4π*10 -7 H.m -1 , the magnetic permeability of the free space. In that case µr = µ/µo the relative permeability is equal to 1. The speed of the waves will therefore be essentially dependent on the dielectric permittivity, some values of which for rocks are given in [START_REF] Annan | Electromagnetic Principles of Ground Penetrarting Radar[END_REF]. (Eqns. 1.1a) characterized by the vector Helmholtz equation. In electromagnetics, the vector Helmholtz equation is the frequencydomain equivalent of the lossy wave equation (Eqns. 1.1b). The first term (1) is called the Laplacian (𝛻𝛻 2 ). The second term (2), which contains a first order time derivative, controls the diffusive behavior of the electromagnetic signal and the third term (3), which contains a second order time derivative, represents an energy conservation term in (Eqns. 1.1b). The electromagnetic waves will be able to propagate in the medium when the diffusive term is weak compared to the propagative term, that is, in a weakly conductive medium and at high frequencies. Indeed, the behavior of the waves is depending on the frequency and the physical properties of the medium. At low frequencies, the behavior of the waves will be diffusive, and the wave will diffuse with different speeds depending on the frequency (dispersion). At high frequencies (> 10 MHz), the speed of the waves becomes independent of the frequency; this is the case of ground penetrating radar (GPR) method. The transition frequency ft between diffusive/propagative regimes for a simple material, is given by equation [START_REF] Annan | Electromagnetic Principles of Ground Penetrarting Radar[END_REF]:

𝑓𝑓 𝜕𝜕 = 𝜎𝜎 2𝜋𝜋𝜋𝜋 1.2
For frequencies higher than this value, all frequency components have the same propagation speed and attenuation; the medium is non-dispersive. The propagation speed V of an impulsive signal in the absence of dispersion is given by equation [START_REF] Annan | Electromagnetic Principles of Ground Penetrarting Radar[END_REF]:

𝑉𝑉 = 1 √ 𝜇𝜇𝜋𝜋 = 𝑐𝑐 √𝜅𝜅 1.3
Since µ =µo, the speed of the waves will therefore be essentially dependent on the dielectric permittivity, c is the speed of light (3*10 8 m/s). During its propagation, the amplitude of the radar signal decreases exponentially with the propagation distance z (e -αz ) in the medium, where the attenuation α is given by [START_REF] Annan | Electromagnetic Principles of Ground Penetrarting Radar[END_REF]:

𝛼𝛼 = � 𝜇𝜇 𝜋𝜋 • 𝜎𝜎 2 1.4
In addition to the intrinsic attenuation α (ohmic or geometrical attenuation), there is another attenuation (scattering attenuation) due to the presence of heterogeneities (scatters) which diffract or reflect the energy in all directions. Another characteristic of the EM wave propagation is the impedance Z which describes the resistance of the medium to the passage of the wave and is given by [START_REF] Annan | Electromagnetic Principles of Ground Penetrarting Radar[END_REF]:

𝑍𝑍 = � 𝜇𝜇 𝜋𝜋 = 𝑍𝑍 0 √𝜅𝜅 1.5
𝑍𝑍 0 is the impedance of free space and 𝜅𝜅 is the dielectric constant (κ = 𝜀𝜀 𝜀𝜀 0 � , where 𝜀𝜀 0 is the permittivity of vacuum, 8.89*10 -12 F/m), Equation (1.5) is valid in the case of GPR frequency range between 10 MHz -2 GHz (Cassidy N. , 2009). An impedance contrast between two layers describes an interface on which the reflections are characterized by their reflectivity; a crucial property for reflection imaging.

Table 1-1 Typical values of relative permittivity and conductivity for materials conventional sub-surface for an antenna frequency of 100 MHz, after [START_REF] Annan | Electromagnetic Principles of Ground Penetrarting Radar[END_REF]. , generally equal to 1. For an antenna of center frequency fc = 100 MHz, the transmitted signal will therefore have a frequency band B of 100 MHz centered in 100 MHz, and a duration of 10 ns. The ratio R tends to be maximized, so to widen the frequency band while reducing the center frequency and thus increasing both the resolution and penetration depth. The transmitting antenna emits in all directions defined by the radiation diagram close to the air-ground interface. Therefore, part of the energy will be transmitted into the ground, and some energy will travel through the air. When the source is located exactly at the air-ground interface, the entire downward energy is transmitted into the ground. When the source is above the air-ground interface most of the energy is reflected at the air-ground interface. Therefore, a good coupling between the transmitting antenna and the ground is therefore essential for a maximum signal to be transmitted into the ground.

Reflection, diffraction, refraction, transmission -The emitted EM waves will then follow the principles of reflection, transmission and refraction according to Snell-Descartes and Huygens-Fresnel diffraction laws (Figure 1.1). The reflection and transmission coefficients (Eqns. 1.6, 1.7, 1.8 and 1.9, [START_REF] Cassidy | Ground Penetrating Radar Data Processing, Modelling and Analysis[END_REF] describe how the amplitude of EM waves is varying when waves meet an interface between two different media depending on the angle of incidence (Zoepritz equations). These coefficients are different depending on the transverse electrical (TE) or transverse magnetic (TM) component of the EM field.

𝑅𝑅 𝑇𝑇𝜕𝜕 = 𝑍𝑍 1 -1 𝑐𝑐𝑐𝑐𝑐𝑐 𝜃𝜃 1 -𝑍𝑍 2 -1 𝑐𝑐𝑐𝑐𝑐𝑐 𝜃𝜃 2 𝑍𝑍 1 -1 𝑐𝑐𝑐𝑐𝑐𝑐 𝜃𝜃 1 +𝑍𝑍 2 -1 𝑐𝑐𝑐𝑐𝑐𝑐 𝜃𝜃 2 1.6 𝑅𝑅 𝑇𝑇𝑇𝑇 = 𝑍𝑍 1 𝑐𝑐𝑐𝑐𝑐𝑐 𝜃𝜃 1 -𝑍𝑍 2 𝑐𝑐𝑐𝑐𝑐𝑐 𝜃𝜃 2 𝑍𝑍 1 𝑐𝑐𝑐𝑐𝑐𝑐 𝜃𝜃 1 +𝑍𝑍 2 𝑐𝑐𝑐𝑐𝑐𝑐 𝜃𝜃 2 1.7 𝑇𝑇 𝑇𝑇𝜕𝜕 = 1 -𝑅𝑅 𝑇𝑇𝜕𝜕 1.8 𝑇𝑇 𝑇𝑇𝑇𝑇 = 1 -𝑅𝑅 𝑇𝑇𝑇𝑇 1.9
Where 𝑍𝑍 1 is the impedance in the first medium, 𝑍𝑍 2 is the impedance in the second medium, 𝜃𝜃 1 is the incident/reflection angle, and 𝜃𝜃 2 is the transmission angle.

Received Signal -The receiving antenna first receives the direct wave in the air (V = 0.3 m/ns), then the direct wave in the ground (V ≈ 0.1 m/ns), and then the various reflected and diffracted waves in air and in the ground. When the source-to-receiver distance is small (offset and angle of incidence almost zero), which is generally the case for ground penetrating radar acquisition in common offset, the arrival times are close, and some arrivals may interfere with each other.

The received EM signals are converted into an electrical signal which is digitally reduced under the shape of an amplitude as a function of time called the trace shown in Figure 1.2. These traces are then juxtaposed to obtain an image of the reflectivity (radar section) of the subsoil.

Figure 1.1 Path of propagation of electromagnetic waves in the ground in the case of a twolayer medium with contrasting dielectric permittivity (𝜀𝜀 1 and 𝜀𝜀 2 ), after [START_REF] Sperl | Determination of spatial and temporal variations of soil water content in an agro-ecosystem with ground penetrating radar[END_REF].

Resolution and penetration depth

Ground penetrating radar performance can be described in terms of two interrelated characteristics: maximum penetration depth and resolution [START_REF] Davis | Ground penetrating radar for high resolution mapping of soil and rock stratigraphy[END_REF]. The maximum penetration depth refers to the maximum depth at which a buried target can be detected, and the resolution is the minimum separation between two buried targets so that they remain discernible. These two characteristics are strongly influenced by the electromagnetic properties of the medium [START_REF] Noon | A frequency-independent characterisation of GPR penetration and resolution performance[END_REF]. In addition, there is a trade-off between penetration and resolution due to the dependence opposite of these two characteristics to frequency. For example, to penetrate the ground efficiently, low frequency signals are needed because the subsoil acts as a low pass filter. However, if the targets to be imaged are relatively close to each other, a wide bandwidth with a good resolving power is necessary, high-frequency signals.

Resolution is a concept common to all detection methods based on wave propagation and can be described in terms of received echoes: echoes can happen simultaneously, overlap or be separated in time [START_REF] Annan | Electromagnetic Principles of Ground Penetrarting Radar[END_REF]. The resolution therefore depends directly on the width (or center frequency) of the pulse propagating in the soil and is divided into two components, one lateral and one deep (Figure 1.3).

Figure 1.2 Signal received by the receiving antenna and the graphic profile display (radar section), after [START_REF] Benson | Geophysical Techniques for Sensing Buried Wastes and Waste Migration[END_REF].

Figure 1.3 Ground penetrating radar resolution is divided into two components, depth resolution and lateral resolution, after [START_REF] Annan | Electromagnetic Principles of Ground Penetrarting Radar[END_REF].

In-depth (or vertical) resolution (∆r) is defined according to the Rayleigh criterion:

∆𝑟𝑟 ≥ 𝑊𝑊 𝜐𝜐 4 1.10
With W the width of the pulse and V the speed of EM waves. The width of the pulse is directly linked to the frequency band B which is equal to the center frequency fc of the antenna, thus we have.

𝑊𝑊 = 1 𝐵𝐵 = 1 𝑓𝑓 𝑐𝑐 1.11
Therefore, the vertical resolution can then be expressed:

∆𝑟𝑟 ≥ 𝜐𝜐 4𝑓𝑓 𝑐𝑐 = 𝜆𝜆 𝑐𝑐 4 1.12
With λc the wavelength of the center frequency.

In an ideal case of a medium without attenuation, the vertical resolution is independent of the distance to the source. In practice, the attenuation causes a shift in the spectrum towards the low frequencies and therefore a decrease in fc (or frequency band B) with depth. The lateral resolution depends on the speed, the pulse width and distance to the source.

The lateral resolution (∆l) is equal to the radius of the first Fresnel zone for a monochromatic signal of frequency fc and can be expressed [START_REF] Annan | Electromagnetic Principles of Ground Penetrarting Radar[END_REF]:

𝛥𝛥𝛥𝛥 = � 𝑑𝑑𝜆𝜆 𝑐𝑐 2 1.13
Where d is the depth of the target. If we calculate these resolutions for different values of antenna frequencies on the market, we obtain the values described in.

The maximum penetration depth is governed by the attenuation which characterizes the exponential decay of the signal as it propagates in the soil. For the EM waves, attenuation is mainly controlled by the conductivity of the medium. More the medium is conductive (like clays, or rocks saturated with water), the stronger the attenuation. Part of the energy is also dissipated by the phenomena of diffraction (scattering) in very heterogeneous media. The attenuation also increases with the frequency. Finally, the penetration depth also depends on the strength of the signal emitted, but this is limited by legislation so as not to interfere with other instruments (TV, radio, navigation instruments, etc.).

In the first order, the penetration depth (δ) of an GPR wave is given by:

𝛿𝛿 = 2 𝜎𝜎 * � 𝜋𝜋 𝜇𝜇 1.14
with µ the magnetic permeability, σ the conductivity, ε the dielectric permittivity. The penetration depth is of the order of 2 m at 800 MHz, 10 m at 250 MHz and may reach 30 m at 100 MHz, if the medium is moderately electrically resistive (see [START_REF] Annan | Electromagnetic Principles of Ground Penetrarting Radar[END_REF].

The success of a GPR survey therefore depends on an important factor, the prior knowledge of the target. The choice of acquisition parameters will depend on the depth, size, and nature of the target's cash flow, depending on the capacities and limits of GPR method that we have just described. We can also bypass this duality between resolution and penetration depth by performing multiscale imaging, i.e., multi-resolution, thanks to broadband or multifrequency radar measurements.

Acquisition and topographic data

Topographic data has two interests in our approach. The first is from accurately position GPR profiles and correct topographic variations along the profiles. For 3D and pseudo 3D campaigns, precise positioning of GPR profiles relative to each other is vital for making correlations of a side to side or horizontal sections. For a GPR acquisition at 100 MHz and 250

MHz, topographic data should have a sampling interval of 2 m and 1 m, respectively, and a spatial precision of 5 cm and 3 cm [START_REF] Cassidy | Ground Penetrating Radar Data Processing, Modelling and Analysis[END_REF]. The second interest is knowing the topography of the surface in detail to perform a morphological analysis marker present and then compare them with information from the data GPR.

During all acquisition missions, we used differential GPS systems for the topographic surveys, RTK survey method. We used this information during morpho-tectonic analyzes, and for topographic corrections of our GPR profiles. The data are in the form of point clouds with XYZ coordinates. The topographic data corresponding to the GPR profiles are linearly interpolated to get a point every 20 cm. The 3D set of points is interpolated by the kriging method on grids, according to the density of points, of 2 x 2 m2 for GPS data using SURFER software, in order to obtain digital elevation models (DEM) that can be used in GIS software.

Differential RTK GPS

Principle -As its name suggests, the principle of differential GPS is based on difference of two signals received at two neighboring points which makes it possible to reduce the effects measurement errors. It is therefore enough to observe at a known point the fluctuations of measurements and subtract them from a second point observing the same satellites Figure 1.4.

This corrects a large part of the measurement errors, whether they are related satellites (clock, orbits), propagation conditions (tropospheric effects, ionospheric, atmospheric) or to voluntary fluctuations in the transmitted signal. The positioning accuracy then drops from around 10-20 meters to a few centimeters and more so as the distance between the measuring points is small.

The "differential" mode exists in several variants. The most elaborate uses the measure of several phase differences, thus increasing the accuracy of the measurements. The RTK GPS we used, is that the receivers communicate by radio to transmit the corrections in real time and the data is processed also in real time. A disadvantage of RTK GPS is that it uses radio waves like ground penetrating radar. RTK GPS systems must therefore stay at an enough distance from ground penetrating radar not to disturb the measurements.

GPS data acquisition and processing -During the GPR mission, we had two Trimble 5700 RTK GPS systems and Piksi Multi RTK GNSS system, each made up of a rover, a base and a controller, transmitting corrections and data to each other in real time by radio. That system is dedicated to locating GPR profiles, and the other to topographic surveying. Indeed, even though the relative positions of the RTK GPS data are known to an accuracy of a centimeters, it is necessary to precisely know their absolute position in order to import them into GIS software and superimpose them on other geo referenced data (high resolution). The relative position of the mobile RTK GPS data is already corrected in real time, their position is simply translated by the same quantity as the translation of the base position after correction.

The relative positions of the mobile receiver with respect to the base are calculated in a second step for the data from kinematic systems.

GPR data acquisition

The type of GPR system and the acquisition geometry are parameters to be chosen before to start the acquisition according to the depth of the target, its geometry (size, composition, orientation), the nature of the target and its surrounding.

Description of the GPR system. A ground penetrating radar system is made up of two antennas and their electronics, a control (recorder, analog-to-digital converter), a trigger, a computer, and a power supply Figure 1.5.

There are different types of GPR antennas; we can distinguish them according to:

• Their center frequency generally offered by manufacturers between 25 MHz and 2.5 GHz.

The lower limit is constrained by the physical limits of propagation EM waves in sub-surface materials. Depending on the size of the target, the center frequency, on which the resolution depends, should be minimized to avoid energy losses associated with attenuation of high frequencies.

Mode of acquisition

The purpose of GPR is to receive the signal reflected by the interfaces of the subsoil. It The second acquisition mode in the common midpoint (multiple offsets, CMP, Figure 1.6) is carried out by gradually moving the antennae away from each other at regular intervals relative to a central point (Mid-Point). The reflections then appear in the form of hyperbolas and the equation for a tabular medium can be expressed as follows:

𝑡𝑡 2 = 𝑡𝑡 0 2 + 𝑥𝑥 2 𝜐𝜐 2
1.15

with t the arrival time of the reflection, t0 the arrival time at zero offset, x the offset and V the average speed between two interfaces, the unknowns being the speed and the time arrival at zero offset (t0).

CMPs are mainly used to determine speed of the radar waves in the medium as a function of depth and are achieved at a few points of the investigated area. The ideal would be to know the speed at any point in the medium, this is called multi-offset acquisitions or multiple coverage reflection. This method of acquisition (as in seismic reflection) reduces the signal / noise ratio by summing the traces and providing knowledge of the speed at any point in the medium. This type of data can be inverted in order to find the physical properties of the medium (dielectric permittivity, conductivity). However, given the distances considered here and the number of profiles, this heavy acquisition had to be abandoned. Only a few (3-4) CMPs were therefore acquired in order to estimate the speeds of radar waves. [START_REF] Bano | Imagerie de la proche surface par géoradar[END_REF][START_REF] Steelman | Comparison of petrophysical relationships for soil moisture estimation using GPR ground waves[END_REF].

Acquisition parameters

The triggering of the acquisition is controlled by a coding wheel (pre-calibrated)

attached to the antenna. The center frequencies of the chosen antennas impose a sampling in time (∆t) and in space (∆x) respecting the Nyquist criterion:

𝛥𝛥𝑡𝑡 ≤ 1 2𝑓𝑓 𝑚𝑚𝑚𝑚𝑚𝑚 1.16 ∆𝑥𝑥 ≤ 𝜐𝜐 2𝑓𝑓 𝑚𝑚𝑚𝑚𝑚𝑚
1.17

Given the relationship between frequency band and center frequency, it is recommended to use a sampling frequency greater than 6 to 10 times the center frequency (no anti-aliasing filter),

i.e., a sampling frequency of 1000 MHz and of 2500 MHz for the selected antennas (∆t ≤ 1 ns and 400 ps respectively). For an average speed of 0.1 m.ns-1, the sampling interval between the measurement points must be less than 4 cm. In order to optimize the speed acquisition time, we also adapt the acquisition time and therefore the number of samples per trace. In addition, at each point, a trace is issued and received several times, then stacked (summed). By increasing the number of traces summed up, the signal/noise ratio is increased but the acquisition speed is reduced. These last two parameters are adapted to each site investigated, depending on the estimated penetration depth from initial test of measurements.

Acquisition geometry

The acquisition geometry depends on the size and orientation of the target. It is one the originalities of our application of ground penetrating radar to paleo seismology. Indeed, the first objective is to image buried morphological markers on both sides of a fault and not the fault plane itself. More specifically, the purpose of this application is to measure the amount of displacement accumulated on buried markers. We are therefore looking for markers oriented perpendicularly at the fault plane, or close to perpendicular. In order to cut these markers perpendicularly, we make the GPR profiles parallel to the fault. These profiles have been conducted closest to the fault trace in order to determine precisely where the markers morphological intersect it and cover minimum 50 m on either side of the fault in order to define the geometry of these markers. We also performed 3 to 4 perpendicular profiles to the fault on each site in order to image the fault zone and help us correlating reflectors on both sides of the fault. We have just described the different types of data acquired as well as the signals and sources of noise and EM wave attenuation. The objective of Signal processing described below is to reduce these effects in order to highlight the reflections.

Processing

GPR data processing

The processing of GPR data includes a subjective part because it depends on the opinion and the experience of the interpreter as well as the nature of the dataset and the targets. We have followed the classic steps of advanced GPR data processing as described in reference books and articles (give a reference here). The goal of these treatments is to increase the signal to noise ratio, to image as correctly as possible the various complex interfaces, while avoiding or correcting artifacts. The flow chart of the processing is given in Figure 1.7.

Time-zero corrections

The data is first converted from commercial Ramac format to Seismic Unix format, software that we used for data processing. The arrival time of the direct wave (T0) is then aligned in all the profiles to correct the offsets or drifts in recording time that may result from stopping/resuming the acquisition, too low battery problem, the length and tension of the optical fibers, etc. In order to detect and realign the arrival times we have chosen as a criterion the first passage from a positive amplitude (in black in Figure 1.8a) to a negative amplitude (in white in

Editing data

This step is essential and tedious. It asks beforehand to have the location and topography of profiles obtained from GPS data and LiDAR. Due to acquisition conditions, errors, and obstacles in the field, the profiles must be combined, reversed, sorted. White traces must be added in order to fill acquisition stops when crossing fences, streams and other obstacles. When all the GPR profiles are corrected, it is necessary to check and correct the acquisition parameters entered in the headers of each trace as well as adding the position in XYZ of each track obtained from GPS data. Like GPR profiles and GPS were acquired separately, landmarks were placed in the field every 30-50 m and read by the two systems in order to readjust the two data sets.

The localization traces can also be used to correct the triggering irregularity problems associated with the roughness of the ground, the jamming of the encoder wheel etc. This is done by interpolating the traces to ensure regular spacing, necessary during subsequent processing steps such as migration.

De-wow, ringing reduction and lateral equalization of the traces

This step consists of removing the noise created by the fitting and the acquisition. The "wow" is the result of the saturation of the signal by the first arrivals of the direct waves and inductive coupling [START_REF] Annan | Practical processing of GPR data[END_REF]. The ringing is due to the multiple reflections on the shielded antenna or the air/ground interface due to poor antenna coupling with the ground. In addition, as this coupling is not regular along the profile, the amplitude of the signal transmitted in the soil varies laterally and is not related to the changes of physical properties of the medium (see Figure 1.9).

The "de-wow" consists in reducing the DC component of the signal and its drift in time by subtracting the average value of each trace and applying a high pass filter, respectively. The ringing is reduced by subtracting from each trace, the trace averaged over a sliding window of a given width, this also reduces the amplitude of the horizontal reflectors. [START_REF] Kim | Removal of ringing noise in GPR data by signal processing[END_REF] propose other methods more sophisticated in order to suppress the ringing.

Figure 1.9 Effect of de-wow filter. After [START_REF] Cassidy | Ground Penetrating Radar Data Processing, Modelling and Analysis[END_REF].

The amplitude of the traces is equalized laterally by normalizing each trace by its maximum amplitude value or its RMS value. In (Figure 1.10), we notice in particular the reduction of the wave in the air which increases the dynamics of the image: the reflections appear better, as well as the beats which have been reduced but not suppressed. GPR profile after De-wow processing.

Filtering

Filters are typically applied to remove human induced noise and to improve the visual quality of the data. They are also useful to highlight certain aspects of the data to facilitate interpretation. There are many filter types, 1D (temporal or spatial) or 2D, simple or sophisticated, in time or frequency domain. We used a first filter, high pass during the de-wow, and a band pass filter to remove all signals of lower frequencies or greater than the signal frequency band determined from the data spectrum. Depending on the medium, the frequency band will be translated towards the low frequencies relative to the frequency band of the source signal. We also apply a spatial filter, in the form of a weighted sliding average, which improves the continuity of reflectors. This sliding average manages to reduce air diffraction hyperbolas when applied to a window wide enough but tends to smooth the data and therefore reduce the resolution. [START_REF] Bano | Ground penetrating radar study of dry aeolian environment[END_REF][START_REF] Sun | Recognizing surface scattering in ground-penetrating radar data[END_REF] have proposed a method to remove these diffractions in air but this method cannot be fully automated and requires the presence of the vertex of the diffraction hyperbola on the profile.

Flat Reflection Filter

Ground penetrating radar data often presents noise that appears as coherent horizontal reflections and periodic. This is a ringing noise effect, due to the instrumentation. This noise appears when the signal-to-noise ratio is less than 1. If it is not subtracted, this noise can completely mask reflections from deep geological structures. These exists several methods to remove these flat reflections [START_REF] Kim | Removal of ringing noise in GPR data by signal processing[END_REF]. The method used by Radlab (Software developed at EOST, Strasbourg) is a subtraction of an average trace, or background removal.

As this noise is consistent over the entire length of the section (real reflections are much less consistent), if we take the average of the traces on a sliding window in x, we thus obtain an average trace containing only resonant noise. By subtracting this mean trace from the data, we thus remove the flat reflections noise (Figure 1.11). However, there is always a risk of removing real horizontal reflections appearing at the same time and coming from geological structures, such as water tables. In our case, however, this has an advantage because it allows sloping reflectors to appear better. Figure 1.11 shows a GPR profile with and without the Flat Reflection Filter applied. The data is much less noisy, and we see appearing the reflections coming from a channel structure.

Amplification -Time gain

The signal is then amplified in order to better see the low amplitudes. We give a value of 80 dB: the factor between signals of smaller amplitude and signals of higher amplitude is 40 in this case (Figure 1.12).

Automatic gain control

This filter attempts to adjust the gain of each trace by equalizing the mean amplitudes observed in a sliding time window. A short window gives a more pronounced effect, the extreme of which would a one-sample window which would cause all amplitudes to be equal.

The other extreme would be a time window of the same length as the trace. This would have no effect on the trace. After equalization a constant multiplier is applied to the trace to make the resulting amplitudes reasonable (Figure 1.14, Figure 1.17).

Figure 1.14 The effect of an AGC function applied to a radar trace. See [START_REF] Inc | EKKO-for-DVL pulseEKKO 100 User's Guide Version 1[END_REF][START_REF] Inc | From Sensors & Software Inc Web site[END_REF].

Velocity analysis

In order to convert the time radar section into depth radar section we need to know the speed of radar waves in the medium. A common method (coming from seismic) is by analyzing the common mid-point CMP) profiles. For a range of velocities, the CMP data are corrected for NMO (normal move out) with a constant velocity and then a summation (stack) is preformed (See Figure 1.15 below, [START_REF] Yilmaz | Seismic data processing[END_REF]).

A 2010 GPR study jointly by Jilin University and Tohoku University of the Deren active fault determined that the velocity of radar waves within the subsurface sediment was 0.12 m/ns [START_REF] Lu | Characterization of the shallow structures of the Deren Fault by ground penetrating radar[END_REF].

Figure 1.15 Velocity analysis by calculating the semblance over reflections hyperbolas [START_REF] Yilmaz | Seismic data processing[END_REF] the maximum of the energy gives the value of the velocity for the given time.

Figure 1.16 Radar velocity analysis by diffraction hyperbola fitting. After (Cassidy N. , 2009).

Another kind of velocity analysis is by analyzing the diffractions hyperbolas on the common offset (CO) GPR data. This type of analysis provides an estimate of speeds with an uncertainty of 10% (Figure 1.16) [START_REF] Cassidy | Ground Penetrating Radar Data Processing, Modelling and Analysis[END_REF].

A velocity estimation by diffraction analysis (not shown here) on CO GPR data gives a mean velocity of 0.135 and 0.095 m/ns for the data collected in all the GPR survey. The difference in velocity is explained by the soil humidity for the weather condition period during the GPR survey. We have been used a constant average speed for all active fault study area, because we have just CO GPR data (Figure 1.18). (Table 1-3) studies describe surface ruptures with different lengths and slides from one earthquake to another (Figure 2.1a) [START_REF] Daëron | 12,000year long record of 10 to 13 paleoearthquakes on the Yammouneh Fault, Levant fault system, Lebanon[END_REF][START_REF] Weldon | Wrightwood and the earthquake cycle: What a long recurrence record tell us about how faults work[END_REF], others describe repetitive behaviors with a more uniform slip distribution (Figure 2.1b) [START_REF] Sieh | A Review of Geological Evidence for Recurrence Times of Large Earthquakes[END_REF], or even characteristic behaviors (Figure 2.1c) [START_REF] Schwartz | Fault behavior and characteristic earthquakes: Examples from the Wasatch and San Andreas Fault Zones[END_REF][START_REF] Rubin | Long dormancy, low slip rate, and similar slip-per-event for the Emerson fault, eastern California shear zone[END_REF]Tapponnier, et al., 2001a).

A study compiling the results obtained from 45 trenches carried out in a single site of the San Andreas fault (displacements and ages of the last 15 strong earthquakes) suggests the possible role of a threshold of relative deformation accommodated on the fault, in the ' triggering of phases of strong seismic activity [START_REF] Weldon | Wrightwood and the earthquake cycle: What a long recurrence record tell us about how faults work[END_REF]) (Figure 2.3). Similar observations have been made very recently on a normal fault in Italy [START_REF] Schlagenhauf | Earthquake supercycles in Central Italy, inferred from 36CI exposure dating[END_REF]. In these two studies, it appears that the quantity of stresses released by each earthquake varies from one earthquake to another and that the earthquakes do not release all the stresses accumulated during the interseismic period which precedes them. The repetition of earthquakes also does not appear regular over time but seems to alternate between periods of quiescence during which the stresses accumulate up to a certain threshold and the apparent sliding speed is therefore relatively low, and periods of high activity during which a large part of the stresses are relaxed during several strong earthquakes following each other over a short period of time.

During these seismic swarm periods, the apparent slip speed over the fault is therefore much higher (Figure 2.3), [START_REF] Weldon | Wrightwood and the earthquake cycle: What a long recurrence record tell us about how faults work[END_REF]. The intense activity phases would possibly be triggered by the fact that the fault has reached a maximum threshold of accumulation of relative deformation.

Whatever the interpretations, these studies confirm that knowledge of the earthquake slip as well as the age of earthquakes are two essential parameters for a better understanding of the repetition modes of earthquakes. On the other hand, due to the lack of regular behavior of earthquakes in the medium term, this study suggests the need to know more precisely the displacement and the date of the last 10-20 strong earthquakes at several points of a fault in order to better understand the mechanisms responsible for triggering, repetition and pruning earthquakes. Finally, this study also suggests the need to develop new methods, less expensive in time and less destructive than the trenches, allowing to find this information preserved in the first few tens of meters of the ground. Depending on the time since the last break, the amount of slip is predictable, after [START_REF] Shimazaki | Time-predictable recurrence model for large earthquakes[END_REF].

Figure 2.3 Identification of the last 15 strong earthquakes on the San Andreas fault reveals differences between each event, and variations in accommodation and relaxation slip over short periods of time including several seismic cycles, after [START_REF] Weldon | Wrightwood and the earthquake cycle: What a long recurrence record tell us about how faults work[END_REF]. 'Seismic cycles' are nevertheless suggested, no longer on the scale of a single event, but to that of several events. [START_REF] Natsag-Yïm | Mongolian earthquakes, in Seismic Regionalization of Ulan Bator[END_REF][START_REF] Khil'ko | Strong earthquakes, paleoseismological and macroseismic data, in Earthquakes and the Bases of Seismogenic Zoning of Mongolia[END_REF][START_REF] Huang | Source mechanisms of the Mogod earthquake sequence of 1967 and the event of 1974 July 4 in Mongolia[END_REF][START_REF] Baljinnyam | Ruptures of major Earthquakes and active deformation in Mongolia and its surroundings[END_REF]. Right-lateral strike-slip faulting also occurs in western Mongolia, parallel to the NW-SE ranges of the Mongolian Altay (Bayasgalan & Jackson, 1999b).

This chapter provides a brief overview of how

The main co-seismic surface ruptures in 1967 occurred in four segments or zones. The [START_REF] Baljinnyam | Ruptures of major Earthquakes and active deformation in Mongolia and its surroundings[END_REF]. The three main shock sub events (marked 1, 2 and 3) and the mechanism of the principal aftershock ( 670120) are taken from our analysis of the P and S waveforms. Slip vectors are shown as large white arrows, after (Bayasgalan & Jackson, 1999b). In 2017, as part of the 60th anniversary conference of the Institute of Astronomy and Geophysics, a field trip to the Mogod active fault zone of the conference team was planned organize. The paleo-seismic trenches were excavating after a detailed geophysical study was carried out in the area where the paleo-seismic trench was planned to be excavated. Geophysical surveys were conducted from June 13 to June 18 by IAG geophysical survey team, while trenches were surveyed from July 5 to July 18 by French scientist, IAG and Russian scientist team.

Data acquisition method and survey parameters

In these three zones, detailed MALA ProEx GPR system (MALA Swedish trademark) measurements were conducted with three different frequencies, MALA 500 MHz and 250 MHz shielded, and unshielded MALA RTA 50MHz antennas with 3-4 parallel profiles each one, used with XV11 field control computer. We have chosen the distance between two profiles 1 m in all zones.

We have also performed seismic surveying and carried out one profile measurement at each trench excavation site. The seismic results are not shown here. The trace interval (∆x) was 3 cm for both antennas and the investigation depth are approximately of 7 m and 5 m for 250 and 500 MHz antenna, respectively. However, due to the proximity of the trench excavation site with a Mongolian family's ger, we chose to perform three profiles with GPR RTA 50 MHz antennas at a distance of 50 m to the west of the point.

The distance between profiles was 15 m, the trace interval was 10 cm, their lengths were 70 and 115 m, and the penetration depth was approximately of 20 m (see Table 2-1). The topography of all GPR profiles was recorded using differential GPS (DGPS) system. The GPS antenna was mounted on a backpack carried by an operator who was following GPR paths. The recording step of the GPS was set to 1 second.

Results and interpretation

Mogod fault rupture in the main shock occurred sequentially on two N-S right-lateral strike-slip segments, ending in thrust motion on a NW-SE fault at the southern end of the strikeslip faulting (Bayasgalan & Jackson, 1999b).

According to the results of GPR studies conducted in three parts of Mogod fault, the angle of inclination of the fault is almost 90⁰ due to the strike slip structure of the first zone (Figure 2.6b), and it is not possible to distinguish the location of the fault plane due to almost 100% scattering.

However, the location of the fault plane is not well recorded because of the scattering of GPR waves due to the high angle structure and reverse segment in the third study zone (Figure 2.6d). This was also seen in the three profiles of IAG and French team, 2012 GPR survey.

The GPR results and interpretations are included in the Figure 2.7-Figure 2.11. In these figures we show the results of the surveys conducted in the transition zone (second zone) between the two different type fault structures, as the soil cracks is well defined. This transition zone is broken into three segments, and our study was conducted on a segment in the middle of those three segments. This segment is located at the foot of the mountain back side, and the fault scarp is folded to the northeast. It has folds that rise from the northeast or along the direction of movement of the hanging wall block. From this analysis, it can be assumed that the dipping of this segment is from southwest to northeast. However, it can be clearly seen that this structure is formed in the opposite direction of the GPR results shown in Figure 2.

3.2

The Mungunmorit (Kherlen) active fault zone

Location and Access

The Mungunmorit active fault is located on the Mungunmorit sum of the Tuv province at 190 km East of Ulaanbaatar capital city, Mongolia. There is a very nice road of 130 km asphalt and 60 km countryside road to access to the Mungunmorit sum center (see location on According to some researchers, the Mungunmorit fault is called the Kherlen fault because it is in the Kherlen river basin. The western margin of the basin is bounded by neo-tectonic fault separating it from mountain ranges (> 1800 m above sea level) that form the core part of the Khentii-daurian mega-dome [START_REF] Smekalin | Paleoseismogenic dislocations in the Upper Kerulen basin (southern Henteyn-Daurian mega-arch)[END_REF]. The tectonic activity of the Khentii upland reached a maximum during the Mesozoic and then rapidly decreased toward the Cenozoic [START_REF] Smekalin | Paleoseismogenic dislocations in the Upper Kerulen basin (southern Henteyn-Daurian mega-arch)[END_REF]. Determining the long-term strain rate would help in better determining its quaternary deformation history. Most of the paleo-seismologic studies in the Khentii upland are limited to trenching, usually constraining the earthquake events, which occurred during the Holocene or late quaternary due to limitations in sediments suitable for conventional radiocarbon dating [START_REF] Ferry | Tectonic Morphology of the Hustai Fault (Northern Mongolia), A Source of Seismic Hazard for the city of Ulaanbaatar[END_REF][START_REF] Imaev | Seismic-hazard assessment for Ulaanbaatar (Mongolia) on the basis of seismogeological studies[END_REF][START_REF] Smekalin | Paleoseismogenic dislocations in the Upper Kerulen basin (southern Henteyn-Daurian mega-arch)[END_REF][START_REF] Smekalin | Seismogeology of Verkhnekerulen basin (Khentei, Northern Mongolia)[END_REF].

The Mungunmorit fault is ~40 km long and borders the Kherlen river basin to the east.

It has been identified as thrust in trenched locations [START_REF] Smekalin | Paleoseismogenic dislocations in the Upper Kerulen basin (southern Henteyn-Daurian mega-arch)[END_REF][START_REF] Smekalin | Seismogeology of Verkhnekerulen basin (Khentei, Northern Mongolia)[END_REF], but the deflection of active channels implies that motion along this fault also has a strikeslip with dextral shift component.

Description & Survey Grids

The Mungunmorit active fault studying area of the 2013 year is located within the

Survey design

We have used MALA 2-2).

The topography of all GPR profiles was recorded using differential GPS (DGPS) system. The GPS antenna was mounted on a backpack carried by an operator who was following GPR paths. The recording step of the GPS was set to 1 second. Saw-tooth effects were observed on the topographic profiles due to the small movements of the operator while he was walking along the profile. To filter out these unwanted effects, we have used a high degree polynomial curve fitting our data and used the topographic profiles filtered.

Results and interpretation

The Mungunmorit fault is located along the northwestern foothills of the Kherlen river basin. Therefore, the azimuth direction varies between approximately 20⁰ and 171⁰. [START_REF] Smekalin | Seismogeology of Verkhnekerulen basin (Khentei, Northern Mongolia)[END_REF], b)

Result of GPR image and interpretation of fault dipping and movement, location of profile 1a

(Pr1a) shown in Figure 2.12.

The result of GPR image and the log of the right wall of the trench are arranged to have the same scale in Figure 2.14. Figure 2.14a shows a paleo-seismological trench with geological scarps were formed by several strong earthquakes that occurred at this fault [START_REF] Odonbaatar | Earthquake hazard assessment and micro-zoning survey: Bayankhongor soum of the Bayankhongor province center[END_REF].

Detailed paleo-seismological studies, based on the analysis of satellite images, show a more pronounced fault and a visible scarp at the foot of a thick mountain slope where the fault sediments have been accumulated (see Figure 2.15b).

Data acquisition method and survey parameters

In this study, we performed the MALA ProEx GPR system (MALA Geoscience, Sweden) measurements in the Common Offset (CO) acquisition mode, in which the distance (offset) between the transmitter and receiver antennas is kept constant. We used two types for

GPR Results and interpretation

During the GPR campaign conducted on this fault (Figure 2.15), we have used the 250

MHz and RTA 50 MHz antennas. The profile's coordinates and acquisition parameters are given in Table 2-3 andTable 2- The Bayankhongor fault is considered to be a normal fault [START_REF] Cunningham | Cenozoic normal faulting and regional doming in the southern Hangay region, Central Mongolia: implications for the origin of the Baikal rift province[END_REF], and our GPR images of this fault show that the fault dipping, sediment accumulation pattern, and direction of movement are characteristic of low-angle normal fault structure or Listric fault (Figure 2.18) (Nyambayar, et al., 2018).

The scarps, which show an average of 1.5-3 m in height, do not present partial fault damage, but are often strewn with broad bands along the fault zone [START_REF] Odonbaatar | Earthquake hazard assessment and micro-zoning survey: Bayankhongor soum of the Bayankhongor province center[END_REF]. This is a signification of a normal fault structure, and landslides are common on the eastern slopes of the Jargalant Range, which are stepped along the expansion movement of the fault. between the two broken sections (Nyambayar, et al., 2018).

Another key feature of the Listric fault structure is the formation of a Paleo-lake in the depression below the fault, and the Galuut Paleo-Lake depression, a continuation of the Bayankhongor fault, 18 km from our study area to northwest side. Also, the location of the fault plane is unknown. The angle of the first dropped section of the Listric fault plane, or the part closest to the surface, is vertical or high angled structure of inclination gradually changes to a lower angle. Due to the structure, the GPR signal may not be scattered laterally from the fault plane. Ulaanbaatar; their length and morphology indicate they can produce earthquakes of magnitude 6.5 to 7.5 [START_REF] Schlupp | Investigation of active faults near Ulaanbaataar. Implication for seismic hazard assesment[END_REF][START_REF] Ferry | Tectonic Morphology of the Hustai Fault (Northern Mongolia), A Source of Seismic Hazard for the city of Ulaanbaatar[END_REF][START_REF] Nyambayar | Studying active faults by combing GPR images with morphotectonic and trenching results[END_REF]. Most of the Mongolian population (1.5 million over 3.2 million) is concentrated at Ulaanbaatar, which is the main political and economic center of the country [START_REF] Dugarmaa | Earthquake hazard assessment of Ulaanbaatar, the capital city of Mongolia. Earthquake micro-zoning map[END_REF][START_REF] Nyambayar | Studying active faults by combing GPR images with morphotectonic and trenching results[END_REF].

In One of the main studies of these geophysical surveys was the detailed study of MALA ProEx GPR system (MALA Swedish trademark). Measurements were made with MALA 500MHz shielded and MALA RTA 50MHz unshielded antennas to visualize the depth structure of Emeelt fault, and 500 MHz antennas were used in detailed paleo river channel studies to determine horizontal displacement.

Data acquisition and survey parameters

The Emeelt fault was geomorphologically recognized in 2009 and a preliminary trench (T1) was dug, after which we decided to perform GPR measurements to study the underground structure that may be affected by the fault. GPR survey should help us determine the optimal location of future trenches. Our first goal was to study any sediment near the T1 (47.909015°, 106.605226°) trench for three reasons. First, sediment provides stratigraphy, and if it is cut by a fault, it provides information on the geometry and dynamics of the fault, such as fault displacement, amplitude, and direction. Second, the sediment is usually an environment suitable for EM wave propagation, and third, the proximity of the trench allows us to directly compare the two data sets (geology and GPR). In this study, we mapped the fault depth and structure using a 50 MHz antenna. We recorded 1 profile 200 m long adjacent to the T1 trench to determine the fault zone (see Table 2- We also continued GPR studies on Emeelt fault in 2011 and 2013, the objective was to identify the traces and the geometry of Emeelt fault near the subsurface. However, from the survey planning to the end of the 2013 GPR survey. I did all the work myself, including GPR measurements, data processing, interpretation and satellite image analyses, and GPS measurements. Among these measurements, 7 profiles conducted in 2013 with a RTA50 MHz antenna were very effective (Table 2345). The site showing a lot of incisions was selected to detect the faults as deep as possible by selecting areas with low environmental impact that would be suitable for measurement after the traces of faults were first identified and mapped on the satellite image. Figure 2.20 shows the location of the sixth profile (P-6) and the locations of starting (47.940885°, 106.583281°) and ending points (47.935084°,106.574615°). This profile was selected following the mountain ridge, and because the sediment accumulation is on a thin rocky soil, the deep structure of the fault can be clearly seen. In addition, the direction of measurement was chosen from the top of the mountain to the bottom.

GPR Results and interpretation

The Although the sediment and rock boundaries are the same, the GPR output maps differ depending on the RTA50 MHz and 500 MHz antenna penetration depths. In addition, the dipping of the fault determined by the GPR results corresponds to the fault trace on the surface. The 2010-2011 Integrated Geological and Geophysical Survey identified the Emeelt Fault as a right-lateral strike slip fault [START_REF] Schlupp | Investigation of active faults near Ulaanbaataar. Implication for seismic hazard assesment[END_REF]. The results of GPR measurements conducted in 2013 on the Emeelt active fault are well mapped and provide a lot of in-depth information. Because the sections of these measurements are long, they contain a large amount of information. As an example, we show the result of the profile 6, which presents sub structures of several parallel fractures. The length of the profile-6 is approximately of 1000 m, and in

The Avdar active fault zone

Location and Access

Avdar Mountain Fault is located in the Tuv aimag, just south of the Zuunmod province center, 45 km straight from Ulaanbaatar capital city of the Mongolia, 24 km from Zuunmod province center, and about 20 km from the new international airport. The Avdar fault surface rupture extends from southwest to northeast, similar to Khustai and Sharkhai faults (Figure 2.4).

There are two faults structure along the southern and northern slopes of the mountain on the satellite image, and we have mainly studied the faults in the southern part of the Avdar mountain fault (Figure 2.23).

In terms of seismic activity, earthquakes of 1.5-4.2 magnitude are quite common, and because of their proximity to the new international airport, they are also of interest to researchers. Avdar mountain faults have been identified as having a left lateral strike slip, like Khustai and Sharkhai faults [START_REF] Abeer | Tectonique active de la région d'Oulan Bator, Mongolie: Analyse morphotectonique et paléosismologique des failles actives de Sharkhai et Avdar[END_REF].

Data acquisition and survey parameters

In 2011, when the Avdar mountain fault was discovered, a detailed study of the that active fault began. Topographic survey measurements of the sections were also performed using a trimble 5700 differential GPS system with RTK mode.

GPR Results and interpretation

The Z2 fault zone is the area where the fault is most clearly visible on the surface (Figure Zoomed map of the T1 trench and 500MHz GPR profiles, after [START_REF] Dujardin | Imagerie géoradar (GPR) en milieu hétérogène Application aux failles actives en Mongolie et aux dépôts pyroclastiques du Tungurahua (Equateur)[END_REF].

Description & Survey Grids

The vertical resolution depends on the velocity of EM waves and the frequency of the antennae used. Following the λ/4 criterion [START_REF] Wides | How thin is a thin bed?[END_REF][START_REF] Jol | Ground-penetrating radar antennae frequencies and transmitter powers compared for penetration depth, resolution and reflection continuity[END_REF][START_REF] Zeng | How thin is a thin bed? An alternative perspective[END_REF], it varies from 70 to 5 cm for frequencies of 50-500 MHz and velocities of 0.1-0.14 m/ns. In general, features such as sedimentary structures, lithological boundaries, fractures and/or faults are clearly visible with GPR [START_REF] Neal | Ground-penetrating radar and its use in sedimentology: principles, problems and progress[END_REF][START_REF] Gross | Location and geometry of the Wellington Fault (New Zealand) defined by detailed three-dimensional georadar data[END_REF][START_REF] Mcclymont | Shallow fault segmentation of the Alpine fault zone, New Zealand, revealed from 2and 3-D GPR surveying[END_REF], even when these features differ only by small changes in the nature, size, shape, orientation and packing of grains [START_REF] Guillemoteau | Influence of grain size, shape and compaction on georadar waves: examples of aeolian dunes[END_REF]. After the geomorphologic recognition of the Emeelt fault and a preliminary trench (T1) realized in 2009 (Figure 2.33), we decided to use GPR measurements to investigate subsurface deposits potentially affected by the fault on a wider area. The GPR observations should help us to decide location of future trenches. Our first objective was to study an alluvial fan situated close to the trench T1 (referred to as Z1 area) for three reasons.

First, the sedimentary deposits provide a stratigraphy that, if offset by the fault, can give us information on the geometry and dynamic of the fault, such as the dip, amplitude and direction of displacement with a non-destructive method. Second, sediments are usually favorable to EM propagation and third, the proximity of the trench will allow us to perform a direct comparison of both datasets (geology and GPR). In this survey we used a MALA 500 MHz shielded antenna to get detailed features at shallow depth and to have a depth of penetration and a wavelength consistent with the trench observations. We recorded 25 profiles of 40 m long on both sides of the trench T1 and parallel to it with the aim to cut through the fault. The space between profiles was 1 m and the recoding step was 0.03 m. The main objective of this survey was to image the paleochannel in 3D in order to characterize any horizontal/vertical displacement caused by the fault. The pseudo-3D approach has become a common procedure and has successfully been used in the case of active fault studies [START_REF] Malik | Ground penetrating radar investigation along Pinjore Garden Fault: implication toward identification of shallow sub-surface deformation along active fault, NW Himalaya[END_REF][START_REF] Beauprêtre | Finding the buried record of past earthquakes with GPR based palaeoseismology: a case study on the Hope fault, New Zealand[END_REF]. A full 3D acquisition schema requires an interval of a quarter wavelength grid spacing [START_REF] Grasmueck | Full resolution 3D GPR imaging[END_REF] in both in-line and crossline directions of the 3D cube. This would require one profile every five centimeters (for a mean velocity of 0.1 m/ns) with a 500 MHz antenna. The number of profiles would then be multiplied by five compared to our current survey. As a result, and due to time constraints on the field, we have chosen the pseudo-3D approach rather than a full 3D schema. The profiles were parallel to the fault direction and spaced each 25 cm. (see Table 2-8 for more details about the acquisition geometry).

The topography of all GPR profiles was recorded using differential GPS. The GPS antenna was mounted on a backpack carried by an operator who was following GPR paths. The recording step of the GPS was set to 20 cm. Saw-tooth effects were observed on the topographic profiles due to the small movements of the operator while he was walking. In order to get rid of these undesirable effects, a high degree polynomial curve fitting for the data was used, instead of using raw topographic profiles.

3D GPR processing

The processing of all GPR profiles has been performed with in-house software [START_REF] Girard | Imagerie géoradar et modélisation des diffractions multiples[END_REF] written in MATLAB. We used a common flow procedure involving a constant shift to for the topography [START_REF] Lehmann | Topographic migration of georadar data: implications for acquisition and processing[END_REF][START_REF] Heincke | Acquisition and processing strategies for 3D georadar surveying a region characterized by rugged topography[END_REF], and [START_REF] Dujardin | Topographic migration of GPR data: Examples from Chad and Mongolia[END_REF], has been applied before loading the data into seismic interpretation software (OpendTect). The topographic migration of each profile has been performed with a constant velocity.

GPR results and interpretation.

On the north of the Z1 area we recorded eleven profiles of 80 m long across the two streams to verify the presence of the paleo-channel in the GPR data and to help choose a location for the precise 3D work. One of these profiles is presented in After processing, all the profiles were merged in one single survey and loaded into seismic interpretation software (OpendTect) which allows 3D representations, depth slices and horizon picking. We tried depth slices representation and attribute analysis to improve the quality of the interpretation, but the results were disappointing due to the space between the profiles. Horizon picking was much more time consuming but gave much more interesting results. Owing to the strong disparity of the reflections, which did not allow a semi-automatic picking, we picked the channel's flanks manually. is almost perfectly aligned with a reflection in the GPR profile. The left flank of the channel (limit between the yellow and the orange unit) is as well matching a reflection (arrows) and the yellow unit corresponds to an attenuated signal. On the east side, the unit's interfaces seem to match with dipping reflections but with a lateral offset that can be explained by the distance between the trench and the GPR profile (2.5 m) after [START_REF] Dujardin | Imagerie géoradar (GPR) en milieu hétérogène Application aux failles actives en Mongolie et aux dépôts pyroclastiques du Tungurahua (Equateur)[END_REF].

During this step, crossline profiles were of great importance as they permit a strong correlation of the reflections from one profile to the next one. The result of the manual picking is the 3D surface presented in Figure 2.36 in relation with GPR profiles (three in-lines and one crossline). The depth (from the most elevated point in the topography) is presented by color scale. Green lines on the profiles represent the picking, thus the intersection between the surface and the profiles. The red arrow indicates the flow direction. The displayed surface is obtained by interpolating the picked profiles taken from cubes C2, C3 and C4. Afterwards, the main slope of the channel was subtracted to provide a better interpretability of the data. The removal of the main slope straightens the channel, and the geometry of the flanks is greatly enhanced.

The result is superimposed on the aerial view (Figure 2.37). The channel appears very heterogeneous, and many bumps are observed at its bottom. The penetration depth, from 1.5 to shows the relative position of the profiles, after [START_REF] Dujardin | Imagerie géoradar (GPR) en milieu hétérogène Application aux failles actives en Mongolie et aux dépôts pyroclastiques du Tungurahua (Equateur)[END_REF].

The picture in Figure 2.37b has been taken inside the trench T2 (area Z2), at the position of the pink asterisk. It shows evidence of the fault within a gravel layer with a dip toward the NNE. This dip direction was also determined at the surface as the fault trace moves upstream when crossing a local valley.

The locally apparent vertical offsets of reflections on some of these profiles cannot be directly related to a vertical component of the slip. Firstly, it is impossible to associate the reflections observed on each side of the fault zone because of the shallow penetration of 500

MHz signal and the very few sub-horizontal reflections. On the other hand, the right lateral component can bring itself apparent vertical offsets in a context of interlocked alluvial deposits.

However, a vertical component is suspected by the morphology and the dip of the fault near the surface, but its amplitude cannot be quantified by our GPR data.

On Z2 area, we observed a deep paleo-channel with a few 500 MHz profiles (down to about 2.5 m depth) fossilized by the filling material and crossing the fault. As we suspected a horizontal slip component from the linearity of the seismicity and the fault morphology, we decided to map the channel flanks across the fault by a pseudo-3D approach with numerous 500 MHz in-line and 82 crossline profiles that were acquired during 2010 and 2011 summers. A right lateral offset of about 2 m was imaged on the right downstream flank; it was preserved on the downstream right flank while it has been eroded on the downstream left flank (schema, Figure 2.37c). Afterwards, the filling of the paleo-channel fossilizes the paleo-morphology.

In the 3D channel reconstruction, we did not observe any clear vertical offset.

Nevertheless, it does not prove its absence as the bottom of the channel was locally difficult to follow due the low penetration depth, which was of same depth as the bottom of the channel.

After GPR standard processing for the every profiles, the data of pseudo-3D analysis, we created a pseudo-3D data set and produced indepth-slice 2D result images using Matlab software. In the Matlab program, a pseudo-3D GPR data set of 107 profiles of the C1 cube was created, and the pseudo between them were interpolated and rehabilitated, and the depth-slices were made in 0.01 m or 1 cm increments. In the Figure 2.38 shows a three-dimensional entire data set image of the C1 cube between the T3 and T4 trenches of the Emeelt fault study area.

The depth incisions were made in 1 cm increments to allow for animation of soil changes in the paleo river channel.

We continued this depth-slice processing to a depth of 1.31 m below ground surface and obtained a total of 122 horizontal profiles of the resulting images, which were combined to produce a moving animation of soil depth changes. In this animation, we can see the changes in the depth of the soil and how the paleo river channel flows through the water.

In addition, the results of the depth-slice show that the traces of soil eroded by the faults are not very clear. It also appears that some of the horizontal displacements along these fault boundaries can be identified. 123

The results of the above two methods of the pseudo-3D GPR processing, imaging and analysis show that both methods are more effective in active fault studies or paleoseismological studies. In particular, it is clearly shown that the horizontal displacement of the active fault and the location of the fault trace are predominant. It is also possible to identify traces or geometry of vertical displacements after further development of pseudo-3D GPR survey design and methodology. The combination of 500MHz, 250MHz with RTA 50MHz antenna produces three complementary and consistent datasets as they allow the imaging of different structures. A good consistency is observed between the 500 MHz antenna GPR profiles and trenching results.

Despite the low penetration depth of the GPR up to 15 m for the 50 MHz antenna, 5m

for the 250 MHz antenna and 3 m for the 500 MHz antenna, it clearly provided several important pieces of information that improve our understanding of the active fault geometry and horizontal displacement.

One of the main results of this study is that it is possible to distinguish between compression and tensile effects of the active fault by combining the geomorphology of active faults and the imaging of underground structures imaged by GPR measurements.

It is also possible to reconstruct the structure of paleo-seismically active faults hidden under alluvial sediment accumulation and to determine the extent of the displacement with greater accuracy than traditional paleo-seismological methods.. The high efficiency of GPR studies in determining the optimal location for trench excavation has been reaffirmed.

Pseudo-3D profiles, recorded with a 500 MHz antenna over a paleo-channel crossing the Emeelt fault, provided information about the lateral displacement of 2 m caused by the fault and active fault trace locations. It could be associated to an earthquake with magnitude Mw of about 7 or several with magnitude Mw≥ 6.

The 50 MHz GPR profiles show a direct reflection, coming from the fault plane, giving access to the location, the dip angle and direction of the fault. In contrast, the linearity of the actual seismicity indicates a near vertical fault plane examples in GPR result of the Avdar, Bayankhongor and Emeelt active faults. To clarify that point, we need to investigate more in depth the active structure, by combining GPR with high resolution seismic profiles and very precise 3D relocation of the seismicity.

Chapter 3 : The characterization GPR-based technique of the Songino active fault, Ulaanbaatar, Mongolia.

Introduction

During the 20th century, Mongolia was one of the most seismically active intracontinental areas in the world with four large earthquakes (Magnitude around 8) along its active faults in the western part of the country [START_REF] Schlupp | Investigation of active faults near Ulaanbaataar. Implication for seismic hazard assesment[END_REF].

However, there is very low on the damages sustained by these strong earthquakes, which is due to Mongolia's small population, low urbanization, and low nomadic lifestyle. However, according to the reports of strong earthquakes around the world, the magnitude of the earthquake in our country is less than the magnitude of the earthquake of the other country, but many people were death and a lot of economic damage was caused (Table 3

-1, Table 3-2.)
However, it is clearly related to the urbanization, population density and settlement of these countries.

The capital city of Mongolia, Ulaanbaatar, is in a relatively low seismic zone, but at a time when the city's population and urbanization are increasing rapidly, it is impossible not to talk about the hazards of the earthquakes. Today, Ulaanbaatar is home to more than 50 percent of Mongolia's population approximately 1.6 million and is the only major economic, cultural, and commercial center, indicating that it is a high-risk area due to strong earthquakes [START_REF] Dugarmaa | Earthquake hazard assessment of Ulaanbaatar, the capital city of Mongolia. Earthquake micro-zoning map[END_REF]. The seismic activity observed in the vicinity of Ulaanbaatar (UB), capital of Mongolia, is relatively low compared to the activity observed in western Mongolia.

Nevertheless, since 2005, the seismic activity around UB has increased [START_REF] Dugarmaa | Earthquake hazard assessment of Ulaanbaatar, the capital city of Mongolia. Earthquake micro-zoning map[END_REF], and is organized at the west of UB along two perpendicular directions: the recently discovered Emeelt fault (about 25 km long) situated at about 10 km West of UB (Schlupp, et al., 2012;[START_REF] Munkhsaikhan | Seismic activity near Ulaanbaatar: Implication for seismic hazard assessment[END_REF] and Hustai fault (the main segment has a length of ~80 km)

with its nearest at about 15 km West of UB [START_REF] Ferry | Tectonic Morphology of the Hustai Fault (Northern Mongolia), A Source of Seismic Hazard for the city of Ulaanbaatar[END_REF][START_REF] Ferry | Tectonic Morphology of the Hustai Fault (Northern Mongolia)[END_REF].

The Songino fault (20 km long, situated 20 km West of UB) is located between Emeelt and Hustai faults. The length and morphology of these faults indicate that they can produce earthquakes of magnitude 6.5-7.5 [START_REF] Schlupp | Investigation of active faults near Ulaanbaataar. Implication for seismic hazard assesment[END_REF][START_REF] Dujardin | Imagerie géoradar (GPR) en milieu hétérogène Application aux failles actives en Mongolie et aux dépôts pyroclastiques du Tungurahua (Equateur)[END_REF][START_REF] Munkhsaikhan | Seismic activity near Ulaanbaatar: Implication for seismic hazard assessment[END_REF]. Therefore, the study of seismic hazard and the estimation of the probability of future destructive earthquakes are of first importance for the country. Since the last large earthquake, still unknown on the Songino fault, the fault geomorphology has been smoothed due to erosion processes and a low slip rate (most likely less than 1 mm per year), thus the exact location of the fault is hidden in a several meter strip.

In such a context, the Ground Penetrating Radar (GPR) method gives good and useful results, can help to characterize faults by identifying offsets of radar reflections, and buried fluvial channel deposits [START_REF] Beauprêtre | Finding the buried record of past earthquakes with GPR based palaeoseismology: a case study on the Hope fault, New Zealand[END_REF]. The investigation depth depends on the electromagnetic (EM) attenuation of the medium and the frequency of the antenna used. The lower the frequency, the greater the penetration depth, which varies from a few centimeters in conductive materials up to 50 m for low conductivity (less than 1 mS m -1 ) media [START_REF] Jol | Ground Penetrating Radar: Theory and Applications[END_REF].

The vertical resolution depends on the velocity of EM waves and the frequency used. In general, features such as sedimentary structures, lithological boundaries, fractures and/or faults are clearly visible with GPR [START_REF] Deparis | On the potential of ground penetrating radar to help rock fall hazard assessment: a case study of a limestone slab, Gorges de la Bourne (French Alps)[END_REF][START_REF] Christie | Assessing fault displacement and off-fault deformation in an extensional tectonic setting using 3-D groundpenetrating radar imaging[END_REF][START_REF] Mcclymont | Shallow fault segmentation of the Alpine fault zone, New Zealand, revealed from 2and 3-D GPR surveying[END_REF], even when these features differ only by small changes in the nature, size, shape, orientation and packing of grains [START_REF] Guillemoteau | Influence of grain size, shape and compaction on georadar waves: examples of aeolian dunes[END_REF].

Table 3-

Tectonic setting around UB area

The Hustai fault (west area surrounded by red line in Figure 3.1) northeastern tip is located at 15 km west from Ulaanbaatar. This main fault of 80 km long may produce an M 7.5 earthquake. It displays continuous microseismicity with five M 4+ and an M 5.4 since 1974. Exploratory trenches located along the central section of the active trace reflect the transtentional nature of the fault with mixed normal and left lateral strike slip faulting geometries [START_REF] Ferry | Tectonic Morphology of the Hustai Fault (Northern Mongolia), A Source of Seismic Hazard for the city of Ulaanbaatar[END_REF][START_REF] Ferry | Tectonic Morphology of the Hustai Fault (Northern Mongolia)[END_REF][START_REF] Schlupp | Investigation of active faults near Ulaanbaataar. Implication for seismic hazard assesment[END_REF]. A detailed morphotectonic view of this fault is provided by combining highresolution satellite images, digital elevation models, GPR images, magnetic mapping, geomorphology, and trenching investigations [START_REF] Ferry | Tectonic Morphology of the Hustai Fault (Northern Mongolia), A Source of Seismic Hazard for the city of Ulaanbaatar[END_REF][START_REF] Schlupp | Investigation of active faults near Ulaanbaataar. Implication for seismic hazard assesment[END_REF][START_REF] Dujardin | Imagerie géoradar (GPR) en milieu hétérogène Application aux failles actives en Mongolie et aux dépôts pyroclastiques du Tungurahua (Equateur)[END_REF]. The Emeelt fault (swarm area in Figure 3.1) located less than 10 km from the westernmost area of UB and its surface trace can be mapped over about 5 km along a N146 direction (Schlupp, et al., 2012;[START_REF] Dujardin | Imagerie géoradar (GPR) en milieu hétérogène Application aux failles actives en Mongolie et aux dépôts pyroclastiques du Tungurahua (Equateur)[END_REF][START_REF] Munkhsaikhan | Seismic activity near Ulaanbaatar: Implication for seismic hazard assessment[END_REF], and shows a right lateral strike slip with a small reverse vertical component (compression). It is capable to produce an earthquake with magnitude up to 6.7 [START_REF] Schlupp | Investigation of active faults near Ulaanbaataar. Implication for seismic hazard assesment[END_REF][START_REF] Ferry | Tectonic Morphology of the Hustai Fault (Northern Mongolia), A Source of Seismic Hazard for the city of Ulaanbaatar[END_REF][START_REF] Dujardin | Imagerie géoradar (GPR) en milieu hétérogène Application aux failles actives en Mongolie et aux dépôts pyroclastiques du Tungurahua (Equateur)[END_REF][START_REF] Munkhsaikhan | Seismic activity near Ulaanbaatar: Implication for seismic hazard assessment[END_REF].

The Songino fault (Blue Square of [START_REF] Ferry | Tectonic Morphology of the Hustai Fault (Northern Mongolia), A Source of Seismic Hazard for the city of Ulaanbaatar[END_REF][START_REF] Ferry | Tectonic Morphology of the Hustai Fault (Northern Mongolia)[END_REF]Al Ashkar 2015). The Hustai northeastern tip is located at 15 km west from UB and is of 80 km long; the Emeelt fault (swarm area) of 40 km long is situated less than 10 km from the westernmost area of UB; the

Songino fault located between Emeelt and Hustai faults is about 20 km long and situated 20 km

West of UB.

Methodology

Ground Penetrating Radar (GPR) is a high-resolution, near surface, geophysical method based on the propagation, reflection and scattering of high frequency (from 10 MHz to 2 GHz) electromagnetic (EM) waves in the Earth [START_REF] Daniels | Introduction to subsurface radar[END_REF][START_REF] Jol | Ground Penetrating Radar: Theory and Applications[END_REF][START_REF] Van Dam | Landform characterization using geophysics -Recent advances, applications and emerging tools[END_REF]. EM waves sent into the ground are reflected at interfaces of electric and dielectric contrast and the reflected signals are recorded. Those discontinuities can result from changes in the water content, the chemical composition, or the texture of the deposits. In sedimentary deposits, variations in grain size and water saturation are the main factors triggering reflections.

GPR has proven its efficiency in many fields of geosciences but seems generally most efficient in sedimentary deposits, marked lithological boundaries, fractures and/or faults [START_REF] Gross | Location and geometry of the Wellington Fault (New Zealand) defined by detailed three-dimensional georadar data[END_REF][START_REF] Deparis | On the potential of ground penetrating radar to help rock fall hazard assessment: a case study of a limestone slab, Gorges de la Bourne (French Alps)[END_REF][START_REF] Mcclymont | Shallow fault segmentation of the Alpine fault zone, New Zealand, revealed from 2and 3-D GPR surveying[END_REF], even for subtle changes in the nature, size, shape, orientation and packing of grains [START_REF] Guillemoteau | Influence of grain size, shape and compaction on georadar waves: examples of aeolian dunes[END_REF]. In the sedimentological context, interpretation generally uses GPR-facies analysis, analogous to their use in seismic studies [START_REF] Jol | GPR in sediments: advice on data collection, basic processing and interpretation, a good practice guide[END_REF][START_REF] Neal | Ground-penetrating radar and its use in sedimentology: principles, problems and progress[END_REF]. Ground Penetrating Radar (GPR) is a geophysical imaging method that detects reflections and diffractions of electromagnetic waves occurring at the dielectric, electrical and magnetic contrasts of the sub-surface within an electrically resistive medium. This is a relatively recent geophysical method which has found a wide field of applications over the last twenty years [START_REF] Annan | GPR-History, Trends, and Future Developments[END_REF] for a background, and [START_REF] Jol | Ground Penetrating Radar: Theory and Applications[END_REF][START_REF] Beres | Application of Ground-Penetrating-Radar Methods in Hydrogeo-logie Studies[END_REF] for a review. description of the main applications). By describing in the following the principle of propagation of electromagnetic waves, we will introduce the notions and the vocabulary inherent in this method, as well as the signal and noise sources that we will try to amplify and reduce, respectively, during the treatment. Details of the Ground Penetrating Radar survey measurements performed on the Songino fault can be found in Table 3-3, Table 3-4. The processing of all GPR profiles has been performed with an in-house software written in MATLAB (RadLab) [START_REF] Girard | Imagerie géoradar et modélisation des diffractions multiples[END_REF][START_REF] Dujardin | Imagerie géoradar (GPR) en milieu hétérogène Application aux failles actives en Mongolie et aux dépôts pyroclastiques du Tungurahua (Equateur)[END_REF]. The high quality of the data allows a very light workflow in four standard steps, which are described in detail elsewhere [START_REF] Fisher | Processing ground penetrating radar (GPR) data[END_REF][START_REF] Yilmaz | Seismic data analysis : processing, inversion, and interpretation of seismic data[END_REF][START_REF] Annan | Ground Penetrating Radar Work-shop Notes[END_REF][START_REF] Annan | GPR-History, Trends, and Future Developments[END_REF][START_REF] Jol | GPR in sediments: advice on data collection, basic processing and interpretation, a good practice guide[END_REF][START_REF] Neal | Ground-penetrating radar and its use in sedimentology: principles, problems and progress[END_REF][START_REF] Annan | Electromagnetic Principles of Ground Penetrarting Radar[END_REF]. We used a common flow procedure including a constant shift to adjust the time zero, a dc filter to remove the low frequencies, a flat reflections filter to remove continuous flat reflections noise, a band-pass filter, a time varying gain function and finally the topographic corrections [START_REF] Annan | Electromagnetic Principles of Ground Penetrarting Radar[END_REF]Cassidy N. , 2009). Band-pass filters are chosen to be 50-450 and 100-800 MHz for the 250 and 500 MHz antenna, respectively.

GPR Data acquisition and processing

Analysis and visualization of the GPR images results

The results of the first Ground Penetrating Radar measurements conducted in 2012 on the Songino fault are also interesting. These measurements are collected by overlapping the 250MHz shielded and RTA 50MHz unshielded antennas on each profiles so that different levels of depth data can be seen. In August 2013, we completed a trench survey on the fault, and we performed GPR measurements inside the trench. Due to the fact that the excavation team of this study did not dig in the planned place, the results of the trench survey were not as we expected to be.

Conclusions

The GPR survey of Songino faults was carried out in several stages in a total of seven zones where horizontal and vertical displacements could be determined. Based on the results of the above detailed GPR studies and their interpretations, the following conclusions are made.

• GPR studies have been reaffirmed as a highly effective and important geophysical method for the study of fault zone structures and alluvial sediment accumulation.

• The results of GPR studies conducted in seven Songino fault zones show that the measurements made in the geodynamic environment of compression and tension are more effective. It means that the smaller the angle of dipping of faults and fractures, the clearer the position of their planes and fracture zones. Especially in the normal fault structure, the displacement of the horizontal layers is strongly reflected by the GPR.

• Songino faults have a compressed structure on the northwest edge and have the highest scarp area (approximately 21 meters high from geomorphology analyze).

• The GPR study did not reveal any significant anomalies of the fault plane in the strike slip structured zone named S3 zone, and most of the radar waves may be irreversible due to the high angle of incidence of the fault plane in this zone.

• The GPR signal reflection is strongly anomalous in the area of the fault tensional zone or normal structure, and the structure below the fault scarp is clearly visible. The resulting image (Figure 3.11) provides a complete analysis of the movement in the area.

• Most of the budget for paleo-seismic surveys is spent on trench surveys, and GPR surveys have been shown to be important for the efficient use of the survey budget by determining the optimal location for trench excavations and making trench surveys more effective.

• It has been reaffirmed that it is possible to accurately determine the extent of their displacement by reconstructing paleo river channels that have been buried and hidden since the last earthquake.

A detailed GPR study of the Songino fault revealed a wealth of information on the structure, direction of movement, and alluvial sediment accumulation of the soil disturbances caused by the fault. Based on all of these data, a unified interpretation reveals that the Songino fault plane is a left-lateral strike slip from southeast to northwest. The compressional effect on the west part, or the high scarp sedimentary structure formed on the ground surface, is the effect of excavation at an angle of approximately 40° to the movement direction of the fault. The sediment with a normal-structured scarp on the east is the effect of the tensional of the structure, which bends in the direction of the Songino fault movement and forms an angle of about 21°.

Chapter 4 : The geological context of the Songino active fault, Ulaanbaatar, Mongolia.

Introduction

Mongolia's regional geology is commonly approached by employing tectonostratigraphic terrane analysis to aid in interpreting its structure and history. In this sense, a terrane is defined as a fault-bounded geologic entity or fragment that is characterized by a regional size and a distinctive geologic history that differs markedly from that of neighboring crustal blocks [START_REF] Tomurtogoo | A brief explanatory note to the Tectonic Map of Mongolia at a scale of 1:1,000,000[END_REF]. Thereby, workers have divided Mongolia into numerous individual terranes and super terranes, many extending beyond Mongolia's political borders [START_REF] Badarch | A new terrane subdivision for Mongolia; implications for the Phanerozoic crustal growth of Central Asia[END_REF][START_REF] Tomurtogoo | A brief explanatory note to the Tectonic Map of Mongolia at a scale of 1:1,000,000[END_REF].

Two prevalent terrane division schemes frequently appear within the literature [START_REF] Badarch | A new terrane subdivision for Mongolia; implications for the Phanerozoic crustal growth of Central Asia[END_REF] The boundary between the northern and southern Mongolian tectonic domains has been interpreted as a suture formed via closure of the Paleo-Asian Ocean due to the collision of the Sino-Korean craton with the Siberian craton [START_REF] Ziegler | Mesozoic assembly of Asia: Constrains from fossil floras, tectonics, and paleomagnetism[END_REF][START_REF] Zorin | Geodynamic of the western part of the Mongolia-Okhotsk collisional belt, Trans-Baikal region (Russia) and Mongolia[END_REF][START_REF] Badarch | A new terrane subdivision for Mongolia; implications for the Phanerozoic crustal growth of Central Asia[END_REF][START_REF] Li | Permian geodynamic setting of northeast China and adjacent regions: closure of the Paleo-Asian Ocean and subduction of the Paleo-Pacific plate[END_REF]. The position of this important tectonic lineament is of great importance because it provides some of the first-order tectonic constraints on the geologic evolution represented by rocks found in Mongolia.

The Khustai fault is centrally situated east of the Mongolian Altai and Hangay Dome areas, south of the BF, and north of the TB where it serves as the tectonic boundary between [START_REF] Badarch | A new terrane subdivision for Mongolia; implications for the Phanerozoic crustal growth of Central Asia[END_REF][START_REF] Lehmann | Structural constraints on the evolution of the Central Asian Orogenic Belt in SW Mongolia[END_REF].

Since 1957, seismologists have been using a modern seismograph network to track the positions of earthquake hypocenters throughout the region. French and Mongolian geologists then used the resulting seismic data set (detectable events dating to between 1900 and 2000) to compile a map of earthquake epicenters located throughout Mongolia. A review of their map shows that the greatest concentrations of seismic events occur in the western half of the country and throughout the Lake Baikal region, whereas seismicity in the eastern half of the country has been much more subdued during this recent time period. The eastern region of the country has originated several recent earthquake clusters throughout the Gobi Desert, Hentey Mountains, and grassy steppe, most notably the Deren fault rupture near the town of Mandalgobi, with singular, smaller-magnitude events widely distributed but ebbing towards the east. The TRV is centrally located between these two seismic regions, approximately 300-km east of the January 5, 1968, Mogod Earthquake (M=7.0) epicenter [START_REF] Bayasgalan | Lithosphere rheology and active tectonics in Mongolia; relations between earthquake source parameters, gravity and GPS measurements[END_REF]. The depths of earthquake hypocenters throughout Mongolia are also of great importance as they provide insights on crustal rheology as well as the information used for producing earthquake models of the region. [START_REF] Bayasgalan | Lithosphere rheology and active tectonics in Mongolia; relations between earthquake source parameters, gravity and GPS measurements[END_REF] reports that none of the earthquakes they analyzed originated at depths greater than 20 km, and that in central Mongolia, the depths were generally 8 events each resulting in hundreds of kilometers of surface rupture with an average offset of over 5 m [START_REF] Pollitz | Fault interaction and stress triggering of twentieth century earthquakes in Mongolia[END_REF]. Interestingly, the recurrence interval for large earthquakes (M 8+) on each of the aforementioned faults is between 2,000 and 8,000 years, therefore the presence of four 8+ magnitude earthquakes within a very short time span (~60 years) is likely anomalous [START_REF] Calais | Are post-seismic effects of the M = 8.4 Bolnay earthquake (1905 July 23) still influencing GPS velocities in the Mongolia-Baikal area?[END_REF][START_REF] Calais | Continental deformation in Asia from a combined GPS solution[END_REF][START_REF] Dugarmaa | Seismic map of Mongolia and site effect microzoning at the capital, Ulaanbaatar[END_REF][START_REF] Pollitz | Fault interaction and stress triggering of twentieth century earthquakes in Mongolia[END_REF]. For example, the afore mentioned Bulnai Earthquake was closely preceded by the July 9, 1905 Tsetserleg Earthquake epicentered a short distance to the east (M=7.9-8.0).

Though coincidental epicenter temporal and spatial clustering is a possibility, it is statistically unlikely. Furthermore, this clustering behavior resembles that of the Big Bear and Hector Mine earthquakes (epicentered in southern California during the 1990's), which were triggered by stress changes caused by the preceding Landers earthquake [START_REF] Pollitz | Fault interaction and stress triggering of twentieth century earthquakes in Mongolia[END_REF]. The other two M 8+ events were located further away and occurred several years later. [START_REF] Dorjsuren | Geological settings of the Ulaanbaatar terrane in the Hangay-Hentey zone of the Devonian accretionary complex, Central Asian orogenic belt[END_REF].

The stress transfer models proposed by [START_REF] Pollitz | Fault interaction and stress triggering of twentieth century earthquakes in Mongolia[END_REF] suggests that the Tsetserleg and Bulnai events likely triggered these later events as well, showing that these very large earthquakes could have far-reaching seismic "triggering" effects on fault systems throughout Mongolia for many years. The resulting temporal large earthquake cluster would transfer stress throughout the region, but with long periods of tectonic quiescence between seismic event periods [START_REF] Dugarmaa | Seismic map of Mongolia and site effect microzoning at the capital, Ulaanbaatar[END_REF]. The duration of these large earthquake clusters has not been extensively explored, but it has been suggested by one source that these changing stress fields may still be in place and is likely influencing GPS derived deformation rates [START_REF] Calais | Are post-seismic effects of the M = 8.4 Bolnay earthquake (1905 July 23) still influencing GPS velocities in the Mongolia-Baikal area?[END_REF][START_REF] Calais | GPS measurements of crustal deformation in the Baïkal-Mongolia area (1994-2002): Implication for current kinematics of Asia[END_REF][START_REF] Calais | Continental deformation in Asia from a combined GPS solution[END_REF]. Evidence for this post-seismic stress transfer is found within the apparent discrepancy between long-term fault movement rates, earthquake tensors, and the GPS data which has been collected since the mid-1990's [START_REF] Calais | Are post-seismic effects of the M = 8.4 Bolnay earthquake (1905 July 23) still influencing GPS velocities in the Mongolia-Baikal area?[END_REF][START_REF] Calais | GPS measurements of crustal deformation in the Baïkal-Mongolia area (1994-2002): Implication for current kinematics of Asia[END_REF][START_REF] Calais | Continental deformation in Asia from a combined GPS solution[END_REF].

The seismicity of the region around Ulaanbaatar from 1900-2000 and all earthquakes occur at depths of < 10 km. That the largest earthquakes to have occurred near Ulaanbaatar allows us to study the distribution of earthquakes in space and over thousands and tens of thousands of years. Such long paleo-seismic stories, in turn, help us understand many aspects of neotectonics, such as earthquakes and tectonic regional patterns. Paleoseismology is a broad field of earthquake geology that includes the use of modern advanced tools and technologies, such as seismology, tectonic and structural geology, historical surface deformation (geodesy), and tectonic landscape geomorphology (tectonic geomorphology) [START_REF] James | Introduction to Paleoseismology[END_REF].

The main driving force behind most paleoismic studies is the social need to assess the likelihood, severity, and danger of future earthquakes [START_REF] Reiter | Paleoseismology-A user's perspective[END_REF][START_REF] Gurpinar | The importance of paleoseismology in seismic hazard studies for critical facilities[END_REF]. Evidence of past earthquakes ranges from local deformation along the crustal fault (fault scarps, sag ponds, laterally offset stream valleys, monoclinic folded marine terraces, scarp-dammed lakes)

to large-scale uplift and subsidence, can be. Features (deposits or landforms) formed during an earthquake are described as coseismic and are commonly contrasted with nonseismic features formed by processes of erosion, deposition, and deformation unrelated to large earthquakes.

For example, seismogenic faults may creep between earthquakes or slip small amounts during small to moderate earthquakes that leave no signs of sudden slip. For this reason, nontectonic and nonseismic are better adjectives than aseismic (no detectable seismicity) for features unrelated to fault slip or strong earthquake shaking. The term aseismic should be restricted to seismology [START_REF] James | Introduction to Paleoseismology[END_REF].

Paleoseismologists can only study and establish the paleoearthquakes that produce recognizable deformations (in the form of deformed stratigraphic units, displaced landforms, or earthquake-induced sedimentation) [START_REF] James | Introduction to Paleoseismology[END_REF].

Songino fault trench

A detailed study of the trench is planned for August 2013 on the Songino active fault.

In preparation for this, detailed GPR measurements were conducted in June 72"E). It is wrong, but because the cost of digging a trench is high, we will not be able to dig another trench again, so we will make a logging on the right wall of the trench and take an sample for dating (Figure 4.5).

As you can see from the photo mosaic of this trench (T1) wall, it is composed of dark brown soil, light yellow loam, orange clay and dark blue fine clay layers from top to the bottom.

Beneath the dark blue fine-grained clay layer, a layer of highly fragmented and damaged bedrock is located below the fault scarp (

Optical luminescence dating results

As a sediment dating technology, optical luminescence technology has been rapidly developed in the last 20 years [START_REF] Murray | Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol[END_REF][START_REF] Wintle | A review of quartz optically stimulated luminescence characteristics and their relevance in single-aliquot regeneration dating protocols[END_REF]. In recent years, optical luminescence dating technology has been widely used in active fault study [START_REF] Chen | Preliminary results of long-term slip rates of 1999 earthquake fault by luminescence and radiocarbon dating[END_REF][START_REF] Chen | Optical dating of a sedimentary sequence in a trenching site on the source dault of the 1999 Chi-Chi earthquake, Taiwan[END_REF][START_REF] Cheong | Determination of slip rate by optical dating of fluvial deposits from the Wangsan Fault, SE Korea[END_REF][START_REF] Liu | Paleoearthquake studies along the Nalinlaka Fault of the Late Pleistocene in Lhasa, Tibet[END_REF][START_REF] Nogueira | Quaternary fault kinematics and chronology in intraplate northeastern Brazil[END_REF].

The pretreatment and determination of the optical luminescence samples in this study were all completed in the optical luminescence laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences. Among them, the equivalent dose of coarse-grained (90~ [START_REF] Prescott | Cosmic-ray contributions to dose-rates for luminescence and ESR dating: Large depth and long-term time variations[END_REF][START_REF] Adamiec | Dose-rate conversion factors: Update[END_REF].

Structural geological analysis.

We measured on the georadar image result of the Songino fault profile (Profile-2), vertical displacement of the fault scarp was approximately 1.5 m, and then vertical displacement was 0.8 m at the crossed profile (Cross-1). The directions is same of movement of the two normal structured scarp, but the length of the horizontal displacement of the subsurface two layers is approximately 5.2 m (Figure 4.9).

The main distinguishing feature of the Songino fault satellite map is that it consists of three main types of substructures, which can be compressional, tensional and strike slip (Figure 4.10). To the far west, at the tip with the compressional effect, there are three subsections with a stepped structure, which are clearly visible with a shear zone in the middle of two subsections.

It is also look like horizontal folding effect. In addition, the three subsections of the hierarchy are also stepped in azimuth, moving from South-southeast to Southeast. The scarp formed in this compressional effect area is more than 22 m high at the northern tip and disappears as it and it is clear that the compressed sediment was lifted up to form a high scarp, but there is less displacement compared to the existing scarp height over 20 m. It is not possible for scarp to form because the strike slip is structurally moving along its own axis. In terms of structural differences, it is estimated that a tensional displacement should have occurred at a distance of 11.22 m, which is twice as much as the displacement, which is not yet consistent with the results of satellite image and georadar studies. Thus, it is clear that the main movement of Songino fault is not only a straight movement along the strike slip structure, but also a combined movement with a rotational moment. But the estimated value of the horizontal displacement in the normal structured zone is 11.22m (Figure 4.12), and then a measured horizontal displacement value on the GPR result image of the profile P2 is approximately 5.2m (Figure 4.9), difference between that values is approximately 6m.

This great difference is confirmed by the following. This is: Two big paleo river channels are located between the GPR P2 profile (acquired the 14 June 2013) and the beginning point location of the normal structure. All the substructures on these two paleo-river channels around and contains all the different migrations of their own migration. Also, the substructures of our predicts are evidence to divide this stretching defective deformer power and multiple substructures fracture. with push-up oblique thrust structure (Burg, 2017).

An explanation of the origin and development of the northwestern thrust structure of the Songino fault is given in detail in the work of (Burg, 2017). The mechanical development of folds-and-thrusts-structure is compared to the piling up of loose sand in front of a bulldozer as it is pushed up a slope (Burg, 2017).

In the critically tapered, stable wedge, equilibrium between three main elements exists:

-Frictional resistance to sliding along the base, which refers to the basal traction of the wedge.

-Forces pushing at the rear of the wedge, which express the regional tectonics.

-The shape of the wedge, which is controlled by various factors such as frontal or basal accretion, internal deformation, sedimentation, surface, and tectonic erosion (Burg, 2017).

A change of one or more of these factors generates within-wedge deformation caused by internal stress release to regain or maintain stability. The gravitational potential energy, due to elevation of the hinterland, creates both horizontal and vertical stresses. If the surface of the wedge becomes too steep because of excessive thickening, then the basal plane is unable to support the load and the wedge collapses forward. If the surface of the wedge is too gentle, then not enough gravitational force is transmitted down to the basal plane to allow slip to occur and the topography builds up while the wedge deforms internally by forming folds, faults, and penetrative strain (Burg, 2017).

It is clear that mount Ergent moved northwest due to Khustai fault, spawning sediments in the northwestern part of Songino fault to form the highest scarp in the that area. This can be seen from the folds of the highest scarp on the northwest side of the Songino Fault and the river channel formed behind it, and the structure of the scarp structure is similar to the ramp-1 layer structure of the thrust structure shown in Figure 4.16.

The western boundary of mount Ergent, which pushes the sediment, is the boundary of the bulldozer push-up plane. During the push-up, the push-up plane is likely to rise as slowly along the sliding plane as shown in Figure 4.17.

The northwest thrust structure of the Songino-fault is a wedge-shaped bulldozer effect, and the materials in this wedge-shaped area are composed of Quaternary soft sedimentary material, which folds at the ends to form a highest topographic scarp structure. one (Burg, 2017).

Figure 4.17 Geometry and growth of a wedge structure and bulldozer effect (Burg, 2017).

Conclusions

This chapter seeks to identify the geological environment of the Songino fault and the factors that contribute to it, and this study believe that this goal has been achieved to some extent. These include: By carrying out optical luminescence chronology research on the trench sections of the Songino, Emeelt and Khustai active faults for Paleo-seismologic dating, combined with the faults of the formation, the following preliminary understandings are obtained:

1) Based on the optical luminescence age of the fault dislocation strata and its overlying strata on the ET trench, it is inferred that the time of the latest activity of the Emeelt active fault is between (9.6±1.0) ka and (11.3±1. 3) ka.

2) Based on the optical luminescence ages of the fault dislocation strata and the overlying deposits on the KT trench, it is inferred that the time of the latest fault activity of the Khustai active fault is between (0.87±0.08) ka and (24.2±4.9) ka, it may be closer to (0.87 ± 0.08) ka from today.

3) Based on the optical luminescence age of the fault dislocation strata and its normal structured overlying strata on the T-1 trench, it is inferred that the time of the latest activity and age of last top layer of the Songino active fault is (5.77±0.04) ka.

The vertical displacement of the scarp of the normal-structured section was approximately 1.5 m in the perpendicular profile to the Songino fault trace, and the vertical displacement was approximately 0.8 m in the parallel profile along the, while the height of the vertical displacement in the northwest thrust structure of the Songino fault was the highest. At its highest point, it was approximately 22 m high.

There is a difference about 20.5⁰ between the normal and strike slip structures, and about 37.1⁰ difference between the strike slip and thrust structures, and a total of about 58⁰ difference between the normal and thrust structures.

In addition, the horizontal displacement of the normal structured section was 33 m, and from this value, the geometric calculations were used to determine the displacement on the other structures, including 19.32 m for the thrust structured section and then 32.03 m on the left-lateral strike slip section, which is the main transition of the Songino fault.

Finally, the correction between the geological map and the traces of fault line elongation determined the direction of the Songino fault movement and the surrounding sediments, the cause of the accumulation damage, and the relationship.

since 2010, we have been working to introduce and localize this survey methodology throughout Mongolia, in many fields. In particular, the aim is to usefully in paleoseismology surveys, and in the future to use a combination of many advanced techniques and technologies to improve the results and quality of surveys.

The structure of underground surveillance for GPR is divided into three main parts: 1. The location of the fault and fracture plane are not exactly visible in the example of a normal fault structure with a high angle, but the contrasting layers on both sides of the normal fault structure are almost parallel to the ground surface, which gives a very clear and strong reflection. However, since the vertical structure of the high angle does not any reflection and attenuated completely for the GPR signal, the location of the layers that give a very strong reflection reveals the vertical structure between them for the GPR facies (Figure 5.2). Also, on a low angle normal fault structure, due to the high angle structure near the surface, it also has the effect of completely attenuated and chaotic propagation, which does not any reflections the fault plane. This is especially true when the GPR signal energy intensity is high near to the ground surface, and which given a chaotic propagation effect (Figure 5.2), and when the downward GPR signal energy intensity decreases, it have been a complete attenuation effect for the GPR facies. In such an environment, the location of the fault and fracture plane is determined by the location and reflections of the lateral accumulated sedimentary layers (Figure 5.3). The section of the ET trench on the Emeelt fault shows that the fault activity caused the light-yellow silt layer (P3) to be dislocated; during the fault activity, the material in the gravel layer (P2) was involved in the fault fracture zone and became the material in the fault zone; The brown gravel-bearing silt layer (P1) has no faults and is not affected by fault activity. Therefore, the time of the latest fault activity on this fault should be somewhere in between. The sample ET-1 was collected from the bottom of the dark brown gravel-bearing silt layer (P1), and its age is (9.6±1.0) ka. The samples ET-3 and ET-4 were collected from the light-yellow silt layer (P3). Among them, the sampling point of ET-3 is located at the top of the formation, and its optical luminescence age represents the minimum age of formation of the dislocation formation, namely (11.3±1.3) ka. Therefore, the time of the latest fault activity of the Emeelt active fault is between (9.6±1.0) ka (ET-1) and (11.3±1.3) ka (ET-3).

The section of the KT trough on the Khustai active fault shows that the fault activity caused the gray-white gravel-bearing silt layer (L2), the gray-brown silt layer (L3), the reddish palaeo-soil layer (L4) and the gravel layer (L5) to produce dislocations. This indicates that the latest activity of the fault was later than the formation of the gray-white gravel-bearing silt layer (L2). The KT trench shows that the fault is a normal fault. When the fault is active, the two disks of the fault are relatively dislocated, resulting in a height difference on the surface. In addition to the inclination of the stratum itself, a local depression is formed there. The surface material is mixed with a large amount of organic matter to fill the depressions, forming a black loess layer (L1). Therefore, the fault activity is earlier than the formation of the black loess soil layer (L1). The dating results of the three optical luminescence samples KT-1, KT-2, and KT-3 on the KT profile are (0.87±0.8) ka and 35 volumes of paleo-seismic geology (24.2±4.9) ka respectively and (37.8±5.1) ka. Since the sample KT-1 was collected from the bottom of the black loess soil, its optical luminescence age represents the age when the black loess soil began to form. Therefore, this age can be used to define the lower limit of the fault activity time. The samples KT-2 and KT-3 come from the gray-white gravel-bearing silt layer (L2) and the graybrown silt layer (L3), respectively, and their optical luminescence ages represent the formation ages of the corresponding fault strata. Therefore, the optical luminescence age of KT-2 can define the upper limit of the fault activity time. The formation of the black loess soil layer may be very close in time to the fault activity. Therefore, the latest activity time of the Khustai active fault is between (0.87±0.8) ka and (24.2±4.9) ka and may be closer to (0.87±0.8) ka. The combination of 500MHz, 250MHz with RTA 50MHz antenna produces three complementary and consistent datasets as they allow the imaging of different structures. A good consistency is observed between the 500 MHz antenna GPR profiles and trenching results.

One of the main results of this study is that it is possible to distinguish between active fault compression and tensile effects by combining the geomorphology of active faults and the imaging of underground structures imaged by GPR measurements.

It is also possible to reconstruct the structure of paleo seismic active faults under alluvial sediments and to determine the extent of horizontal displacement more accurately than traditional paleo-seismological methods.

Most of the budget for palaeoseismological research is spent on trenching, and GPR studies have been shown to be important in determining the optimal location for trench excavation. The GPR survey of Songino faults was carried out in several stages in a total of seven zones where horizontal and vertical displacements could be determined. Based on the results of the above detailed GPR studies and their interpretations, the following conclusions are made.

GPR studies have been reaffirmed as a highly effective and important geophysical method for the study of fault zone structures and alluvial sediment accumulation.

The results of GPR studies conducted in seven Songino fault zones show that the measurements made in the geodynamic environment of compression and tension are more effective. It means that the smaller the angle of dipping of faults and fractures, the clearer the position of their planes and fracture zones. Especially in the normal fault structure, the displacement of the horizontal layers is strongly reflected by the GPR.

Songino faults have a compressed structure on the northwest edge and have the highest scarp area (approximately 21 meters high).

GPR studies do not reveal strong anomalies in the strike slip structured zone, which is likely to have a high angle of collapse in this zone.

The GPR signal reflection is strongly anomalous in the area of the fault tensional zone or normal structure, and the structure below the fault scarp is clearly visible. The resulting image provides a complete analysis of the movement in the area.

It has been reaffirmed that it is possible to accurately determine the extent of their displacement by reconstructing paleo river channels that have been buried and hidden since the last earthquake.

A detailed GPR study of the Songino fault revealed a wealth of information on the structure, direction of movement, and alluvial sediment accumulation of the soil disturbances caused by the fault. Based on all of these data, a unified interpretation reveals that the Songino fault plane is a left-lateral strike slip from southeast to northwest. The compressional effect on the west part, or the high scarp sedimentary structure formed on the ground surface, is the effect of excavation at an angle of approximately 40° to the movement direction of the fault. The sediment with a normal-structured scarp on the east is the effect of the tensional of the structure, which bends in the direction of the Songino fault movement and forms an angle of about 21°.

Identify the geological environment of the Songino fault and the factors that contribute to it, and this study believe that this goal has been achieved to some extent. These include: By carrying out optical luminescence chronology research on the trench sections of the Songino, Emeelt and Khustai active faults for Paleo-seismologic dating, combined with the faults of the formation, the following preliminary understandings are obtained:

Based on the optical luminescence age of the fault dislocation strata and its overlying strata on the ET trench, it is inferred that the time of the latest activity of the Emeelt active fault is between (9.6±1.0) ka and (11.3±1. 3) ka. On the KT trench, it is inferred that the time of the latest fault activity of the Khustai active fault is between (0.87±0.08) ka and (24.2±4.9) ka, it may be closer to (0.87 ± 0.08) ka from today. The T-1 trench, it is inferred that the time of the latest activity and age of last top layer of the Songino active fault is (5.77±0.04) ka.

The vertical displacement of the scarp of the normal-structured section was approximately 1.5 m in the perpendicular profile to the Songino fault trace, and the vertical displacement was approximately 0.8 m in the parallel profile along the, while the height of the vertical displacement in the northwest thrust structure of the Songino fault was the highest. At its highest point, it was approximately 22 m high.

There is a difference about 20.5⁰ between the normal and strike slip structures, and about 37.1⁰ difference between the strike slip and thrust structures, and a total of about 58⁰ difference between the normal and thrust structures.

In addition, the horizontal displacement of the normal structured section was 33 m, and from this value, the geometric calculations were used to determine the displacement on the other structures, including 19.32 m for the thrust structured section and then 32.03 m on the left-lateral strike slip section, which is the main transition of the Songino fault.

The main object of this doctoral dissertation, the main structure of the Songino fault, is a left lateral strike slip displacement fault, and the within two side effects of this movement are:

the thrust structure of the north-west side structure will have a rotational thrust structure and the normal structure of the south-east side structure will have the same rotational normal structure. However, the stress that causes this Songino fault is defined as the compressive effect of the "Khustai" and "Emeelt" active faults. The purpose of this doctoral dissertation is to determine and explain the characteristics and parameters of active faults using modern advanced technology, such as 2D and 3D imaging techniques of GPR. was performed using 500, 250 and RTA 50 MHz antennas. The GPR 2D and 3D imaging techniques wer satellite image analysis, quaternary geological survey, and palaeoseismological compression effect is shown by the example of Mogod and Mungunmorit ure of tensional effect is presented by Bayankhongor active fault, and the structure of by the examples of Avdar and Emeelt active faults. In addition, the results of by depth slices on the example of the Emeelt active fault between the Hustai and Emeelt paleoseismic faults located identified three different types of surface fractured structures and Songino fault have a basic left lateral strike slip movement from southeast to northwest, and this movement has been found to be caused by compression between the Hustai and Emeelt active faults and its thrust and normal structure on both sides are due to the curvature structure of the that fault The paleochannel dating samples show that the Emeelt fault is on average 10,000 years, the Hustai fault is about 900 years, and the Songino fault is about 6,000 years for last event. date, en moyenne, de 10 000 6 000 ans pour les derniers in the characteristics and parameters of active faults using modern advanced technology, such as 2D and 3D imaging techniques of GPR. This research 2D and 3D imaging techniques were satellite image analysis, quaternary geological survey, and palaeoseismological by the example of Mogod and Mungunmorit by Bayankhongor active fault, and the structure of Avdar and Emeelt active faults. In addition, the results of mple of the Emeelt active fault. located west of Ulaanbaatar. surface fractured structures and Songino fault have a basic left lateral strike slip movement from southeast to northwest, and this movement has been found to be caused by compression between the Hustai and Emeelt active faults and to the curvature structure of the that fault The paleochannel dating samples show that the Emeelt fault is on average 10,000 years, the Hustai fault is

Figure 1 .

 1 Figure 1. Nombre de séismes cumulé entre 2000 et 2021 (superficie de 140x140 km). La ligne verte est l'heure de début de l'installation de la nouvelle station, en 2000 l'installation du « tableau CTBT Songino » et en 2008 l'installation de « UB-mobile », MNDC data.

Figure 2 .

 2 Figure 2. Schéma de traitement des données GPR. La couleur orange clair correspond à l'étape de filtrage et de traitement, tant disque le bleu clair correspond à l'étape d'entrée, de sortie et d'analyse des données

Figure 3 .

 3 Figure 3. Carte de localisation des failles actives à l'étude autour d'Oulan-Bator et de la région centrale de la Mongolie.

Figure 6 .

 6 Figure 6. Les résultats des études et d'analyses 2D et 3D GPR, menées depuis 2010 sur les

Figure 4 .

 4 Figure 4. Résultat de l'interprétation de l'mage radar du profil-1 (acquis en juin 2017) de la zone 2 (structure chevauchée) de la faille de Mogod. La première figure est obtenue avec l'antenne de 250 MHz. La deuxième figure est obtenue avec l'antenne de 500 MHz. Les flèches jaunes montrent les réflexions des couches, les flèches bleues correspondent au même calque, les lignes orange correspondent aux couches réfléchissantes, les flèches noires indiquent la direction du mouvement.

Figure 5 .

 5 Figure 5. Diagraphie de la paroi ouest de la tranchée paléo-sismique 2. La couche indiquée en jaune a été fracturée et chevauchée. (D'après Laurent Bollinger (CEA), Yann Klinger (IPGP) 2017).

Figure 6 .

 6 Figure 6. Résultat des 'depth slices' du cube de données pseudo-3D C1 (acquis en juin 2010) obtenu avec une antenne blindée de 500 MHz dans le paléo chenal fluvial de la faille active d'Emeelt. a) « Depth slice » à 0,48 m sous la surface du sol, b) « Depth slice » à0,53 m sous la surface du sol, flèches bleues indiquant les traces de faille.

  ont été collectées avec des antennes blindées 250 MHz et RTA non blindées 50 MHz en septembre 2012. Par la suite, en 2013, 2015 et 2017, plusieurs levés GPR 2D et 3D ont été effectués sur la faille Songino (voir figure 7), et en 2013, une étude de tranchée a été réalisée.

Figure7.

  Figure7. Carte de localisation des zones d'étude de la faille de Songino. Les lignes rouges montrent les traces de la faille sur la rupture en surface de la faille de Songino, les carrés bleus montrent les zones étudiées par géoradar en 2012 et 2017

Figure 8 .

 8 Figure 8. Carte de localisation des profils GPR acquis en juin 2013. a) La zone d'étude GPR S5 (voir localisation à la figure 7). b) Une image agrandie de la zone des profils (carré orange) ; les lignes rouges représentent les profils GPR de juin 2012, la ligne bleue est le profil croisé Cr1, le symbole bleu indique la position initiale de la mesure, les symboles rouges (F) indiquent l'intersection des traces de faille en surface avec le profil.

Figure 9 .

 9 Figure 9. Le résultat des images GPR du profil P2 et du profil perpendiculaire Cr1 (acquis le 14 juin 2013) obtenu avec une antenne 250 MHz dans la zone d'étude principale de la faille de Songino S5 (voir localisation sur la Figure 7). a) Résultat de l'image GPR du profil P2, il montre une structure normale claire et un effet d'extension régional. b) Résultat de l'image GPR du profil perpendiculaire Cr1, la flèche rouge montre une zone très réflective. Les flèches noires indiquent la direction du mouvement des deux côtés.

Figure 10 .

 10 Figure 10. Carte de localisation des profils GPR mesurés en aout 2017, la zone d'étude GPR S2 est montrée sur la Figure 7. Les lignes jaune indiquent les profils GPR perpendiculaire à la faille, la ligne rouge est la trace de la faille, les symboles bleus indiquent le début des profils.Les flèches bleues pointent le chevauchement de l'escarpement de la faille.

Figure 11 .

 11 Figure 11. Le résultat de l'image GPR du profile P1 de la zone 3D6 (acquis en 2017, voir aussi Figure 10) obtenu avec l'antenne de 250 MHz (voir la localisation sur la Figure 7). Interprétation de l'image radar du profil P1. L'image radar montre clairement une structure chevauchée (lignes rouges), les flèches noires montrent la direction de la déformation. La hauteur de l'escarpement est, de 2.1 m.

Figure 12 .

 12 Figure 12. La faille active de Songino, a) la ligne rouge indique la première trace de la faille, la ligne bleu est la 2nde trace et la ligne jaune la 3eme trace après le 2eme événement. b) Quatre différents directions de la trace de la faille avec les angles correspondants.

  d'environ 1,5 m dans le profil perpendiculaire à la trace de la faille Songino, et le déplacement vertical était d'environ 0,8 m dans le profil parallèle le long de la faille, tandis que la hauteur du déplacement vertical dans la structure de chevauchement nord-ouest de la faille de Songino était la plus élevée, son point culminant mesurait environ 22 m de haut. Il y a une différence d'environ 20,5⁰ entre les structures normales et décrochantes, et d'environ 37,1⁰ entre les structures décrochantes et poussées, et un total d'environ 58⁰ de différence entre les structures normales et poussées. зөвхөн тектоник хавтангуудын субдукцийн бүсэд төдийгүй тивийн дунд бичил хавтангуудын хооронд тохиолддог тул Монгол улс нь газар хөдлөлтийн эрсдэлтэй орны нэг юм. Монгол улс геологийн хувьд Төв Азийн орогенийн бүсэд багтдаг бөгөөд эдгээр уулс нь гулсалтын хурд багатай хөгжиж байдаг тул газар хөдлөлтийн идэвхтэй байдаг. Өнгөрсөн зууны эхэн үеэс хойш дэлхий дээр 8.0-аас дээш баллын 100 орчим газар хөдлөлт болж, үүний бараг 50% нь Ази тивд болжээ. Эдгээрээс Ази тивд болсон 44 хүчтэй газар хөдлөлтийн дотроос хоёр нь Монгол улсад болсон ба газар хөдлөлтийн магнитудын эрэмбээрээ Булнайн газар хөдлөлт (1905) ес дүгээрт, Говь-Алтай (1957) газар хөдлөлт нь 12 дугаарт бичигджээ. Гэсэн хэдий ч эдгээр газар хөдлөлт нь хүн амын нягтрал багатай, нүүдэлчин амьдралын хэв маягаас шалтгаалан гамшиг, хохирол багатай байсан. Гэвч өнөөдөр Монгол улс дахь хотжилт улам бүр нэмэгдэж, ялангуяа 1.5 сая орчим хүн ам төвлөрсөн нийслэл Улаанбаатар хотод эрчимтэй суурьшилт явагдаж байна. Улаанбаатар хот нь олон идэвхтэй хагарлаар хүрээлэгдсэн байдаг бөгөөд 2005 оноос хойш газар хөдлөлтийн идэвхжил нь огцом нэмэгдэж байгаа нь тулгамдаад буй бодит асуудал болж байна. Зураг 1. 2000-2021 оны хооронд болсон газар хөдлөлтийн нийт тоо (140x140 km талбай). Ногоон шугам нь шинэ станц суурилуулсан хугацаа, 2000 онд "CTBT-ийн Сонгино дахь сүлжээ станц" ба 2008 онд "UB-mobile"станцууд, MNDC-ийн мэдээллийн сан.

2.

  Идэвхитэй хагарлын үндсэн төрлүүд дээрхи бүтцүүдийг хэрхэн тодорхойлох, зураглах арга зүй, хэмжилтийн зарчмийг боловсруулах. 3. Георадарын хэмжилтийн мэдээлэл боловсруулах программ хангамжын зарим модулиудыг бичиж туршин зүгшрүүлэх. 4. Судалгааны гол талбар болох Сонгино хагарлын нарийвчилсан хэмжилт, зураглалын мэдээлэл бүрдүүлж тайлал хийснээр уг хагарлын талаархи палеосейсмологи болон структур геологийн тайлбар хийх.

1.

  Зайнаас тандан судлал, сансрын зургын анализ (Хагарлын ул мөр, мөргөцөг, шилжилт хөдөлгөөнийг илрүүлэх зураглах) 2. Талбайн геологийн судалгаа (Хагарлын ул мөр, мөргөцөг, шилжилт хөдөлгөөн, шалгах цэгүүд, георадарын судалгааны орчин нөхцөл шалгах) 3. Георадарын судалгаа (2D ба 3D георадарын хэмжилт, мэдээлэл цуглуулах, гадаргын топографын хэмжилт хийж гүйцэтгэх) 4. Палеосейсмологийн судалгааны тренч ухах, хагарлын нас тогтоох дээж авах. 5. Бүх хэмжилт судалгааны үр дүн, структур геологи, геодинамикийн нэгдсэн тайлал хийх.

Figure 0 . 1

 01 Figure 0.1 Earthquakes of moment magnitude 8.0 and greater from 1900 to 2018. The apparent 3D volumes of the bubbles are linearly proportional to their respective fatalities. The color indicates the continent, and the legend counts the number of quakes for each, black square surrounds the two earthquakes of Mongolia, (USGS sources). ............ 44 Figure 0.2 Cumulative number of earthquakes between 2000 and 2021 (area of 140x140 km). Green line is starting time of new station installation, in 2000 installation of "CTBT Songino array" and in 2008 installation of "UB-mobile", MNDC data. ................... 45 Figure 0.3 Earthquake magnitude comparison. Each step on the magnitude scale is 10 times more powerful than the previous step. Radiuses of circles represent the seismic energy to scale. The two strongest Mongolian's earthquakes (indicated in red) are compared in magnitude with other major earthquakes (USGS sources). ................................... 47

Figure 3 . 8

 38 Figure 3.8 Location map of the third GPR profile (blue line) P3 acquired in 2012. Blue symbols indicate the initial position of the measurement. a) The GPR survey zone S5 as shown in Figure 3.2b) a magnified image of the profile area, red symbols (F) indicate the intersection of the fault traces on the surface rupture with the profile. .................... 139 Figure 3.9 The result of GPR images of the third profile (acquired the September 2012) obtained with 250 MHz and RTA 50 MHzantenna in the Songino fault study area S5 (see location in Figure 3.8. a) Result of GPR image obtained with 250MHz antenna. b) Result of GPR image obtained with RTA 50 MHz antenna; the blue symbols indicate the initial position of the measurement, red symbols (F) indicate the intersection of the fault traces on the surface with the profile, red arrows are pointing underground high reflected GPR anomalies. ......................................................................................... 140 Figure 3.10 Location Map of the GPR profiles acquired in June 2013. a) The GPR survey zone S5 as shown in Figure 3.2. b) A magnified image of the profiles area (orange square); the red lines show the GPR profiles of June 2012, blue line is crossed profile Cr1, the blue symbol indicates the initial position of the measurement, red symbols (F) indicate the intersection of the fault traces on the surface with the profile. .......................... 140 Figure 3.11 The result of GPR images of the profile P2 and crossed profile Cr1 (acquired the 14 June 2013) obtained with 250 MHz antenna in the Songino fault main study area S5 (see Figure 3.10). a) Result of GPR image of profile P2, it shows a clear normal structure and regional extensional effect with competent layer structure under scarp (red arrows). b) Result of GPR image of the crossed profile Cr1, red arrow is pointing

Figure 5 .

 5 Figure 5.1 GPR target description (GEOSCANNERS, 2021). .............................................. 175 Figure 5.2 The facies used in the interpretation of the 250MHz GPR result images on the high angle normal structure zone. Clear strongly horizontal reflection layer, it is almost parallel to the surface topography. A strongly attenuated signal and don't have any reflection in the high angle vertical structure of the high angle normal structure (black circle). ....................................................................................................................... 176 Figure 5.3 The facies used in the interpretation of the 250MHz GPR result images on the high angle fault structure. a) First example of the interpretation, 1. Clear horizontal reflection layer and chaotic background, 2. Clear horizontal reflection layer and strongly attenuated signals. b) Second example of the interpretation, upside of 1. And 2. Chaotic background with no layer continuity, downside of 1. And 2. Clear horizontal reflection layer and strongly attenuated signals. ...................................................... 176 Figure 5.4 The facies used in the interpretation of the 250MHz GPR result images on the low angle thrust fault structure. a) First example of the interpretation, 1. Clear horizontal reflection layers and phases changed, 2. Clear horizontal reflection diverged layers. b) Second example of the interpretation, 1. And 2. Clear horizontal and low angle reflection layers and strongly attenuated signals. ..................................................... 178 Figure 5.5 Structural evolution of the Songino fault area. a) Past first location, b) Past second location, c) Past third location, d) Present location .................................................. 180

Figure 0 .

 0 Figure 0.1 we remark that among 44 strong earthquakes in Asia, two of them occurred in Mongolia The Bulnai earthquake (1905) is the ninth strongest and the Gobi-Altai earthquake

  ).Although theBulnai (1905) andGobi-Altai (1957) earthquakes caused less damages and victims than the other earthquakes of the same magnitude in the world (Figure0.3), nowadays the urbanization in Mongolia is continually increasing especially in the capital city of UB which presents a high seismic hazard risk. Figure0.2 shows the seismic activity around Ulaanbaatar capital city since 2000 and the dates of installation of the seismic array stations for the seismic monitoring.

Figure 0 . 1

 01 Figure 0.1 Earthquakes of moment magnitude 8.0 and greater from 1900 to 2018. The apparent 3D volumes of the bubbles are linearly proportional to their respective fatalities. The color indicates the continent, and the legend counts the number of quakes for each, black square surrounds the two earthquakes of Mongolia, (USGS sources).

Figure 0 . 3

 03 Figure 0.3 Earthquake magnitude comparison. Each step on the magnitude scale is 10 times more powerful than the previous step. Radiuses of circles represent the seismic energy to scale. The two strongest Mongolian's earthquakes (indicated in red) are compared in magnitude with other major earthquakes (USGS sources).

Figure 0 . 4

 04 Figure 0.4 Block motion model. Thick gray dashed lines are the block boundaries. Red fan shapes and hollow arrows denote block rotation rates and translation rates with respect to the Eurasian plate, respectively. Red thin vector pairs denote principal strain rates. Black arrows indicate motion directions of the Indian plate and Tibetan plateau[START_REF] Wang | Present-Day Crustal Deformation of Continental China Derived From GPS and Its Tectonic Implications[END_REF].

  characterization GPR-based technique of the Songino active fault, Ulaanbaatar, Mongolia., and geological context of the paleo-seismologic characterization of the Songino active fault, Ulaanbaatar, Mongolia. GPR research methods and techniques are the fastest-growing methods and techniques in geophysical research fields and now is widely used in a lot of branches around the world. Mongolia was only a few examples of GPR research before 2010 (Since 1998, GPR survey has been conducted jointly by MUST and Tohoku University) and only one instance used in active fault studies. This was performed on a single profile in the Common Offset (CO) survey design with a RAMAC GPR system (MALA Geoscience, Sweden) 100 MHz antenna on the Deren active fault (Dundgovi, Mongolia) and combined with trench studies (Figure 0.6)[START_REF] Lu | Characterization of the shallow structures of the Deren Fault by ground penetrating radar[END_REF]. The velocity of the electromagnetic wave in the subsurface was measured by Common-Mid-Point (CMP) data[START_REF] Lu | Characterization of the shallow structures of the Deren Fault by ground penetrating radar[END_REF].

Figure 0 . 6

 06 Figure 0.6 Characterization of the Deren fault by GPR. a) GPR result image with trench observations, b) Geological model interpreted from GPR result image[START_REF] Lu | Characterization of the shallow structures of the Deren Fault by ground penetrating radar[END_REF].

2. 2

 2 Reflection, refraction, transmission, and reception of EM signals Transmitted signal -The EM signal transmitted by the transmitting antenna is a pulse signal of a given frequency and duration. It is characterized by a center frequency fc and a bandpass of width B (in Hz), linked by the ratio 𝑅𝑅 = 𝐵𝐵 𝑓𝑓 𝑐𝑐

Figure 1

 1 Figure 1.4 RTK and DGPS/GNSS. Source: after[START_REF] Jan | GPS for Land Surveyors[END_REF]

Figure 1 . 5

 15 Figure 1.5 The different components of a GPR system. Image Source: (Canada: Geoscan)

  is a geophysical method analogous to seismic reflection. Therefore, the modes of acquisition, processing and representation are similar. The constant offset acquisition mode is characterized by a constant distance (S) and the same orientation between transmitting and receiving antennas at each measurement point. These measurement points are equidistant (∆x) and distributed along profiles. In each point, the recorded trace describes the amplitude variations of the reflected signal as a function of time. The tracks are then represented as a function of the distance along the profile in order to obtain an image of the reflectivity (radar section) of the ground (Figure1.6). Variations in amplitude and time reflections indicate variations in speed, attenuation, contrast of impedances and geometry of interfaces.

Figure 1 . 6

 16 Figure 1.6 The different acquisition modes typical of ground penetrating radar, with constant offset at the top, in the CMP mode at the bottom. On the right, an example of the typical image obtained for each of the modes with the different types of arrivals indicated (Figure 1.1) after

Figure 1

 1 Figure 1.7 GPR data processing diagram. Lighter orange is filter and processing step, lighter blue is data in, out and analyzing step.

Figure 1 .

 1 Figure 1.8a) or vice versa.

Figure 1 .

 1 Figure 1.8 a) Example of raw GPR profile before processing; b) GPR profile after time-zero correction.

Figure 1 .

 1 Figure 1.10 De-wow processing. a) Example of GPR profile before De-wow processing; b)

Figure 1 .

 1 Figure 1.11 Flat reflection filter processing. a) Example of GPR profile with Flat reflection filter processing; b) GPR profile without Flat reflection filter processing.

Figure 1 .

 1 Figure 1.12 Amplification processing. a) Example of GPR profile before Amplification processing; b) GPR profile after Amplification processing.

Figure 1 .

 1 Figure 1.13 Frequency filter processing. a) Example of GPR profile before frequency processing; b) GPR profile after frequency processing.

Figure 1 .

 1 Figure 1.17 AGC processing. a) Example of GPR profile before AGC processing; b) GPR profile after AGC processing.

Figure 1 .

 1 Figure 1.18 Hyperbola analyze. Green line is surface reflected air wave hyperbola, air wave velocity is 0.3 m/ns.

Figure 2 . 1

 21 Figure 2.1 Conceptual models of succession of coseismic shifts. Different paleo seismological studies illustrate each of these models. a) the variable slip model describes earthquakes of variable magnitudes occurring randomly on the fault, b) the uniform slip model, after[START_REF] Sieh | A Review of Geological Evidence for Recurrence Times of Large Earthquakes[END_REF] describes earthquakes repeating periodically on the same segment of the fault. Large earthquakes that can break several segments and moderate earthquakes accommodating slip deficits, c) the characteristic model describes earthquakes repeating periodically over the same rupture length and with similar magnitudes, after[START_REF] Schwartz | Fault behavior and characteristic earthquakes: Examples from the Wasatch and San Andreas Fault Zones[END_REF]

  Ground Penetrating Radar survey methods have been used in active faults in Mongolia and the results obtained on them. The main purpose of Ground Penetrating Radar surveys conducted on active faults since 2010 is to develop Ground Penetrating Radar surveys in Mongolia, to determine all types of active fault parameters, and to obtain a methodology for using a combination of Ground Penetrating Radar surveys and other modern technologies. To do this, we used many advanced research methods, such as Ground Penetrating Radar and remote sensing, and high-precision differential positioning system, on several active faults, and as a result, we were able to achieve good results on all types of active faults. The results of these studies are classified by types of active faults and included in this chapter. Also, examples of three-dimensional survey and analysis, an advanced method of Ground Penetrating Radar research, are also presented in this chapter.2 The GPR applications and method development for the active fault structures of the central Mongolia.The GPR technique has been shown to be very effective in determining the horizontal and vertical displacement of faults, and in mapping old channels and ravines. GPR imaging results in the analysis of the boundaries of different layers and the areas where the reflections, refraction, deformation, and distortion of the waves occur, and a conclusion is made concerning the geological structure of the area[START_REF] Davis | Ground penetrating radar for high resolution mapping of soil and rock stratigraphy[END_REF].

Figure 2 . 4

 24 Figure 2.4 Location map of the studying active faults around Ulaanbaatar and the central region of Mongolia.

  segments with the largest and most continuous offsets consist of ~20 km of N-S right-lateral strike-slip faulting (Figure 2.5b) and ~10 km of thrust faulting with a NW-SE strike dipping NE (Figure 2.5c). In addition, less continuous and smaller-offset N-S ruptures (Figure 2.5a) occurred further north of the main strike-slip faulting, following the western side of the range front east of Mogod town. Other minor cracks occurred in the valley floor (Figure 2.5d). A brief description of the main characteristics of these ruptures is necessary to help constrain the seismological analysis and the geo-morphological interpretation.

Figure 2 . 5

 25 Figure 2.5 Summary map of the 1967 Mogod earthquakes (see Figure 2.4 for location). The main surface fault ruptures (b and c) are shown as heavy lines and are taken from(Baljinnyam,

A

  detailed study of Mogod active fault was conducted in August 2012 by a team of scientists from the University of Strasbourg, France, and researchers from the Institute of Astronomy and Geophysics, Mongolian Academy of Sciences. A comprehensive geological field investigation and geophysical survey (GPR, GPS, Magnetic) was conducted over a threeday period from 21 August to 23 August 2012 year.

  At the first trench excavation or strike slip structured zone, three GPR profiles of 90 m were performed by using GPR 250 MHz and 500 MHz antennas (Figure2.6b). The trace interval (∆x) was 3 cm for both antennas and the investigation depth are approximately 7 m and 5 m for 250 and 500 MHz antenna, respectively. The trace interval of GPR RTA50 MHz antenna was 10 cm, and the penetration depth was approximately of 20 m. Table2-1 shows the frequency, the acquisition parameters, and the length for each GPR profile. At 200 m and 1100 m south of the trench excavation site, two electrical resistivity profiles, have been measured, the length of which was 200 m and 280 m, respectively. At the second trench excavation zone or back thrust structured zone, four GPR profiles of 73 m length were performed by using GPR 250 MHz and 500 MHz antenna (Figure 2.6c).

Figure 2 . 6

 26 Figure 2.6 The location map of GPR profiles 2017 on the Mogod active fault. a) Main location of the studying three different structured zones, b) GPR profiles and trench location of the first right lateral strike slip structured zone-1, c) GPR profiles and trench location of the second area, back thrust structured zone-2, d) GPR profiles and trench location of the third area, reverse structured zone-3.

  7 and Figure 2.8. At the top of the scarp, there is a visible demarcation of the fault from northeast to southwest.

Figure 2

 2 Figure 2.7 GPR result image of the Profile-1 (acquired the June 2017) interpretation of the zone 2 (back-thrust structured) of the Mogod active fault (Figure 2.6c). First figure is 250MHz antenna result image, yellow arrows show the reflections from layers. Second figure is 500 MHz antenna result image, blue arrow is same layer, orange line is strong reflected layers, black arrows indicated movement direction.

Figure 2

 2 Figure 2.8 GPR result image of the Profile-3 (acquired the June 2017) interpretation of the zone 2 (back-thrust structured) of the Mogod active fault (Figure 2.6c). First figure is 250MHz antenna result image, yellow arrows show the reflections from layers. Second figure is 500 MHz antenna result image, blue arrow is same layer, orange line is strong reflected layers, black arrows indicated movement direction.

Figure 2

 2 Figure 2.9 GPR result image of the Profile-4 (acquired the June 2017) interpretation of the zone 2 (back-thrust structured) of the Mogod active fault (Figure 2.6c). First figure is 250MHz antenna result image, yellow arrows show the reflections from layers. Second figure is 500 MHz antenna result image, orange line is strong reflected layers.

Figure 2 .

 2 Figure 2.10 West wall logging of the paleo-seismic trench 2. Yellow layer was break up and thrusted structure, after (Laurent Bollinger (CEA), Yann Klinger (IPGP) 2017).

Figure 2 .

 2 Figure 2.11 Photo of the paleo-seismic trench 2 wall. a) West wall photo and movement direction, b) East wall photo movement interpretation, after (Laurent Bollinger (CEA), Yann Klinger (IPGP) 2017).

Figure 2

 2 Figure 2.4). The study area lies within the northwestern flank of the upper kherlen basin.

  Figure 2.12).

Figure 2 .

 2 Figure 2.12 Location map of GPR profiles on the Mungunmorit active fault. a) Main location of GPR and trench study, red arrow is fault trace. b) GPR profiles and trench location, red line is GPR profile in 2013, green line is GPR profile in 2016, white square is paleo seismological trench, blue symbol is profile name and start point.

  ProEx GPR system (MALA Swedish trademark) with MALA RTA 50 MHz unshielded, and MALA 250 MHz shielded antennas and all the profiles are perpendicular to the fault trace (see Figure 2.13). The time window was selected 205 ns for the 250 MHz antenna and 501 ns for the RTA 50 MHz antenna. The trigger interval or distance interval was 0.02 meter in 2013 profiles and 0.05 meter in 2016 profiles for 250 MHz antenna (see Table

  Figure 2.13 GPR result zoomed image of the profile P1a (acquired the August 2013) obtained with 250 MHz antenna in the Mungunmorit fault, location show in Figure 2.12. The GPR image shows a clear thrust structure with several layered structure under scarp (yellow, blue, green, and orange arrows) and ramp of the thrust structure.

Figure 2 .

 2 Figure 2.14 Comparison of the paleo-seismological trench logging and result of GPR image profile 1a. a) Section of the west wall of paleo-tranche T-1. After(Smekalin, et al., 2016), b)

  antennas. That is shielded (MALA 250 MHz) and unshielded (MALA RTA 50 MHz) antennas with MALA ProEx fiber optic control system and MALA XV11 field computer. GPR measurements were collected in August 2014 and acquired three profiles with each antenna in the direction from the top to the bottom of the mountain (Figure 2.15b). Detailed acquisition parameters are shown inTable 2-3 and Table 2-4, respectively.

Figure 2 .

 2 Figure 2.15 The location map of the study area of Bayankhongor fault. a.) Google Earth satellite image showing the survey area. The blue arrows indicate the fault mark, and the yellow rectangle indicates the GPR survey area. b.) An enlarged drawing of the GPR surveyed area. The red line indicates the GPR measurement profiles, the yellow line indicates the starting points and names of the profiles, and the red and blue points indicate the fault trace mark.

  4. Figure 2.16 shows the GPR image of profile 1 obtained with 250 MHz antenna: the first (Figure 2.16a) is shown without AGC applied, while the second one (Figure 2.16b) is shown with AGC applied. In Figure 2.16a we remark a sharply change of reflections around 38 m horizontally indicating two different sub-soils. This sharply changing of reflections remains even after AGC application (Figure 2.16a). The penetrating depth of 250 MHz antenna is approximately 2.5-3 m, while for the RTA 50 MHz antenna it is 12 m (see Figure 2.17a). A comparison of the resulting GPR images obtained with GPR 250MHz and RTA 50MHz of the same profile (Figure 2.17a and Figure 2.17b) show that the fault plane zone is approximately on the same place, approximately at 33 m horizontally. However, in the location map of Figure 2.15b the fault trace mark is indicated by a red symbol.

Figure 2 .

 2 Figure 2.16 GPR image of profile 1 obtained with 250 MHz antenna. a.) The GPR image is obtained without the automatic gain control (AGC). b.) The GPR image is obtained after AGC amplification.

Figure 2 .

 2 Figure 2.17 Interpretation of GPR RTA 50MHz and 250MHz antenna performance maps performed on the profile P1 (acquired the July 2014) in the Bayankhongor fault. The topographic corrections and depth conversion are performed using a velocity of 0.12 m/ns. a.) RTA 50MHz antenna result image, with a yellow arrow roughly outlining the location of the fault plane. b.) The resulting GPR image from 10 m to 85 m of the profile P1 with a 250MHz antenna. The yellow arrow outlines the fault zone. The GPR image shows a clear low angle normal (listric) structure with lot of layered structure near fault zone c.) The result image of the profile P1 performed with 250 MHz antenna from 72 m to 142 m, and a break in sediment accumulation is observed at around 102 m in this profile P1. The same layers that move in both directions are indicated by arrows of the same color. The yellow lines indicate the boundary

Figure 2 .

 2 Figure 2.18 Low-angled normal fault structure or Listric fault model (University of SYDNEY, 2000).

  the Emeelt fault zone, we studied in 2009-2012, the fault scarp and traces are clearly visible on the satellite image. Elsewhere, some traces are visible but very dim. Seismic activity at the Emeelt fault increased sharply in 2013. We have been conducting comprehensive geological and geophysical surveys on the Emeelt active fault since 2010, and some research and monitoring is still ongoing. We have been conducting a comprehensive geological and geophysical study in this area for 4 years in collaboration with researchers from the Strasbourg University of France, Institute of the Earth's crust Russia and Institute of Astronomy and Geophysics Mongolia. The first trench was excavated in 2009, and in 2010 a large threesegmented trench was excavated, and various geophysical surveys were conducted around it.

  5). The location of the profiles and their GPR results together with comparison of the trench photo mosaic are shown in Figure 2.21a.

Figure 2 .

 2 Figure 2.19 Seismic activity map around Ulaanbaatar capital city. Emeelt active fault location is white square. Red line is fault trace and red dots represent the seismic events since 2005 (after NDC data, IAG, MAS).

Figure 2 .

 2 Figure 2.20 Location map Emeelt fault 2013 GPR survey. a) Main location GPR profile P6 and fault trace. Green square GPR survey 2010-2011 area. b) Zoomed detailed map of the GPR profile P6. Red line is RTA profile and yellow symbols are fault segments on the surface.

  results of GPR measurements on active faults such as Emeelt and Hustai in 2010-2011 were used in the doctoral dissertation of one doctoral student at the University of Strasbourg, France ( (Dujardin J.-R. , 2014). The results of the GPR measurements performed on the Emeelt active fault in 2010 are of good quality due to the low impact of soil moisture and provide significant information about the depth structure of the fault plane. From the results of the RTA50 MHz antenna, it is possible to clearly see the location of the fault and the dipping of the fault (Figure 2.21).

Figure 2 .

 2 Figure 2.22 GPR result image of the profile P6 (acquired the May 2013) obtained with RTA 50 MHz antenna in the Emeelt fault. The GPR image shows a clear strong reflected and dipping angle structure (yellow arrows point soil fractured zone on the surface). First figure is first section of the profile P6 and second one is next section of the profile P6.

Figure 2 .

 2 Figure 2.22 we show only from 0 to 525 m in two parts. Also, at the beginning of the profile, between 80-130 m, the dipping of fractures indicated by 1 and 2 is in the different direction (Figure 2.22). However, between 250-510 m, the dipping is in same direction and there are several signs of fractures with different angles (Figure 2.22). The result of GPR image reveals the structure of the deep fault then locates it on the surface and places it on the line of the profile, which shows that it corresponds to the soil fault traces observed on the satellite image (Figure 2.20).

Figure 2 .

 2 Figure 2.23 GPR survey locations map of the Avdar fault. Z1-Z2 are GPR zones surveyed in 2011. Z3 is surveyed in 2012 and Z4-Z5 are surveyed in 2016. Two red lines are Avdar fault traces.

  2.23). In Z4 zone, 4 parallel profiles of 160-180 m in length were collected at a distance perpendicular to the fault plane and 55-70 m between two profiles (Figure2.30), apart to ensure underground fault information.

  2.30). This is due to the fact that the water from the ravine is blocked by the wall at the bottom of the fault, and the accumulation of mud and sediments is clearly visible. Layers of sediments accumulated on both sides of the fault plane through three profiles in this section gave a strong reflection of the Ground Penetrating Radar wave (Figure2.30). The fault plane is not very well visible, but some structures are observed (Figure2.30). In the GPR profiles made in Z1, the structure of the fault is clearly visible by giving a strong reflection of the Ground Penetrating Radar wave from the fault plane, for which the angle of incidence can be determined. Profile 8 intersects the two segments (red mark) of the fault (Figure2.30), and the Ground Penetrating Radar result image shows the structure of these faults below the surface by the distortion of the Ground Penetrating Radar wave reflection (Figure2.30). In addition, there is a large accumulation of sediment between the two segments of the fault, and there are signs of compression from the bottom or from the side of the second segment of the fault. The structure of this accumulation is similar to that of the Z2 sediment accumulation.

Figure 2 .

 2 Figure 2.24 Location map of ground penetrating radar profiles in Z2 zone. The zone location is shown in Figure 2.23. Light blue lines indicate the profiles at the RTA50 MHz, 500 MH zand 250 MHz antenna. The blue symbols are indicating the start points and name of profiles. The red symbols are representing fault trace.

Figure 2 .

 2 Figure 2.25 Result of GPR image of the profile P1 and P2 (acquired the October 2011) obtained with RTA 50 MHz antenna in the Avdar active fault. The GPR result image shows a clear layer structure (yellow arrows), and surface rupture location is red colored symbols (F), the blue symbols are measurement start point.

Figure 2 .

 2 Figure 2.26 Location map of ground penetrating radar profiles in Z1 zone. The zone location is in Figure 2.23. Light blue lines indicate the profiles at the RTA50 MHz, 500 MHz,and 250 MHz antenna. The blue symbols are indicating the start points and name of profiles. The red symbols are representing fault trace.

Figure 2 .

 2 Figure 2.27 Result of GPR image of the profile P8 (acquired the October 2011) obtained with RTA 50 MHz antenna in the Avdar mountain fault. The GPR result image shows an

Figure 2 .

 2 Figure 2.28 Location map of ground penetrating radar profiles in Z1 zone. The zone location is shown in Figure 2.23. Light blue lines indicate the profiles at the RTA50 MHz, 500 MHz, and 250 MHz antenna. The blue symbols are indicating the start points and name of profiles. The red symbols are representing fault trace.

Figure 2 .

 2 Figure 2.29 Result of GPR image of the profile P9 (acquired the October 2011) obtained with RTA 50 MHz antenna in the Avdar fault. The GPR image shows a clear fault structure (red arrows), and surface rupture location is red colored symbols (F), two dipping angles approximately of 18.4⁰, yellow arrows are pointing bed rock layer.

Figure 2 .

 2 Figure 2.30 Location Map of GPR profiles in Z4 zone. The zone location is shown in Figure 2.23. Yellow lines indicate the profiles obtained with RTA 50 MHz and 250 MHz antenna. The blue symbols are indicating the start points and name of profiles. The red symbols (F) are representing surface rupture locations.

Figure 2 .

 2 Figure 2.31 Result of GPR image of the profile P2 (acquired in August 2016) obtained with RTA 50 MHz antennae in the Avdar active fault. The GPR image shows a clear underground structure (red arrows), and surface rupture locations are shown by red colored symbol (F), the blue symbols indicate the starting points of measurements.

Figure 2 .

 2 Figure 2.32 Result of GPR image of the profile P3 (acquired in October 2011) obtained with RTA 50 MHz antenna in the Avdar mountain fault. The GPR image shows a clear underground structure (red arrows), and surface rupture locations are shown by red colored symbol (F), the blue symbols indicate the starting points of measurements.

Figure 2 .

 2 Figure 2.33 Location of ground penetrating radar profiles in Z1 and Z2 zones. The main location is in Figure 2.20. a) red lines indicate the profiles at the RTA50 MHz antenna. The black and white arrows are indicating the surface ruptures. b) Zoomed location map of the trenches (T2, T3 and T4) and GPR 3D cubes (C1, C2, C3 and C4), blue line is fault trace. c)

Figure 2 .

 2 Figure 2.34. Example of a 500 MHz profile crossing paleo-channels and recorded in 2010 to verify the presence of the paleo-channel in the GPR data before the precise work on "cubes".Its location is displayed in Figure2.33 (blue line). Black arrows highlight dipping reflections, which are related to the flanks of the channels, after[START_REF] Dujardin | Imagerie géoradar (GPR) en milieu hétérogène Application aux failles actives en Mongolie et aux dépôts pyroclastiques du Tungurahua (Equateur)[END_REF].

  adjust the time zero; a DC filter to remove the low frequencies; a flat reflections filter to remove some clutter noise (ringing caused by multiple reflections between shielded antenna and the ground surface); a time varying gain function and finally a band-pass filter (elliptic tapered filter). The time varying gain function is a smoothed version of the trace envelope calculated by Hilbert transform. Band-pass filters are of 20-150 MHz and 100-800 MHz for the 50 and 500 MHz antennae, respectively. A velocity analysis, which is not presented here, has been done over the surveying area by analyzing diffraction hyperbolae present in the GPR data. It gives a mean velocity of 0.135 and 0.095 m/ns for the data collected in 2010 and 2011, respectively. These values are constant and consistent for both frequencies (500 and 50 MHz antenna). The difference in velocity is explained by the unusual humid period during the survey in 2011 compared to the 2010 dry summer. Afterwards a Kirchhoff migration, which accounts

  Figure 2.34. The dipping reflections showing moderate continuity are indicated by black arrows. The middle and the right ones clearly define the flanks of a paleo-channel. Sub-horizontal reflections with moderate continuity and many diffraction hyperbolas lie in-between and are related to the sedimentary filling of the channel and the presence of many rocks. On the left side, another dipping reflection is observed. It is related to the edge of a second paleo-channel crossed by the profile. Fromthose results, we decided to investigate the first channel in depth with the pseudo-3D surveys[START_REF] Dujardin | Imagerie géoradar (GPR) en milieu hétérogène Application aux failles actives en Mongolie et aux dépôts pyroclastiques du Tungurahua (Equateur)[END_REF]. The acquisition geometry is presented in Table2-8.

Figure 2 .

 2 Figure 2.35a present a photomosaic from the north wall of trench T4. White wires in vertical and horizontal directions highlight the 1 m spacing gridding of the trench. The profile shown in Figure2.35b is the northern most profile of the cube C1 and is 2.5 m away from the trench T4 wall. The photomosaic and the GPR profile have the same scale, and there is no vertical exaggeration. Three main units were identified in the trench and highlighted here by brown, yellow and orange colors. The brown unit is characterized by a silty-sandy material with some gravel scattered inside it. Locally, gravels are gathered in thin horizontal layers, highlighted by red lines. It corresponds to the filling of the channel. The two next units are characterized by yellowish coarser material. The first one (the yellow unit) is a layer of around 50 cm thick with centimetric stones. The last unit (orange one) is very similar to the yellow unit, but with many scattered centimetric stones.

Figure 2 .

 2 Figure 2.35 a) Photomosaic of the north wall of T4 trench and (b) northernmost profile of cube C1, 2.5 m away from the trench wall. Three main different units were identified in the trench highlighted here by brown, yellow and orange colors. They are superimposed on the trench andthe profile for comparison. The brown is a thin brownish unit filling the channel, the yellow is a yellowish coarse deposits unit (centimetric sized), and the orange is a yellowish unit with fine material. Within the brown unit, a gravel lens is observed and highlighted by the red lines. A good consistency is observed between the GPR data and the trench. The gravel lens(red lines)

  2 m, is often lower than the depth of the channel. The SE flank is straight on its upper part and starts enlarging at around 27 m distance from the SE corner of cube C1 (scale on Figure 2.37a). This enlargement is linked to the arrival in the sedimentary basin. The NW flank is as well straight in its deepest part except for the shift at around 49 m distance with right lateral amplitude of about 2 m (black arrow, Figure 2.37).

Figure 2 .

 2 Figure 2.36 Four GPR profiles with the paleo-channel picked in green. The 3D surface (in colors) of the paleo-channel was deduced from the picking on all the profiles of the cube C2, C3 and C4. The red arrow shows the paleo-flow direction. The map on the down-left corner

Figure 2 .

 2 Figure 2.37 (a) Interpretation map of Z2 area. The interpolated 3D surface of the channel (after subtraction of its main slope) is superimposed on the satellite image. An offset of 2 m horizontally is observed on the NW flank of the channel (highlighted by the red lines and the black arrow), which is consistent with a right lateral strike-slip. The pink asterisk shows the location of picture (b) in the trench where evidence of the fault is observed. The closest RTA profile (upper left corner) shows the record and the location of the fault plane in depth. (c) Illustration of the evolution of the channel flanks due to the right lateral strike slip. The down left flank is preserved while the down right flank is eroded after the shift. The filling of the paleo-channel fossilizes the paleo-morphology, after[START_REF] Dujardin | Imagerie géoradar (GPR) en milieu hétérogène Application aux failles actives en Mongolie et aux dépôts pyroclastiques du Tungurahua (Equateur)[END_REF].

Figure 2 .

 2 Figure 2.38 Pseudo-3D data cube of the C1 cube obtained with 500 MHz shielded antenna in the paleo river channel of the Emeelt active fault. Inline arrow pointed measurements direction of the GPR profiles parallel to the T3 and T4 trenches (see Figure 2.33 for the locations).Crossline arrow pointed measurement direction of the GPR crossed profiles.

Figure 2 .

 2 Figure 2.39 Result of depth slices images of the C1 pseudo-3D data cube (acquired the June 2010) obtained with 500 MHz shielded antenna in the paleo river channel of the Emeelt active fault. a) Depth slice 0.48 m below ground surface, b) Depth slice 0.53m below ground surface, blue arrows indicating fault traces. Channel upside is rainwater inlet side and channel downside are rainwater outlet side.

  1 List of deadliest 21 st century earthquakes We have conducted many GPR campaigns in 2012, 2013, 2015 and 2017 on active fault areas around Ulaanbaatar city. In our study, we present only some preliminary results of GPR measurements performed along the Songino fault in 2013 and 2017 and discuss the results of a trench dug perpendicular to Songino fault in august 2013. The GPR profile's objective is to identify the near surface geometry of this fault (precise location, dip), describe underneath sedimentary sequence affected by past earthquakes, and the type of displacements and amplitude. In this survey, we used 250 and 500 MHz shielded antenna and a 50 MHz rough terrain unshielded antenna.

  Figure 3.1) is located between Emeelt and Hustai faults; it has a length of ~20 km and is situated 20 km West of UB. In the area of the Songino fault, we have conducted manyGPR campaigns in 2012GPR campaigns in , 2013GPR campaigns in , 2015GPR campaigns in and 2017. . In this study, we present results of 2D GPR measurements obtained in 2013 and 2017.The Songino fault was first discovered in 2012 as part of a seismic hazard project around Ulaanbaatar capital city. The Songino fault is a fault between two large active faults the Hustai and the Emeelt. It also aims to confirm the results of research on the two active faults and to contribute to the unraveling of the geodynamic structure around Ulaanbaatar. Therefore, the main purpose of this work is to conduct a detailed study on the Songino fault and to establish its geodynamic correlation using the experience and knowledge gained in the study of active faults. This chapter presents all the results of a detailed Ground Penetrating Radar survey of the Songino fault.The Songino Fault (Blue Square of Figure3.1), discovered in 2012, is located approximately 40 km west of Ulaanbaatar capital city of the Mongolia, between two large seismically active faults, that is Hustai and Emeelt. Figure3.1shows the area around the Songino Fault, a study area, surrounded by a blue square. The main part of the Songino fault is branched at the eastern end of the Hustai active fault. The first azimuth of the Songino fault stretch starts at 166º or about 14º from south, then changes direction and the next azimuth continues at about 148º or southeast (SE), then also changes direction again and continues to 129º or southeast. The azimuth is reversed, and the azimuth is curved at 108º from south to east, and finally splits into 3-5 sections with a straight eastward direction.The geomorphology of the Songino fault is clearly visible on the satellite image, and a high height scarp structure is observed at the beginning, at the branching point of the Hustai active fault. However, as the height of the Songino fault scarp gradually decreases, the fault scarp disappears in some places in the middle zone, and then the Songino faults scarp gradually increases again low rate and disappears.

Figure 3 . 1

 31 Figure 3.1 Seismicity maps (red dots) around Ulaanbaatar city from 1994 to 2014, after (Adiya 2016 and NDC data, IAG, MAS). The green asterisk indicates the UB urban area. Red lines indicate the trace of main active faults, after[START_REF] Ferry | Tectonic Morphology of the Hustai Fault (Northern Mongolia), A Source of Seismic Hazard for the city of Ulaanbaatar[END_REF][START_REF] Ferry | Tectonic Morphology of the Hustai Fault (Northern Mongolia)[END_REF] Al Ashkar 2015). The

Figure 3 . 2

 32 Figure 3.2 Location map of Songino fault and Ground Penetrating Radar survey areas. The red line shows the traces of the fault on the surface rupture of the Songino fault, the blue squares show the Ground Penetrating Radar measurements, the surveyed areas, and the Ground Penetrating Radarprofiles between 2012 and 2017 difference by color each year.

  Figure 3.4 shows the location map of the first profile P1. This picture shows the traces of fracture along the road, and it is interesting to know whether the fracture was caused by fault or caused by the road. Before analyzing underground fracture structure on a Ground Penetrating Radar result image, we check and mark the satellite image analyze for signs of fracturethat intersect with the GPR profile. This is marked on the GPR profile of Figure 3.4b with the red symbol (F), and then the points are analyzed to see how they correspond to the traces of fault at depth. Based on the mapping and interpretation of the results of the first GPR profile on the study area, two parallel fractured structures are observed at 60 and more than 70 meters on the profiles (Figure 3.5).

Figure 3 . 4

 34 Figure 3.4 Location map of the first profile P1 acquired in 2012. a) Northeast corner of zone S1 (see Figure 3.2), b.) a magnified image of the profile area, blue line shows the profile acquired in 2012, blue symbols indicates the initial position of the measurement, red symbols (F) indicates the point of intersection with the traces of damaged structure on the surface rupture and the profile.

Figure 3 . 5

 35 Figure 3.5 The result of GPR images of the first profile (acquired the September 2012) obtained with 250 MHz and RTA 50MHz antenna in the Songino fault study area S1 (see Figure 3.4). a.) result of GPR image obtained with 250 MHz antenna, b.) result of GPR image obtained with RTA 50 MHz antenna; blue symbol indicates the initial position of the measurement, red symbol (F) indicates the point of intersection with the fault traces on the surface rupture and the profile, red arrows are pointing underground high reflected GPR anomaly.

Figure 3 . 6

 36 Figure 3.6 Location map of the GPR profile P2 acquired in 2012, S4 zone as shown in Figure 3.2. Blue line shows the second profile of 2012, blue symbol indicates the initial position of the measurement, red symbols (F) indicate the intersection of the fault traces on the surface rupture with the profile.

Figure 3 . 7

 37 Figure 3.7 The result of GPR images of the second profile (acquired the September 2012) obtained with 250 MHz and RTA 50 MHz antenna in the Songino fault study area S4 (see Figure 3.6). a.) GPR profile obtained with RTA 50 MHz antenna, b.) GPR profile obtained with 250MHz antenna. Blue symbol indicates the initial position of the measurement, red symbols (F)indicate the point of intersection with the traces of damaged structure on the surface rupture and the profile, red arrows are pointing underground high reflected GPR anomalies.

Figure 3 .

 3 Figure 3.11a shows the profile-2 of 14 June 2013, and the structure of the hidden layer below the ground surface shows a very strong reflection anomaly. From this S5 zone, it is possible to see the structure of the soil erosion below the fault scarp, but the location of the fault plane and the dipping angle of the fault plane are not visible. From the 250 MHz shielded

Figure 3 . 8

 38 Figure 3.8 Location map of the third GPR profile (blue line) P3 acquired in 2012. Blue symbols indicate the initial position of the measurement. a) The GPR survey zone S5 as shown in Figure 3.2b) a magnified image of the profile area, red symbols (F) indicate the intersection of the fault traces on the surface rupture with the profile.

  However, the results of GPR images conducted in the trench floor provided some additional information about the movement of the fault zone sediments. The GPR images obtained with 250 MHz shielded antenna inside the trench (Figure 3.14a, location in Figure 3.13) show many parallel layers structure having the same dipping angle. However, the two layers around the fault scarp give a stronger reflection (indicated by the yellow arrow in Figure 3.14). Figure 3.14b shows the result of GPR image obtained with 500 MHz shielded antenna which is similar to the GPR image of previous Figure 3.14a. The vertical resolution in Figure 3.14b is higher than the resolution Figure 3.14a. From the result of these two GPR images the direction of dipping angle of the strongly reflected layers coincides with the direction of dipping angle shown in Figure 3.11.

Figure 3 . 9

 39 Figure 3.9 The result of GPR images of the third profile (acquired the September 2012) obtained with 250 MHz and RTA 50 MHz antenna in the Songino fault study area S5 (see location in Figure 3.8. a) Result of GPR image obtained with 250MHz antenna. b) Result of GPR image obtained with RTA 50 MHz antenna; the blue symbols indicate the initial position of the measurement, red symbols (F) indicate the intersection of the fault traces on the surface with the profile, red arrows are pointing underground high reflected GPR anomalies.

Figure 3 .

 3 Figure 3.10 Location Map of the GPR profiles acquired in June 2013. a) The GPR survey zone S5 as shown in Figure 3.2. b) A magnified image of the profiles area (orange square); the red

Figure 3 .

 3 Figure 3.11 The result of GPR images of the profile P2 and crossed profile Cr1 (acquired the 14 June 2013) obtained with 250 MHz antenna in the Songino fault main study area S5 (see Figure 3.10). a) Result of GPR image of profile P2, it shows a clear normal structure and regional extensional effect with competent layer structure under scarp (red arrows). b) Result of GPR image of the crossed profile Cr1, red arrow is pointing underground high reflected GPR anomalies. The blue symbols indicate the initial position of the measurements, red symbols (F) indicate the intersection of the fault traces on the surface with the profile. The black arrows indicate movement direction of the two sides.

Figure 3 .

 3 Figure 3.12 The result of GPR image of the profile P2 (acquired the 16 June 2013) obtained with 500MHz antenna in the Songino fault main study area S5 (see location in Figure 3.10). The GPR image shows a normal structure and regional extensional effect with competent layer structure under scarp (yellow arrows). The blue symbol indicates the initial position of the measurement, symbols (F) indicate the intersection of the fault traces on the surface with the profile.

Figure 3 .

 3 Figure 3.13 Location Map of the GPR profiles acquired in August 2013. The GPR survey zone S5 as shown in Figure 3.2. The red lines show the fault traces, white square shows trenching area.

Figure 3 .

 3 Figure 3.14 The result of GPR images of the profile P2 inside the trench (acquired the August 2013) obtained with 500MHz and 250 MHz antenna in the Songino fault study area S5 (see location in Figure 3.13 ). a) Result of GPR image of profile P2 obtained with 250 MHz. b) Result of GPR image of profile P2 obtained with 500 MHz antenna. The blue symbols indicate the initial position of the measurements, yellow arrows are pointing underground high reflected GPR anomalies.

Figure 3 .

 3 Figure3.17 shows the location map of the two profiles performed during the trench survey in August 2013. The satellite images of these zones S6 and S7 show very distinct traces of the surface rupture (indicated by red symbols in Figure3.17). However, in the resulting images of these two profiles with a RTA 50 MHz unshielded antenna, the traces of these fractures are not strongly reflected, but are clearly recognizable (see Figure3.18). Additionally, in the first 20 meters of the profile P1 we remark a very strong reflection. It is likely that there are traces of any sub-fractured structure at the beginning of the profile (Figure3.18a).

Figure 3 .

 3 Figure 3.15 Location Map of the GPR profiles acquired on 14 June 2013. The GPR survey zone S4 as shown in Figure 3.2. The green lines show the GPR profiles of 14 June 2014, the blue symbols indicate the initial position of the measurements, red symbols (F) indicate the intersection of the fault traces on the surface with the profile.

Figure 3 .

 3 Figure 3.16 The result of GPR images of the profile P4 and crossed profile Cr3 (acquired in June 2013) obtained with 250 MHz antenna in the Songino fault study area S4 (see location inFigure 3.15). a) Result of GPR image of the profile P4. b) Result of GPR image of the crossed profile Cr3. The blue symbol indicates the initial position of the measurement, red symbols (F) indicate the intersection of the fault traces on the surface with the profile. The yellow arrows are pointing underground high reflected GPR anomalies.

Figure 3 .

 3 Figure 3.17 Location Map of the GPR profiles acquired in August 2013. a) GPR profile P1 of the survey area S6 shown in Figure 3.2. b) GPR profile P2 of the survey area S7 shown in Figure 3.2. The red lines show the GPR profiles, the blue symbol indicates the initial position of the measurement, red symbols (F) indicate the intersection of the fault traces on the surface rupture with the profiles.

Figure 3 .

 3 Figure 3.18 The results of GPR images of the profiles P1 and P2 (acquired in August 2013) obtained with RTA 50 MHz antenna in the Songino fault study areas S6 and S7 (see Figure 3.17). a.) GPR image of the profile P1, S6 zone (Figure 3.17a). b) GPR image of the profile P2, S7 zone (Figure 3.17b). The blue symbols indicate the initial position of the measurements, red symbols (F) indicate the intersection of the fault traces on the surface with the profiles. The yellow arrows are pointing underground high reflected GPR anomalies.

Figure 3 .

 3 Figure 3.19 Location Map of the GPR profiles acquired in August 2017, S1 zone. a) The GPR survey zone S1as shown in Figure 3.2. b) A magnified map of the profile area (orange square), yellow lines indicate the GPR profiles crossing the paleo river channel. The red lines show the fault traces, the yellow symbols indicate the initial position of the measurements, red symbols (F) indicate the intersection of the fault traces on the surface rupture with the profile.

Figure 3 .

 3 Figure 3.20 The results of GPR images of the profiles P1, P3 and P7 (acquired in August 2017) obtained with 500 MHz antenna in the Songino fault study area S1 (see Figure 3.19) of the paleo river channel. a) GPR image of the profile P1. b) GPR image of the profile P3, c) GPR image of the profile P7. The red symbols (C) indicate the center points of the paleo river channel.

Figure 3 .

 3 Figure 3.23 The result of GPR image of the profile P1of 3D6 zone (acquired the August 2017) obtained with 250MHz antenna in the Songino fault study area S2 (see location in Figure 3.21). a) GPR image of the profile P7, the blue symbol indicates the initial position of the measurement. b) Interpretation of GPR image of the profile P7, the white lines indicate fracture zones, the black arrows show the movement direction, the height of the scarp is approximately 8.4 meters in this area.

Figure 3 .

 3 Figure 3.24 The result of GPR image of the profile P1 of 3D6 zone (acquired in August 2017) obtained with 250 MHz antenna in the Songino fault thrust structured study area S2 (see location in Figure 3.21). a.) GPR image of the profile P1, the blue symbol indicates the initial position of the measurement. b) Interpretation of GPR image of the profile P1. The GPR image shows a clear thrust structure with several ramp structure under scarp (white curve) and folding effects (blue arrows), the black arrows show the movement direction, the height of the scarp is approximately 2.1 meters in this area.

  scheme, issued by the Mineral Resources Authority of Mongolia, employs seven different categories of Mongolian terranes: passive continental margins, turbidite basins, active continental margins, accretionary wedges, island arcs, ophiolites, and seamounts as shown on Figure4.2. These two terrane zonation schemes reveal that the boundaries, orientations, positions, and categorization of these terranes can be interpreted differently. Thus, the interpretations of these zones, especially the cratonal zones and sedimentary basins, are the subjects of intense debate as they have important implications for constraining Mongolia's geodynamic history.

Figure 4 . 1

 41 Figure 4.1 Simplified terrane map of Mongolia. Modified from[START_REF] Badarch | A new terrane subdivision for Mongolia; implications for the Phanerozoic crustal growth of Central Asia[END_REF]

Figure 4 . 2

 42 Figure 4.2 Tectonic zonation map of Mongolia. Modified from (Mineral Resources Authority of Mongolia, 1998).

Figure 4 . 3

 43 Figure 4.3 Schematic map of the Ulaanbaatar Terrane in the Hangay-Hentey zone of the Devonian Accretionary Complex. Distribution of stratigraphic groups with the Ulaanbaatar terrane.[START_REF] Dorjsuren | Geological settings of the Ulaanbaatar terrane in the Hangay-Hentey zone of the Devonian accretionary complex, Central Asian orogenic belt[END_REF].

  2013 in zone S5 by a research team from the University of Strasbourg, France, and the Institute of Astronomy and Geophysics, marking a trenching line. However, during the excavation of the trench in August 2013, a research team from the Institute of Astronomy and Geophysics was working with a research team from the Institute of the Earth's Crust, Siberian branch of the Russian Academy of Sciences on the Mungunmorit active fault. The trench was excavated at a distance of about 100 m to the east of our planned profile P2 (acquired the 14 June 2013), with a deviation of about 60⁰, not perpendicular (90⁰) to the fault trace line, it is located (47°47'31.06"N, 106°29'11.

  Figure 4.5a, b). The bedrock disappears from the south to the north, and the accumulated layers of clay and earth also thicken and accumulate from the south to north. If we look at the stratification of the layers of these sediments, the oldest layer, the dark blue layer, is at the bottom, on the bedrock, and the youngest layer, the brown soil, is on the surface. According to the results of the dating samples taken from the two locations of the brown soil layer at the top of the trenches, the age of the veins entering the bottom of the soil is 5770 ± 40 (Figure 4.5b). However, the age of the sample taken from the case close to the surface of the accumulated thick part of the brown soil is 775 ± 25, which shows that it is relatively young (Figure 4.5b).

Figure 4 . 5

 45 Figure 4.5 Comparison of the paleo-seismological trench photo mosaic and interpreted logging image of the Songino fault tensional structure, The trench(white square) location shown in Figure 3.13. a) Photo mosaic of the western wall of trench T-1 of the Songino fault. b)interpreted layers logging image and dating results, five main different units were identified in the trench shown here by brown, orange, yellow, blue, and dark blue colours. The brown is the soil layer (youngest), the orange and yellow are coarse and fine deposits unit, respectively, the blue is the clay layer (oldest) and dark blue is bedrock, whereas the grey one is a small cave.

Figure 4 .

 4 Figure 4.6 shows a model for the interpretation of the T1 trench sediment accumulation of the Songino fault normal structure zone S5. The depression formed after the first earthquake

Figure 4 . 6

 46 Figure 4.6 The paleo-seismological trench interpretation of sediment layered structure. The dark blue is 1 st and oldest sediment and mud layer, the light blue is 2 nd sediment layer and the orange one 3 rd and youngest sediment layer. The trench photo mosaic in Figure 4.5.

Figure 4

 4 Figure 4.7 OSL dating result of west wall of the T2 trench, excavated in 2010, sampled in 2011, Emeelt active fault. The T2 trench location shown in Figure 2.33. a.) photo of sampling strategy from west wall of T2 trenching site, b.) interpreted image of sampling strategy for the dating.

Figure 4

 4 Figure 4.8 OSL dating result of east wall of the trench, excavated in 2011, sampled in 2011, Khustai active fault. a.) photo of sampling strategy from east wall of Khustai trenching site, b.) interpreted image of sampling strategy for the dating and fault movement.

  moves forward. The next type of structure is a left-lateral strike slip structure that moves in a horizontal direction. You can see that this strike slip is moving in an almost straight line. There are almost no fault scarp in this horizontal transition area of the left lateral strike slip, and it is not possible to identify the fault zone and traces on the ground surface. The left extensional zone, on the other hand, is tilted to the east at an angle of about 20 ° from the strike slip structure and is gradually curved. The scarp of this normal structure also have a gradually increasing scarp structure from the strike slip structure to the east end. In Figure 4.10a shows the displacement of three earthquakes on the Songino fault, and the location of the first trace of the crack is shown in red. The blue and yellow colors show how the traces of the fault change and move during the next two earthquakes. The black arrow indicates the general direction of the Songino fault, and the three different colored curves and the distance between them indicate the amount of movement in which part of the Songino fault. In Figure 4.10b shows how the elongation direction of the Songino fault changes by how many degrees, and shows which deformation effects occur in which part.

Figure 4 . 9

 49 Figure 4.9 The GPR result image of the profile P2 and crossed profile Cr1 (acquired the 14 June 2013) obtained with 250 MHz antenna in the Songino fault main study area S5 (Figure 3.10). a.) result image of the profile P2, the GPR result image shows a clear normal structure and regional extensional effect with competent layer structure under scarp (red arrows). b.) result image of the crossed profile Cr1, red arrow pointed underground high reflected GPR anomaly. The blue symbol indicates the initial position of the measurement, red symbol (F) indicates the point of intersection with the traces of damaged structure on the surface rupture and the profile. The black arrows indicate movement direction of the two side.

Figure 4 .

 4 Figure 4.10 The Songino active fault modelling. a. red line indicates the 1 st trace location of the Songino fault, Blue line represent 2 nd trace location after 1 st seismic event, Yellow one is 3 rd trace location after 2 nd seismic event. b. four different direction fault trace bearing angle and fault interpretation.

Figure 4 .

 4 Figure 4.11 Paleo River channel displacement on the S5 zone of the Songino active fault. Blue line pointed paleo river channel axis line and displacement is approximately 33m. Black arrow shown fault movement direction and red arrows shown Songino fault trace on the ground surface. The angle of the axis line of the paleo river channel to fault trace line direction, it is 105.6º.

Figure 4 .

 4 Figure 4.13 shows the structure of the effects of sedimentation due to the strike-slip fault and the surrounding fault structure. In this example, the effects on right-lateral strike-slip fault accumulation are shown to be opposite to the Songino fault structure, but the fault line structure and formation effects are the same. From this, it is clear that the structure of the fault caused by the Songino fault on the west side compressional zone is push-up oblique thrust structure and the structure formed on the left side extensional zone has pull-apart oblique normal structure.

Figure 4 .

 4 Figure 4.12 Songino active fault movement modeling. Blue dashed line is axis line of strike slip direction. Black lines shown first and before earthquake location of the Songino fault trace. Red lines shown last and after earthquake location of the Songino fault trace. Thrust dextral structure in the compressional zone and normal sinistral structure in the tensional zone. Brown area (from red line to black line) is indicated compressional sediment zone and yellow area (from red line to black) is indicate tensional sediment zone.

Figure 4 .

 4 Figure 4.13 Reconstruction of the paleo-river channel movement on the left lateral strike-slip zone. Cumulative left lateral strike slip offset is approximately 29meter. Google Earth 2009.

Figure 4 .

 4 Figure 4.14 Strike slip faulting and sedimentation. This is dextral strike-slip fault example. Left side extensional zone with pull-apart oblique normal structure, Right side compressional zone

Figure 4 .

 4 Figure 4.16 Development of a thrust structured scarp. Ramp1 is older and ramp3 is younger

  point targets or object, 2. line targets or cylindrical structure, and 3. planar targets. However, the surface of faults, fractures, and geological sediments forms a planar structure, so it is very well reflected in GPR waves Figure5.1. In determining the location of the fault and fracture plane, its dipping angle and the angle of the accumulated sedimentary layer are very important basements.

Figure 5

 5 Figure 5.1 GPR target description (GEOSCANNERS, 2021).

Figure 5 . 2

 52 Figure 5.2 The facies used in the interpretation of the 250MHz GPR result images on the high angle normal structure zone. Clear strongly horizontal reflection layer, it is almost parallel to the surface topography. A strongly attenuated signal and don't have any reflection in the high angle vertical structure of the high angle normal structure (black circle).In the case of low dipping angle thrust structures, both the plane of the fault, fracture and the accumulation of ambient sediments are easier to detect and imaging because they provide better reflection anomaly on GPR waves. In such structure, the thrust structure and displacement of the sediment layers differ significantly for the GPR signal facies. It is also easy to identify the fault zone because the phase shifts of the GPR waves are clearly visible at the displacement of the sediment layers (Figure5.4).

Figure 5 . 3

 53 Figure 5.3 The facies used in the interpretation of the 250MHz GPR result images on the high angle fault structure. a) First example of the interpretation, 1. Clear horizontal reflection layer and chaotic background, 2. Clear horizontal reflection layer and strongly attenuated signals. b) Second example of the interpretation, upside of 1. And 2. Chaotic background with no layer continuity, downside of 1. And 2. Clear horizontal reflection layer and strongly attenuated signals.The phase shift of the GPR signal in the fault and fracture plane environment and the location of the movement and displacements of the layers of sedimentary deposits were used to

Figure 5 . 4

 54 Figure 5.4 The facies used in the interpretation of the 250MHz GPR result images on the low angle thrust fault structure. a) First example of the interpretation, 1. Clear horizontal reflection layers and phases changed, 2. Clear horizontal reflection diverged layers. b) Second example of the interpretation, 1. And 2. Clear horizontal and low angle reflection layers and strongly attenuated signals.According to the results of a detailed study of Songino fault trenches conducted in August 2013, the accumulated form of sediment accumulation is stacked in from south to north direction, and each layer has a thickened structure in from south to north direction. In many respects, the structure of this part of the Songino fault indicates a normal structure, such as the fact that the younger layers have the structure of the previous older layers leaning from front to back. According to the latest analysis of the accumulated brown soil, the age of the sample taken at a depth of 1.2 m above the ground, where the three old layers were cut, is 5770 ± 40 BP, which may indicate the time of the last activation of the Songino fault. However, the age of the samples taken from the 0.4 m depth of the topsoil brown soil is 775 ± 25 BP, which indicates a much later soil accumulation. The accumulation of Songino fault sediments has four main types, and it would be helpful to determine the relationship between the frequency of earthquakes in the fault if samples were taken from each layer and analyzed for their age.

Figure 5 . 5

 55 Figure 5.5 Structural evolution of the Songino fault area. a) Past first location, b) Past second location, c) Past third location, d) Present location

  Pseudo-3D profiles, recorded with a 500 MHz antenna over a paleo-channel crossing the Emeelt fault, provided information about the lateral displacement of 2 m caused by the fault and active fault trace locations. The 50 MHz GPR profiles show a direct reflection, coming from the fault plane, giving access to the location, the dip angle and direction of the fault. It is clear gives from these examples that different frequency antenna of the GPR images provide information on different geological structures. In contrast, the linearity of the actual seismicity indicates a near vertical fault plane examples in GPR result of the Avdar, Bayankhongor and Emeelt active faults. To clarify that point, we need to investigate more in depth the active structure.

  Actives en 2D et 3D par la technique d'Imagerie géoradar et Interprétations des Résultats de cette thèse de doctorat est de déterminer et d'expliquer les caractéristiques et les paramètres des failles actives à l'aide de technologies de pointe modernes, telles que les techniques d'imagerie 2D et 3D Cette recherche a été réalisée à l'aide d'antennes 500, 25 et RTA 50 MHz 2D et 3D ont été combinées avec les résultats d'analyse d'images satellites, de levés géologiques quaternaires et de sondages paléosismologiques de tranchées pour la datation La structure des failles actives avec effet de compression est montrée par les Mungunmorit, la structure de l'effet d'extension est présentée par la faille active de Bayankhongor, et la structure de l'effet de décrochement est interprétée par les exemples des failles actives d'Avdar et d'Emeelt Songino est située entre les failles paléosismiques de Hustai et d'Ulaanbaatar. Une étude GPR détaillée de cette faille a identifié trois types différents de structures et de Le mouvement principal de la faille de Songino est un décrochement ouest, et ce mouvement s'est avéré être causé par la compression entre les Son chevauchement (compression) et sa structure normale (extension) des deux côtés sont dues à la structure en courbure de cette faille. éo chenaux montrent que la faille Emeelt date, 900 ans et la faille Songino environ de 6 000 ans pour le : Géoradar, faille active, Mongolie, paléoséismologie

  faults, Mongolia, paleoseismology Caractérisation des Failles Actives en 2D et 3D par la technique d'Imagerie géoradar et de cette thèse de doctorat est de déterminer et d'expliquer les caractéristiques et les paramètres des failles actives à l'aide de technologies de pointe modernes, telles que les techniques d'imagerie 2D et 3D et RTA 50 MHz. Les techniques 2D et 3D ont été combinées avec les résultats d'analyse d'images satellites, de levés géologiques quaternaires et de sondages paléosismologiques de tranchées pour la datation. failles actives de Mogod et est présentée par la faille active de Bayankhongor, et la nterprétée par les exemples des failles actives d'Avdar et d'Emeelt. Hustai et d'Emeelt situées à l'ouest fférents de structures et de Le mouvement principal de la faille de Songino est un décrochement ouest, et ce mouvement s'est avéré être causé par la compression entre les chevauchement (compression) et sa structure normale (extension)

  Accomplissement avec succès des travaux prévus dans le cadre de l'objectif du travail d'étude de thèse de doctorat "Caractérisation des failles actives par technique d'imagerie radar à pénétration de sol 2D et 3D et interprétation des résultats". De plus, trois antennes de fréquences différentes ont été utilisées pour les principaux types de failles actives conformément à la tâche de recherche planifiée dès le départ. Et des mesures ont été effectuées afin de déterminer la corrélation des résultats d'imagerie. Les principes de mesure géoradar ont été développés et testés sur plusieurs types de failles actives. Nous avons pu identifier et imager différents types de structures et les résultats obtenus sont pertinents.Des informations détaillées sur les mesures GPR et l'imagerie de la faille de Songino, la principale zone de recherche, ont été compilées et interprétées, et des explications paléosismologique et géologie structure de la faille ont été faites, et les réponses suivantes ont été

répondues. Ceux-ci inclus : La faille Songino n'est pas une faille auto-activant et se déplace uniquement sous l'influence de la faille de Khustai, elle est contrôlée par les failles actives de Khustai et d'Emeelt. L'environnement géodynamique de la faille de Songino est dû au déplacement horizontal et au mouvement de rotation du bloc frontal de la faille de Khustai vers la faille d'Emeelt. La structure arrondie de la faille de Songino crée trois types de structures différente lors du mouvement de décrochement horizontal. Le mouvement principal de la faille de Songino est un décrochement latéral dextre, et son chevauchement (compression) et sa structure normale (extension) des deux côtés sont dues à la structure en courbure de cette faille.

  Хүснэгт 1. 1900 -аас 2018 он хүртэлх газар хөдлөлтийн момент магнитудийн зэрэглэл. Хүснэгтийг USGS-ийн 1900-2018 онуудад бүртгэгдсэн 8.0 ба түүнээс дээш момент магнитудтай хүчтэй газар хөдлөлтийн мэдээллээр үүсгэсэн болно.

	Rank Number	of	Location name	Magnitude
		quakes
	14.	1		Chile	9.5
	15.	1		Prince William Sound, Alaska, USA	9.2
	16.	1		Indian Ocean, off west coast of northern Sumatra, Indonesia	9.1
	17.	2		Tohoku, off east coast of Honshu, Japan; Kamchatka, Russia, USSR	9.0
	18.	2		Ecuador-Colombia; Maule, Chile	8.8
	19.	1		Rat Islands, Alaska, USA	8.7
	20.	4		Assam-Tibet, China; Northern Sumatra	8.6
	21.	5		Kamchatka, Russia; Banda Sea, Dutch East Indies (Indonesia); Kuril Islands, Russia;	8.5
				Southern Sumatra
	22.	4		Bulnai, Mongolia; Sanriku, Japan; Tonga; Near coast of southern Peru	8.4
	23.	9		Mindanao, Philippines; Kuril Islands, Russia; South of Sumbawa, Indonesia; Hokkaido,	8.3
				Japan; Sea of Okhotsk
	24.	19		Panay, Philippines; Kuril Islands, Russia; Kamchatka, USSR; Irian Jaya region, Indonesia	8.2
	25.	20		Gobi-Altai, Mongolia; Ryukyu Islands, Japan; Bihar, India; Nankaido, Japan; Hokkaido,	8.1
				Japan
	26.	26		Taiwan region; Afghanistan; Pakistan; Mindanao, Philippines; New Guinea; Sea of Okhotsk;	8.0
				Kurile Islands; Loyalty Islands
	Судалгааны асуулт
		Монгол улсын нийслэл Улаанбаатар хотын баруун талд орших Хустай болон
	Эмээлт хоёр том идэвхитэй хагарлын дунд орших Сонгино хагарал нь:
		Бие даасан идэвхитэй хагарал мөн үү?
		Ямар геодинамикийн орчинд яагаад үүсэв?
		Яагаад өөр өөр харилцан адилгүй мөргөцөгүүдтэй байгаа вэ?
		Ямар төрлийн хагарал вэ?
		Цаашид хэрхэн яаж, хэзээ хөдлөх магадлалтай вэ?
		Сонгино хагарлын хөдөлгөөн нь дээрхи хоёр том хагарлын шилжилт
	хөдөлгөөнтэй хэрхэн яаж уялдаж байна вэ? зэрэг асуултуудад энэхүү судалгааны ажлын
	хүрээнд хариулах шаардлагатай болно.

  Нэгдүгээр бүлэг нь Георадарын тандан судалгааны арга зүй, мэдээлэл боловсруулалт нэртэйгээр Георадарын тандан судалгааны онол, арга зүйн үндэс болон хэрэглээний талаар оруулж өгсөн. Мөн георадарын хээрийн хэмжлтийн арга зүй, мэдээлэл боловсруулалт, 2 болон 3 хэмжээст хэмжилт, зураглалын ажлын үндэс, зарчим, үр дүнгийн тайллын талаар дэлгэрэнгүй тусгаж өгсөн.Хоёрдугаар бүлэг нь идэвхтэй хагарлын бүтцийн судалгаан дээр Георадарын зураглалын арга техникийг ашиглах, хөгжүүлэх нэртэйгээр Георадарын тандан судалгааны арга зүйг палеосейсмологийн судалгаанд хэрхэн үр дүнтэйгээр ашиглах талаар оруулж хагарлын бүх төрлүүд дээр хийгдсэн хэмжилт зураглалын үр дүнгүүдийг тайлал болон палео-тренчийн судалгааны үр дүнгүүдийн хамтаар оруулж өгсөн. Мөн 3 хэмжээст зураглалын арга зүйг Эмээлтийн хагарлын судалгааны жишээн дээр оруулж тайлбарласан.Гуравдугаар бүлэг нь Монгол улсын нийслэл Улаанбаатар хотын Сонгино хагарлын палеосейсмологийн шинж чанрыг Георадарын зураглалын арга техник ашиглан тодорхойлох нэртэйгээр уг хагарлын сунлын дагуух бүтцүүдийг нарийвчлан зураглаж, гарсан үр дүнгүүдийг нэг бүрчлэн тайлал хийсэн бөгөөд энэхүү судалгааны ажлын талаар мэдээллүүдийг дэлгэрэнгүй оруулсан болно.Дөрөвдүгээр бүлэг нь Монгол улсын нийслэл Улаанбаатар хотын Сонгино идэвхтэй хагарлын палеосейсмологийн шинж чанарын геологийн нөхцөл нэртэйгээр Сонгино хагарлын Георадарын судалгааны үр дүн тайллаар тодорхойлогдсон бүтцүүдийн үүсэх шалтгаан, геологийн орчин нөхцөл болон Хустай, Эмээлт хагарлуудтай хэрхэн уялдах талаар оруулсан. Мөн Хустай, Эмээлт болон Сонгино хагарлууд дээр хийгдсэн нас тогтоох ажлын үр дүнгүүдийг оруулж тайлбарласан байгаа болно.Тавдугаар бүлэг нь ерөнхий хэлэлцүүлэг, дүгнэлт гэсэн нэртэйгээр Сонгино хагарлын судалгааны үр дүнгүүдийн тайлал, дэвшүүлсэн таамаглалын талаар орсон ба энэхүү Георадарын хоёр болон гурван хэмжээст дүрслэлийн арга техникээр идэвхтэй хагарлын шинж чанрыг тодорхойлж тайлбарлах нь сэдэвт докторын нэг сэдэвт судалгааны бүтээлийн нэгдсэн дүгнэлтүүдийг оруулж өгсөн.

  Идэвхитэй хагарлын үндсэн төрлүүд дээрхи өөр өөр бүтцүүдийг хэрхэн тодорхойлох, зураглах арга зүй, хэмжилтийн зарчмийг боловсруулж түүнийхээ дагуу хэд хэдэн идэвхитэй хагарлууд дээр судалгаа хийж зохих үр дүнгүүдийг гаргасан.Георадарын хэмжилтийн мэдээлэл боловсруулах RadLab программ хангамжын зарим модулиудыг MATLAB программын хэл дээр бичиж туршин зүгшрүүлсэн.Сонгино хагарал нь бие даасан идэвхижил бүхий хагарал биш бөгөөд зөвхөн Хустайн хагарлын нөлөөн дор хөдөлдөг ба Хустайн хагарал болон Эмээлтийн хагарлуудын дунд хянагддаг хагарал юм.Сонгино хагарлын геодинамик орчин нь Хустайн зүүн хойд булан нь хагарлын зүүн гарын хэвтээ шилжилт болон Хустайн урд талын блоны эргэлтийн хөдөлгөөнөөр Эмээлтийн хагаралд шахагдан гэдрэг сөрж эвдэрсэн бүтцээс үүсэлтэй бөгөөд хөдөлгөөний чиглэл нь Хустайн хагарлын шилжилтийн эсрэг байна.Сонгино хагарлын дугуйрсан бүтэц нь хэвтээ шилжилт хөдөлгөөн хийх явцдаа 3 төрлийн өөр бүтэц үүсгэж байгаа болно.Сонгино хагарлын үндсэн хөдөлгөөн нь зүүн гарын хэвтээ шилжилттэй хагарал бөгөөд түүний хоёр талын тохрол болон зөрөл бүтэц нь хагарлын сунлын муруй бүтцээс хамаарч үүсэж байгаа юм.

	Судалгааны гол талбар болох Сонгино хагарлын нарийвчилсан хэмжилт,
	зураглалын мэдээлэл бүрдүүлж тайлал хийснээр уг хагарлын талаархи палеосейсмологи
	болон структур геологийн тайлбар хийж дараахи судалгааны асуултууддаа хариулт авсан
	болно. Үүнд:
	Сонгино хагарал нь Хустай хагарлаас шууд хамааралтай бүтэц учраас дахин
	хөдлөх хөдөлгөөн нь Хустайн хөдөлгөөнтэй хамааралтай болно.
	Сонгино хагарлын хөдөлгөөн зүүн гарын шилжилттэй гэдэг нь илт харагдаж
	байгаач уг хэсгийн бичил хавтан нь өөрөө хөдлөхгүй бөгөөд Хустайн хавтан нь зүүн
	тийш хөдөлснөөр Сонгино хагарлын хавтанг шахаж шилжилт хөдөлгөөн үүсгэж байгаа
	юм.

  The two strongest Mongolian's earthquakes (indicated in red) are compared in magnitude with other major earthquakes (USGS sources). ................................... 47 Figure 0.4 Block motion model. Thick gray dashed lines are the block boundaries. Red fan shapes and hollow arrows denote block rotation rates and translation rates with respect to the Eurasian plate, respectively. Red thin vector pairs denote principal strain rates. Lu, Sato, Liu, Zeng, & Feng, 2010). ................................................................................................. 51 De-wow processing. ...................................................................... 71 Figure 1.11 Flat reflection filter processing. a) Example of GPR profile with Flat reflection filter processing; b) GPR profile without Flat reflection filter processing................. 72 Figure 1.12 Amplification processing. a) Example of GPR profile before Amplification processing; b) GPR profile after Amplification processing. ...................................... 73 the maximum of the energy gives the value of the velocity for the given time. ............................................................................................................................ 75 Figure 1.16 Radar velocity analysis by diffraction hyperbola fitting. After (Cassidy N. , 2009). .................................................................................................................................... 75 Figure 1.17 AGC processing. a) Example of GPR profile before AGC processing; b) GPR profile after AGC processing. .................................................................................... 76 Figure 1.18 Hyperbola analyze. Green line is surface reflected air wave hyperbola, velocity is 0.3 m/ns. ..................................................................................................................... 76

	Figure 1.1 Path of propagation of electromagnetic waves in the ground in the case of a two-layer medium with contrasting dielectric permittivity (𝜀𝜀1 and 𝜀𝜀2), after (Sperl, 1999). .................................................................................................................................... 58 Figure 1.2 Signal received by the receiving antenna and the graphic profile display (radar section), after (Benson, Glaccum, & Noel, 1984). ..................................................... 59 Figure 1.3 Ground penetrating radar resolution is divided into two components, depth resolution and lateral resolution, after (Annan, 2009)................................................................. 59 Figure 1.4 RTK and DGPS/GNSS. Source: after (Jan, 2015) .................................................. 63 Figure 1.5 The different components of a GPR system. Image Source: (Canada: Geoscan) .. 64 Figure 1.6 The different acquisition modes typical of ground penetrating radar, with constant offset at the top, in the CMP mode at the bottom. On the right, an example of the typical image obtained for each of the modes with the different types of arrivals indicated (Figure 1.1) after (Bano, 2000), (Steelman & Endres, 2011). .................... 66 Figure 1.7 GPR data processing diagram. Lighter orange is filter and processing step, lighter blue is data in, out and analyzing step. ....................................................................... 68 Figure 1.8 a) Example of raw GPR profile before processing; b) GPR profile after time-zero correction. ................................................................................................................... 69 Figure 1.9 Effect of de-wow filter. After (Cassidy N. J., 2009). .............................................. 70 Figure 1.10 De-wow processing. a) Example of GPR profile before De-wow processing; b) processing; b) GPR profile after frequency processing.............................................. 74 Figure 1.14 The effect of an AGC function applied to a radar trace. See (Inc., 2001), (Inc, 2020) .................................................................................................................................... 74 Figure 1.15 Velocity analysis by calculating the semblance over reflections hyperbolas (Yilmaz O. , 1987) Figure 2.1 Conceptual models of succession of coseismic shifts. Different paleo seismological studies illustrate each of these models. a) the variable slip model describes earthquakes of variable magnitudes occurring randomly on the fault, b) the uniform slip model, GPR profile after Figure 1.13 Frequency filter processing. a) Example of GPR profile before frequency after

Black arrows indicate motion directions of the Indian plate and Tibetan plateau (Wang & Shen, 2020)............................................................................................................. 48 Figure 0.5 One century of seismicity in Mongolia (1900-2000), RCAG and DASE team. ..... 49 Figure 0.6 Characterization of the Deren fault by GPR. a) GPR result image with trench observations, b) Geological model interpreted from GPR result image (

  Slip vectors are shown as large white arrows, after (Bayasgalan & Jackson, A re-assessment of the faulting in the 1967 Mogod earthquake in Mongolia, 1999b). ...................................................................................................... 83 . .......................................................................................................... 95 Figure 2.17 GPR image of profile 1 obtained with 250 MHz antenna. a.) The GPR image is obtained without the automatic gain control (AGC). b.) The GPR image is obtained after AGC amplification. ............................................................................................ 97 .......................................................................................................................... 99 Figure 2.20 Seismic activity map around Ulaanbaatar capital city. Emeelt active fault location is white square. Red line is fault trace and red dots represent the seismic events since 2005 (after NDC data, IAG, MAS). ......................................................................... 101Figure 2.21 Location map Emeelt fault 2013 GPR survey. a) Main location GPR profile P6 and fault trace. Green square GPR survey 2010-2011 area. b) Zoomed detailed map of the GPR profile P6. Red line is RTA profile and yellow symbols are fault segments on the surface. ..................................................................................................................... 102 Two red lines are Avdar fault traces. ............................................................................................................... 105 ), the blue symbols are measurement start point. ................................... 109 Figure 2.27 Location map of ground penetrating radar profiles in Z1 zone. The zone location is in Figure 2.24. Light blue lines indicate the profiles at the RTA50 MHz, 500 MHz,and 250 MHz antenna. The blue symbols are indicating the start points and name of profiles. The red symbols are representing fault trace. ............................................ 110 Yellow lines indicate the profiles obtained with RTA 50 MHz and 250 MHz antenna. The blue symbols are indicating the start points and name of profiles. The red symbols (F) are representing surface rupture locations. ........................................... 113 Figure 2.32 Result of GPR image of the profile P2 (acquired in August 2016) obtained with RTA 50 MHz antennae in the Avdar active fault. The GPR image shows a clear underground structure (red arrows) and surface rupture locations are shown by red colored symbol (F), the blue symbols indicate the starting points of measurements. .................................................................................................................................. 113 ). ................................................................................................................. 115

	Figure 2.6 The location map of GPR profiles 2017 on the Mogod active fault. a) Main location
	of the studying three different structured zones, b) GPR profiles and trench location of the first right lateral strike slip structured zone-1, c) GPR profiles and trench location of the second area, back thrust structured zone-2, d) GPR profiles and trench location of the third area, reverse structured zone-3. ............................................................... 85 Figure 2.7 GPR result image of the Profile-1 (acquired the June 2017) interpretation of the zone 2 (back-thrust structured) of the Mogod active fault (Figure 2.6c). First figure is 250MHz antenna result image, yellow arrows show the reflections from layers. Second figure is 500 MHz antenna result image, blue arrow is same layer, orange line is strong reflected layers, black arrows indicated movement direction. ................................... 87 Figure 2.8 GPR result image of the Profile-3 (acquired the June 2017) interpretation of the zone 2 (back-thrust structured) of the Mogod active fault (Figure 2.6c). First figure is 250MHz antenna result image, yellow arrows show the reflections from layers. Second figure is 500 MHz antenna result image, blue arrow is same layer, orange line is strong reflected layers, black arrows indicated movement direction. ................................... 88 Figure 2.9 GPR result image of the Profile-4 (acquired the June 2017) interpretation of the zone 2 (back-thrust structured) of the Mogod active fault (Figure 2.6c). First figure is 250MHz antenna result image, yellow arrows show the reflections from layers. Second figure is 500 MHz antenna result image, orange line is strong reflected layers. ....... 88 Figure 2.10 West wall logging of the paleo-seismic trench 2. Yellow layer was break up and thrusted structure, after (Laurent Bollinger (CEA), Yann Klinger (IPGP) 2017). ..... 89 Figure 2.11 Photo of the paleo-seismic trench 2 wall. a) West wall photo and movement direction, b) East wall photo movement interpretation, after (Laurent Bollinger (CEA), Yann Klinger (IPGP) 2017). ...................................................................................... 89 Figure 2.12 Location map of GPR profiles on the Mungunmorit active fault. a) Main location of GPR and trench study, red arrow is fault trace. b) GPR profiles and trench location, red line is GPR profile in 2013, green line is GPR profile in 2016, white square is paleo seismological trench, blue symbol is profile name and start point. ........................... 91 Figure 2.13 GPR result zoomed image of the profile P1a (acquired the August 2013) obtained with 250 MHz antenna in the Mungunmorit fault, location show in Figure 2.12. The GPR image shows a clear thrust structure with several layered structure under scarp (yellow, blue, green, and orange arrows) and ramp of the thrust structure. ............... 93 Figure 2.14 Comparison of the paleo-seismological trench logging and result of GPR image profile 1a. a) Section of the west wall of paleo-tranche T-1. After (Smekalin, Chipizubov, & Imaev, 2016), b) Result of GPR image and interpretation of fault dipping and movement, location of profile 1a (Pr1a) shown in Figure 2.12. ............ 93 Figure 2.16 The location map of the study area of Bayankhongor fault. a.) Google Earth satellite image showing the survey area. The blue arrows indicate the fault mark, and the yellow rectangle indicates the GPR survey area. b.) An enlarged drawing of the GPR surveyed area. The red line indicates the GPR measurement profiles, the yellow line indicates the starting points and names of the profiles, and the red and blue points indicate the fault trace markFigure 2.18 Interpretation of GPR RTA 50MHz and 250MHz antenna performance maps performed on the profile P1 (acquired the July 2014) in the Bayankhongor fault. The topographic corrections and depth conversion are performed using a velocity of 0.12 m/ns. a.) RTA 50MHz antenna result image, with a yellow arrow roughly outlining the location of the fault plane. b.) The resulting GPR image from 10 m to 85 m of the profile P1 with a 250MHz antenna. The yellow arrow outlines the fault zone. The GPR image shows a clear low angle normal (listric) structure with lot of layered structure near fault zone c.) The result image of the profile P1 performed with 250 MHz antenna from 72 m to 142 m, and a break in sediment accumulation is observed at around 102 m in this profile P1. The same layers that move in both directions are indicated by arrows of the same color. The yellow lines indicate the boundary between the two broken sections (Nyambayar, Bano, Schlupp, Ulziibat, & Tseedulam, The results of the work determined the type of the active fault by GPR sensing, 2018). ................. 98 Figure 2.19 Low-angled normal fault structure or Listric fault model (University of SYDNEY, 2000). Figure 2.22 Emeelt fault 2010 GPR survey. a) RTA profiles (red lines) with the location of the fault at surface. b) T1 paleo-seismological trench west wall logging (first trench 2009 year). c) 500 MHz profile. d) RTA50MHz profile and black dashed line, surrounded by yellow zone, highlights the fault (after Dujardin, 2014). .................................... 103 Figure 2.23 GPR result image of the profile P6 (acquired the May 2013) obtained with RTA 50 MHz antenna in the Emeelt fault. The GPR image shows a clear strong reflected and dipping angle structure (yellow arrows point soil fractured zone on the surface). First figure is first section of the profile P6 and second one is next section of the profile P6. .................................................................................................................................. 104 Figure 2.24 GPR survey locations map of the Avdar fault. Z1-Z2 are GPR zones surveyed in 2011. Z3 is surveyed in 2012 and Z4-Z5 are surveyed in 2016. Figure 2.25 Location map of ground penetrating radar profiles in Z2 zone. The zone location is shown in Figure 2.24. Light blue lines indicate the profiles at the RTA50 MHz, 500 MH zand 250 MHz antenna. The blue symbols are indicating the start points and name of profiles. The red symbols are representing fault trace. ........................................ 108 Figure 2.26 Result of GPR image of the profile P1 and P2 (acquired the October 2011) obtained with RTA 50 MHz antenna in the Avdar active fault. The GPR result image shows a clear layer structure (yellow arrows), and surface rupture location is red colored RTA 50 MHz antenna in the Avdar mountain fault. The GPR result image shows an underground fault structure (red arrows) and surface rupture location is red colored symbols (F), the blue symbols are measurement start point. ................................... 110 Figure 2.29 Location map of ground penetrating radar profiles in Z1 zone. The zone location is shown in Figure 2.24. Light blue lines indicate the profiles at the RTA50 MHz, 500 MHz, and 250 MHz antenna. The blue symbols are indicating the start points and name of profiles. The red symbols are representing fault trace. ........................................ 111 Figure 2.30 Result of GPR image of the profile P9 (acquired the October 2011) obtained with RTA 50 MHz antenna in the Avdar fault. The GPR image shows a clear fault structure (red arrows) and surface rupture location is red colored symbols (F), two dipping angles approximately of 18.4⁰, yellow arrows are pointing bed rock layer. ............ 112 Figure 2.31 Location Map of GPR profiles in Z4 zone. The zone location is shown in Figure 2.24. Figure 2.33 Result of GPR image of the profile P3 (acquired in October 2011) obtained with RTA 50 MHz antenna in the Avdar mountain fault. The GPR image shows a clear underground structure (red arrows), and surface rupture locations are shown by red colored symbol (F), the blue symbols indicate the starting points of measurements. .................................................................................................................................. 114 Figure 2.34 Location of ground penetrating radar profiles in Z1 and Z2 zones. The main location is in Figure 2.21. a) Red lines indicate the profiles at the RTA50 MHz antenna. The black and white arrows are indicating the surface ruptures. b) Zoomed location map of the trenches (T2, T3 and T4) and GPR 3D cubes (C1, C2, C3 and C4), blue line is fault trace. c) Zoomed map of the T1 trench and 500MHz GPR profiles, after (Dujardin J.-R. , 2014Figure 2.35. Example of a 500 MHz profile crossing paleo-channels and recorded in 2010 to verify the presence of the paleo-channel in the GPR data before the precise work on "cubes". Its location is displayed in Figure 2.34 (blue line). Black arrows highlight dipping reflections, which are related to the flanks of the channels, after (Dujardin J.-R. , 2014). ................................................................................................................. 116 Figure 2.36 a) Photomosaic of the north wall of T4 trench and (b) northernmost profile of cube symbols (FFigure 2.28 Result of GPR image of the profile P8 (acquired the October 2011) obtained with C1, 2

  . .................................................................................................................................. 119 ........................................................................................................................ 120 Illustration of the evolution of the channel flanks due to the right lateral strike slip. The down left flank is preserved while the down right flank is eroded after the shift. The filling of the paleo-channel fossilizes the paleo-morphology, after (Dujardin J.-R. , 2014). ..................................................... 122 ......................................................................................................................... 131 Figure 3.3 2D GPR data processing flow chart. Blue one is data input and output steps, orange one is GPR data processing steps. Black arrows following with velocity analysis, blue arrows following without velocity analysis.............................................................. 134 . ........................................................................................... 136 Figure 3.6 Location map of the GPR profile P2 acquired in 2012, S4 zone as shown in Figure 3.2. Blue line shows the second profile of 2012, blue symbol indicates the initial position of the measurement, red symbols (F) indicate the intersection of the fault traces on the surface rupture with the profile. .......................................................... 137

	Figure 2.37 Four GPR profiles with the paleo-channel picked in green. The 3D surface (in colors) of the paleo-channel was deduced from the picking on all the profiles of the cube C2, C3 and C4. The red arrow shows the paleo-flow direction. The map on the down-left corner shows the relative position of the profiles, after (Dujardin J.-R. , 2014). Figure 2.38 (a) Interpretation map of Z2 area. The interpolated 3D surface of the channel (after subtraction of its main slope) is superimposed on the satellite image. An offset of 2 m horizontally is observed on the NW flank of the channel (highlighted by the red lines and the black arrow), which is consistent with a right lateral strike-slip. The pink asterisk shows the location of picture (b) in the trench where an evidence of the fault is observed. The closest RTA profile (upper left corner) shows the record and the location of the fault plane in depth. (c) Figure 2.39 Pseudo-3D data cube of the C1 cube obtained with 500 MHz shielded antenna in the paleo river channel of the Emeelt active fault. Inline arrow pointed measurements direction of the GPR profiles parallel to the T3 and T4 trenches (see Figure 2.34 for the locations). Crossline arrow pointed measurement direction of the GPR crossed profiles. ..................................................................................................................... 123 Figure 2.40 Result of depth slices images of the C1 pseudo-3D data cube (acquired the June 2010) obtained with 500 MHz shielded antenna in the paleo river channel of the Emeelt active fault. a) Depth slice 0.48 m below ground surface, b) Depth slice 0.53m below ground surface, blue arrows indicating fault traces. Channel upside is rainwater inlet side and channel downside are rainwater outlet side. .............................................. 124 Figure 3.1 Seismicity maps (red dots) around Ulaanbaatar city from 1994 to 2014, after (Adiya 2016 and NDC data, IAG, MAS). The green asterisk indicates the UB urban area. Red lines indicate the trace of main active faults, after (Ferry et al., 2010, 2012; Al Ashkar 2015). The Hustai northeastern tip is located at 15 km west from UB and is of 80 km long; the Emeelt fault (swarm area) of 40 km long is situated less than 10 km from the westernmost area of UB; the Songino fault located between Emeelt and Hustai faults is about 20 km long and situated 20 km West of UB. .............................................. 130 Figure 3.2 Location map of Songino fault and Ground Penetrating Radar survey areas. The red line shows the traces of the fault on the surface rupture of the Songino fault, the blue squares show the Ground Penetrating Radar measurements, the surveyed areas, and the Ground Penetrating Radarprofiles between 2012 and 2017 difference by color each (see Figure 3.2), b.) a magnified image of the profile area, blue line shows the profile acquired in 2012, blue symbols indicates the initial position of the measurement, red symbols (F) indicates the point of intersection with the traces of damaged structure on the surface rupture and the profile. ........................................................................... 135 Figure 3.5 The result of GPR images of the first profile (acquired the September 2012) obtained with 250 MHz and RTA 50MHz antenna in the Songino fault study area S1 (see Figure 3.4). a.) result of GPR image obtained with 250 MHz antenna, b.) result of GPR image obtained with RTA 50 MHz antenna; blue symbol indicates the initial position of the measurement, red symbol (F) indicates the point of intersection with the fault traces on the surface rupture and the profile, red arrows are pointing underground high reflected GPR anomalyFigure 3.7 The result of GPR images of the second profile (acquired the September 2012) obtained with 250 MHz and RTA 50 MHz antenna in the Songino fault study area S4 (see Figure 3.6). a.) GPR profile obtained with RTA 50 MHz antenna, b.) GPR profile obtained with 250MHz antenna. Blue symbol indicates the initial position of the measurement, red symbols (F)indicate the point of intersection with the traces of damaged structure on the surface rupture and the profile, red arrows are pointing year. .Figure 3.4 Location map of the first profile P1 acquired in 2012. a) Northeast corner of zone S1 underground high reflected GPR anomalies. ........................................................... 138

Table 0 -

 0 1 Rank of the earthquake moment magnitude from 1900 to 2018. Table created by USGS data of the Earthquakes of moment magnitude 8.0 and greater from 1900 to 2018, (USGS sources).

	Rank Number	Location name
	of quakes

Table 1 -

 1 2 -Depth resolutions and lateral resolutions as a function of the center of the antenna,

	Frequency (MHz)	Depth resolution (m)	Lateral resolution (m)
	50	0.5	1
	100	0.25	0.7
	250	0.1	0.45
	400	0.06	0.35
	800	0.03	0.25
	1000	0.025	0.22
	1500	0.017	0.18
	2000	0.013	0.16
	3 Data acquisition

calculated from equations 1.12 and 1.13 for a constant speed in the medium of 0.1 m/ns and a target located at a depth of 1 m. These values are only indicative.

Table 2 -

 2 1 Mogod GPR survey acquisition parameters.

		Site name	Profile name Antenna	Distance	Profile	Time
					interval	distance	window
	1.	Zone 1	Pr01-Pr03	500MHz	0.03 m	90 m	103 ns
				250 MHz	0.03 m	90 m	141 ns
				RTA 50 MHz	0.1 m	190 m	391 ns
						215m
	2.	Zone 2	Pr01-Pr04	500MHz	0.03 m	73 m	107 ns
				250 MHz	0.03 m	73 m	142 ns
				RTA 50 MHz	0.1 m	115 m	391 ns
	3.	Zone 3	Pr01-Pr03	500MHz	0.03 m	70 m	107 ns
				250 MHz	0.03 m	70 m	142 ns
				RTA 50 MHz	0.05 m	70 m	391 ns

Table 2 -

 2 2 Mungunmorit GPR survey acquisition parameters.

	Site name	Profile	Antenna	Distance	Profile	Time
		name		interval	distance	window
	1. 2013 Aug-10 Pr01	250 MHz	0.02 m	230 m	205 ns
			RTA 50 MHz	0.3 m	220 m	501 ns
	2. 2016 Sep	Pr01-Pr11	250 MHz	0.05 m	80 m	130 ns

Table 2 -

 2 3 Bayankhongor profiles and their lengths for different GPR antennas.

	Name	250MHz	RTA50MHz	Start point	End point
	Profile-1	200м	200м	46.405970°	46.406862°,
				100.190633°	100.192840°
	Profile -2 181.5м	181.5м	46.406260°	46.407099°
				100.190611°	100.192647°
	Profile -3 115м	150м	46.407918°	46.408546°
				100.189137°	100.190858°

Table 2 -

 2 4 Acquisition parameters of the Bayankhongor GPR profiles.

		250MHz		RTA50MHz
	Name	Step Interval	Time window	Step Interval	Time window
		(m)	(ns)	(m)	(ns)
	Profile-1	0.04	185	0.2	467.4
	Profile -2	0.04	185	0.2	372
	Profile -3	0.04	185	0.2	372

Table 2 -

 2 5 Emeelt GPR survey acquisition parameters.

	Site name	Profile	Antenna	Distance	Profile	Time
		name		interval	distance	window
	1. 2010 June 27 Pr01-Pr04	RTA50 MHz	0.2 m	200 m	535 ns
	2. 2013 May 30 Pr01-Pr07	RTA50 MHz	0.3 m	900-2000 m	500 ns

  The Institute of Astronomy and Geophysics acquired a new GPR survey instrument of the MALA Geosciences Inc in 2011 as part of a detailed project to study active faults around Ulaanbaatar. The first GPR survey measurements were collected in October and -7). For the first time we have used the 250 MHz shielded antenna in a Avdar mountain fault study (Table2-7).After data processing and verifying the first GPR measurements, we conducted a more detailed geological and geophysical study in the southern part of the Avdar mountain fault in the summer 2012. That study was conducted by a joint team of the University of Strasbourg, France and the Institute of Astronomy and Geophysics. This joint survey included seismic,

	November 2011 at Z1 (47.372177°,106.785050°) and Z2 (47.315682°,106.731789°) (Figure
	2.23). On October 7, 2011, we acquired the GPR profiles 6 to 9, and on October 8, GPR profiles
	1-5, 10, and 11 (Figure 2.24), using RTA 50MHz and 500 MHz shielded antennas, respectively
	(see Table 2magnetic, and GPR survey measurements at Z2 and Z3 (47.311460°, 106.726288°), as well as
	a trench survey.
	However, in 2016 another GPR campaign had been planned and did all the field
	measurements myself. This GPR survey was performed with 50 MHz and 250MHz antennas in
	the northern part of the Avdar mountain fault, Z4 (47.415778°, 106.847601°) and Z5
	(47.439106°, 106.884190°) (see
	Table 2-6), and this doctoral dissertation includes only examples of Z4 profiles (Figure

Table 2 -

 2 6 Avdar 2016 GPR survey acquisition parameters.

	Site	Profile	Antenna	Distance	Profile	Time
	name	name		interval	distance	window
	1. 2016 Z4 Pr01-Pr04	RTA50 MHz	0.05 m	160-180 m	391 ns
	2.		250 MHz	0.05 m	160-180 m	160 ns

Table 2 -

 2 7 Avdar 2011 GPR survey acquisition parameters.

	Site	Profile	Antenna	Distance	Profile	Time
	name	name		interval	distance	window
	1. Z2 Pr01-Pr03	500 MHz	0.05 m	335-361 m	142 ns
	2.		RTA50 MHz	0.2 m	335-361 m	528 ns
	3.		250 MHz	0.05 m	335-361 m	111 ns
	4. Z2 Pr04-Pr05	500 MHz	0.05 m	398-485 m	142 ns
	5.		250 MHz	0.05 m	402-485 m	111 ns
	6. Z1 Pr10	500 MHz	0.05 m	397 m	142 ns
	7.		250 MHz	0.05 m	402 m	111 ns
	8. Z1 Pr11	500 MHz	0.05 m	335 m	142 ns
	9.		250 MHz	0.05 m	352 m	111 ns
	10. Z1 Pr06	500 MHz	0.05 m	470 m	161 ns
	11.		RTA50 MHz	0.2 m	461 m	676 ns
	12.		250 MHz	0.05 m	469 m	111 ns
	13. Z1 Pr07	500 MHz	0.05 m	495 m	161 ns
	14.		RTA50 MHz	0.2 m	491 m	676 ns
	15.		250 MHz	0.05 m	498 m	111 ns
	16. Z1 Pr08	500 MHz	0.05 m	475 m	86 ns
	17.		RTA50 MHz	0.2 m	461 m	676 ns
	18.		250 MHz	0.05 m	484 m	111 ns
	19. Z1 Pr09	500 MHz	0.05 m	511 m	86 ns
	20.		RTA50 MHz	0.2 m	513 m	528 ns
	21.		250 MHz	0.05 m	514 m	111 ns

Table 2 -

 2 8. Emeelt 3D GPR survey acquisition parameters.

		Site name	Profile name Antenna	Distance	Profile	Profile	Time
					interval	distance	interval	window
	1. 2010 July 01	Pr01-Pr107	500 MHz 0.02 m	25 m	0.25 m	82 ns
	2.	(C1 cube)	Cr01-Cr26	500 MHz 0.02 m	26 m	1 m	82 ns

Table 3 -

 3 2 List of costliest 21 st century earthquakes.

Table 3 -

 3 3 Table of the GPR survey zone location parameters

		Zone name	Latitude	Longitude
	1.	S1	47.788014°	106.455834°
	2.	S2	47.816440°	106.445671°
	3.	S3	47.797549°	106.464720°
	4.	S4	47.793126°	106.480566°
	5.	S5	47.792083°	106.485843°
	6.	S6	47.790389°	106.494768°
	7.	S7	47.790684°	106.509810°

Table 3 -

 3 4 Songino fault GPR survey acquisition parameters.

		Site name	Profile	Antenna	Distance	Profile	Time	Zone
			name		interval	distance	window
	1. 2.		Pr01	250 MHz RTA50 MHz 0.2 m 0.06 m	97 m 96 m	ns ns	S1
	3. 4.	2012 Sep-29	Pr02	250 MHz RTA50 MHz 0.2 m 0.06 m	115 m 111 m	ns ns	S5
	5. 6.		Pr03	250 MHz RTA50 MHz 0.2 m 0.06 m	90 m 91 m	ns ns	S4
	7. 8.		Pr01 Pr02	250 MHz 250 MHz	0.02 m 0.05 m	64 m 40 m	ns ns	S5
	9. 10. 11.	2013 June-14	Pr03 Pr04 Pr05	250 MHz 250 MHz 250 MHz	0.05 m 0.05 m 0.05 m	97 m 114 m 221 m	ns ns ns	S4
	12. 13.		Cr01 Cr02	250 MHz 250 MHz	0.05 m 0.05 m	95 m 84 m	ns ns	S5
	14.		Cr03	250 MHz	0.05 m	43 m	ns	S4
	15.		Pr01-Pr03 250 MHz	0.05 m	50-52 m	ns
	16. 17.	2013 June-16	Pr01-Pr03 500 MHz Cr01-Cr02 250 MHz	0.02 m 0.05 m	50-52 m 34 m	ns ns	S5
	18.		Cr01-Cr02 500 MHz	0.02 m	34 m	ns
	19.		Pr01	250 MHz	0.05 m	51 m	ns
	20.	2013 June-17	Cr01	250 MHz	0.05 m	35 m	ns	S5
	21.		Cr01	500 MHz	0.02 m	35 m	ns
	22. 2013 Aug-25 Pr01-Pr02 RTA50 MHz 0.3 m	151-171 m ns	S6, S7
	23. 2013 Aug-26 In-trench-01 24.	250 MHz 500 MHz	0.02 m 0.02 m	40 m 40 m	ns ns	S5
	25. 2017 Aug-08 Pr01-Pr07 500 MHz	0.04 m	50 m	ns	S1
	26. 27. 28. 29.	2017 Aug-15	Pr01-Pr04 Pr05-Pr07	250 MHz RTA50 MHz 0.05 m 0.05 m 250 MHz 0.05 m RTA50 MHz 0.05 m	70-73 m 70-73 m 55-53 m 55-53 m	ns ns ns ns	S2

Figure 3.3 2D GPR data processing flow chart. Blue one is data input and output steps, orange one is GPR data processing steps. Black arrows following with velocity analysis, blue arrows following without velocity analysis.

  125μm) quartz samples was measured by the single-piece regeneration method (SAR)[START_REF] Wintle | A review of quartz optically stimulated luminescence characteristics and their relevance in single-aliquot regeneration dating protocols[END_REF]. In the annual dose rate, the test of the element content of uranium, thorium and potassium was completed in the Analysis and Test Research Center of the Beijing Geological Research Institute of Nuclear Industry. The relevant results are shown in Table 4-1 of the book. The dating results of the optical luminescence samples of the ET and KT grooves are shown in Table 4-2 of the book (refer to the calculation of the relevant annual dose (Prescott

Table 4 -

 4 1 Uranium, thorium, and potassium content in the sediment samples

	Detector name Sample	Th/µg*g -1	K/%	U/ µg*g -1
		number
	ET Probe	ET-1	9.00	3.06	2.02
		ET-2	8.40	3.15	2.13
		ET-3	10.00	2.71	2.27
		ET-4	10.70	2.36	1.97
	KI Probe	KI-1	8.67	1.97	5.07
		KI-2	6.24	1.43	3.46
		KI-3	7.35	2.05	2.45

Table 4 -

 4 2 Basic data and OSL dating results of the samples from Emeelt and Khustai active faults.

	Detector	Sample	Sampling	Equivalent	Moisture	Cosmic	Annual	Optical
	name	number	depth (m)	dose (Gy)	content	rays	dose rate	luminescence
					(%)	(Gy/ka)	(Gy*ka -1 )	age (ka)
	ET Probe ET-1	1.1	40.7±4.0	2.68	0.22688 4.23±0.06 9.6±1.0
		ET-2	1.2	49.9±9.0	1.91	0.22086 4.34±0.04 11.5±2.0
		ET-3	1.35	44.6±5.0	3.94	0.22086 3.96±0.04 11.3±1.3

Il est évident que le mouvement de la faille de Songino est un décrochement latéral dextre, mais la structure de la microplaque dans la région ne bouge pas d'elle-même et la plaque frontale de Khustai se déplace vers l'est, comprimant ainsi la faille de Songino et provoquant le mouvement de l'faille Songino.Alors que d'autres études paléosismologiques n'ont comparé les profils GPR du seul type de faille active étudiée avec les résultats de la tranchée, mes travaux de thèse ont montré en pratique qu'il est possible de déterminer la structure de faille active pour tous les types.

Acknowledgements

List of tables

Migration

The aim of migration is to spatially reconstruct the structures visible in the GPR profiles, focusing the diffraction hyperbolas (coming from the subsoil) into one-point source and repositioning sloped or non-planar reflectors in their actual position. This process, which we applied in 2D, uses the speed of the waves in the medium estimated from the CMP/diffractions analysis. We used the migration of Stoltz (or F-K migration) which is one of the most common methods [START_REF] Stolt | Migration by fourier transform[END_REF]. Migration was only kept in the processing chain when it improves the GPR images. Indeed, intended for seismic signals, migration is not very efficient for GPR signals in a complex and heterogeneous medium due to the differences in wave propagation properties [START_REF] Cassidy | Ground Penetrating Radar Data Processing, Modelling and Analysis[END_REF]. Recently, adapted algorithms to GPR data presenting a strong topography have been developed [START_REF] Lahmann | Topographic migration of georadar data: implications for acquisition and processing[END_REF][START_REF] Streich | Vector-migration of standard copolarized 3D GPR data[END_REF][START_REF] Dujardin | Topographic migration of GPR data: Examples from Chad and Mongolia[END_REF]. The Bayankhongor Fault is part of a 350 km long systemic fault called the Southern Khangai Fault, which extends in a latitudinal direction along the latitude of Bayanbulag soum on the southwest side of the Khangai mountain range [START_REF] Odonbaatar | Earthquake hazard assessment and micro-zoning survey: Bayankhongor soum of the Bayankhongor province center[END_REF]. The

Bayankhongor fault begins about 30 km west of Bayankhongor province center, extends approximately 85 km to the Bor Khairkhan mountain [START_REF] Odonbaatar | Earthquake hazard assessment and micro-zoning survey: Bayankhongor soum of the Bayankhongor province center[END_REF].

This fault has already been studied by foreigner and domestic scientists through satellite images analysis and field route research. The Bayankhongor fault has been described as a thrust [START_REF] Baljinnyam | Ruptures of major Earthquakes and active deformation in Mongolia and its surroundings[END_REF], and a normal fault [START_REF] Cunningham | Cenozoic normal faulting and regional doming in the southern Hangay region, Central Mongolia: implications for the origin of the Baikal rift province[END_REF][START_REF] Gantulga | Late Cenozoic landscape evolution in the Khangay Mountains, Mongolia[END_REF].

We conducted our GPR campaign in collaboration with administration of the Bayankhongor province within the framework of a seismic micro-zoning and mapping project.

The Bayankhongor fault section stretches across the territory of Bayan-Ovoo and Galuut soums of Bayankhongor province along the foothills of the Jargalant mountain range. The study area is located 25 km northwest of Bayan-Ovoo soum and about 1 km from the gold mine stockpile (see Figure 2.15a). A geological study along the Bayankhongor fault shows a right lateral strike slip structure with horizontal displacement of 4-4.5 m and a vertical component of 2-8 m displacement scarps at 3 points. However, the main types and geodynamic conditions of the Bayankhongor fault have not yet been determined [START_REF] Odonbaatar | Earthquake hazard assessment and micro-zoning survey: Bayankhongor soum of the Bayankhongor province center[END_REF]. Along the fault trace, the scarp height is unevenly distributed with an average height of 1.5-3 m, and the high

The dipping angle of the Emeelt fault was determined on the GPR result images and changed from 35⁰ at the south part to 23⁰ at the north side (Figure 2.21). Also, 3 crossed profiles were conducted parallel to the fault plane. We processed the day's GPR measurements in the evening and planned the following day's measurements, and because the results of the profiles conducted on S5 were very interesting, we repeated some of the previous profiles and enlarged them. We were very interested in the results of Ground Penetrating Radar measurements conducted in zone S5 in 2013 (see Figure 3.2), so we decided to study the Songino fault in more detail. After selecting the topic of my doctoral dissertation as Songino faults, I planned to conduct a more extensive GPR study of this fault, and in 2017, I conducted GPR measurements in the previous two zones and five other zones. First, satellite imagery was used to identify areas where horizontal and vertical displacements could be identified, and GPR measurement points were identified accordingly. In the GPR survey design, 3-4 parallel profiles at a distance interval of 1-2 m (Table 3-4) perpendicular to the fault trace were obtained to determine the horizontal displacement of the paleo river channel we collected parallel to the fault trace 6-8 profiles at a distance interval of 1-2 meters from each other (Table 34 the Asralt Khairkhan (Haraa of [START_REF] Badarch | A new terrane subdivision for Mongolia; implications for the Phanerozoic crustal growth of Central Asia[END_REF], Figure 4.1) and Ulaanbaatar terranes; a boundary which strikes northeasterly from the eastern Hangay region into Russia Figure 4.2.

When mentioned as a boundary fault between the juxtaposed terranes, this structure is referred to as the Tuul fault. The geology of the Asralt Khairkhan terrane has been variously interpreted as either a late Devonian backarc/forearc basin [START_REF] Badarch | A new terrane subdivision for Mongolia; implications for the Phanerozoic crustal growth of Central Asia[END_REF] or turbidite/accretionary basin [START_REF] Tomurtogoo | A brief explanatory note to the Tectonic Map of Mongolia at a scale of 1:1,000,000[END_REF][START_REF] Kelty | Detritalzircon geochronology of the Hangay-Hentey basin: implications for the tectonics of the Mongol-Okhotsk Ocean, Mongolia[END_REF]. The Ulaanbaatar terrane is regarded as part of the upper Devonian/ lower Carboniferous Hangay-Hentey turbidite basin that filled prior to closure of the Mongol-Okhotsk Ocean and is considered stratigraphically and lithologically correlative with the Tsetserleg terrane located to the west [START_REF] Tomurtogoo | A brief explanatory note to the Tectonic Map of Mongolia at a scale of 1:1,000,000[END_REF]. Both the Asralt Khairkhan and Ulaanbaatar terranes have been extensively intruded by late Triassic/early Jurassic plutons of the Hentey Batholith (stitch complex) [START_REF] Kravchinsky | Evolution of the Mongol-Okhotsk Ocean as constrained by new palaeomagnetic data from the Mongol-Okhotsk suture zone, Siberia[END_REF][START_REF] Batulzii | Timing, composition, and tectonic setting of igneous complex with and around the Hangay-Hentey flysch basin[END_REF].

During the twentieth century Mongolia experienced at least four 8+ magnitude (M) earthquakes, a fact that effectively highlights the country's geological penchant for very large magnitude earthquakes and overall high seismic activity despite being in an intracontinental setting. The causes of seismicity may be attributable to the Indo-Eurasian collision, uplifted regions possessing high heat-flow (i.e. Hangay dome), mantle plume phenomena, Baikalian extension, Pacific Plate subduction, or a combination of these possibilities [START_REF] Tapponnier | Propagating extrusion tectonics in Asia; new insights from simple experiments with plasticine[END_REF][START_REF] Zorin | Cenozoic upper mantle plumes in east Siberia and central Mongolia and subduction of the Pacific Plate[END_REF]. That Ulaanbaatar is at risk is well demonstrated by the close proximities of the September 24, 1998 Deren Earthquake (magnitude between 5.6-5.3, depending on the reference) and January 9, 2010 aftershock, both epicentered 150-km south of the Khustai fault, all events felt by residents in the city (Figure 4.4) [START_REF] Dugarmaa | Seismic map of Mongolia and site effect microzoning at the capital, Ulaanbaatar[END_REF].

Furthermore, a local seismic monitoring network operable since 1994 has shown the presence of microquake clusters epicentered around Ulaanbaatar [START_REF] Dugarmaa | Seismic map of Mongolia and site effect microzoning at the capital, Ulaanbaatar[END_REF].

during the twentieth century were the Mogod (1968) and Deren (1998) Earthquakes, both .

2 Paleoseismologycal analyze.

The Paleoseismology studies prehistoric earthquakes, their location, time, and magnitude. Seismologists work with instrumental data during earthquakes, while paleoseismologists interpret the geological evidence of paleoearthquakes. Paleoseismology

Emeelt and Khustai trench overview and sample collection

The ET Emeelt trench (47°54'47"N, 106°36'32"E, 1347m above sea level) is located on the Emeelt fault. The details of this section are shown in Figure 4.7. The surface layer of P1 is a deep brown gravel-bearing silt layer with a thickness of about 1.5 m, which is not affected by fault activity. The gravel is poorly sorted, and the maximum particle size can reach 15 cm. P2 is a gray-white gravel layer with a lot of silt sand, and the gravel size is generally 5-10 cm. The formation is thin and partially filled into the fracture. P3 Light yellow silt layer with uniform lithology and no gravel distribution. The stratum was affected by the fault activity and was horizontally broken. Four sampling points are set in the section of the ET trench (Figure 4.7).

The sampling point of sample ET-1 is located at the bottom of the dark brown pebbly silt layer (P1), 1.1m from the surface. Sample ET-2 is a fault interstitial between the light-yellow silt layer (P3) and the overlying dark brown gravelly silt layer (P1), 1.2m from the ground surface.

The sampling points of samples ET-3 and ET-4 are located at the top and middle and lower parts of the dislocated light-yellow silt layer (P3), 1.35m and 1.6m from the surface, respectively.

The KT Khustai trench (47°46'38.7"N, 106°14'45"E, 1248m above sea level) is located on the Khustai fault. The stratum of this section is inclined, as shown in Figure 4.8.

The section of the KT prospecting trough reveals the strata as follows:

L1 gray-black, black loess soil layer, the deposit is a fine-grained filling of the fault ridge, which has a clear boundary with the underlying L2 layer.

L2 is a gray-white gravel-bearing silt layer, the gravel size is generally 5-10cm, and the gravel size reaches 15cm, and the layer is dislocated.

L3 gray-brown silt layer with uniform lithology and no gravel distribution.

L4 reddish paleosol layer, which is relatively thin.

L5 gravel layer.

Three sampling points are set on the KT trench (Figure 4.8). The sampling point of sample KT-1 is located at the bottom of the black loess soil layer (L1), 0.15m from the surface.

Sample KT-2 is in the middle of the gray-white gravelly silt layer (L2) on the hanging wall of the fault, while KT-3 is in the middle of the gray-brown silt layer (L3) in the footwall of the fault.

Chapter 5 General discussions and conclusions 1 General discussion

The objective of this thesis work, and more broadly of the framework of a seismic micro-zoning and mapping project of the Ulaanbaatar capital city, was to develop a new paleoseismological approach based on high-resolution GPR imagery, which would make it possible to find the traces of a larger number of strong past earthquakes. That has been possible to image shallow structure to 2-3 meter and but limited for the deep structure to 5-30 meter until now with existing paleo-seismological methods. That it is possible to do so until now with existing GPR method for deep structure. The corollary objective was to use these traces to document the displacements produced by these past earthquakes, and to use these quantifications to better estimate the seismic hazard relating to the faults analyzed. The approach followed consisted in exploring the near surface. The first 5-30 meters of the ground, using GPR, which is a nondestructive, inexpensive, and rapid implementation. Previous studies had demonstrated the potential of GPR imagery to detect zones of active deformation, in particular fault planes, in the first 10 meters of the ground [START_REF] Smith | Ground penetrating radar : antenna frequencies and maximum probable depths of penetration in Quaternary sediments[END_REF][START_REF] Audru | GPR investigations on active faults in urban areas: the Georisc-NZ project in Wellington, New Zealand[END_REF][START_REF] Gross | Location and geometry of the Wellington Fault (New Zealand) defined by detailed three-dimensional georadar data[END_REF]McClymont, et al., 2009a). In this work, we demonstrated its additional potential to detect morphological markers buried and preserved in the subsurface. Our work is leading to new information on the history of the surface deformation along the Songino, Khustai, Emeelt and other faults, which will help to better constrain seismic hazard estimates for these major faults of the Ulaanbaatar capital city.

GPR has proven its efficiency in many fields of geosciences but seems generally most efficient in sedimentary deposits, marked lithological boundaries, fractures and/or faults [START_REF] Gross | Location and geometry of the Wellington Fault (New Zealand) defined by detailed three-dimensional georadar data[END_REF][START_REF] Deparis | On the potential of ground penetrating radar to help rock fall hazard assessment: a case study of a limestone slab, Gorges de la Bourne (French Alps)[END_REF][START_REF] Mcclymont | Shallow fault segmentation of the Alpine fault zone, New Zealand, revealed from 2and 3-D GPR surveying[END_REF], even for subtle changes in the nature, size, shape, orientation and packing of grains [START_REF] Guillemoteau | Influence of grain size, shape and compaction on georadar waves: examples of aeolian dunes[END_REF].

In the sedimentological context, interpretation generally uses GPR-facies analysis, analogous to their use in seismic studies [START_REF] Jol | GPR in sediments: advice on data collection, basic processing and interpretation, a good practice guide[END_REF][START_REF] Neal | Ground-penetrating radar and its use in sedimentology: principles, problems and progress[END_REF]. Ground Penetrating Radar (GPR) is a relatively recent geophysical method which has found a wide field of applications over the last twenty years [START_REF] Annan | GPR-History, Trends, and Future Developments[END_REF] for a background, and [START_REF] Jol | Ground Penetrating Radar: Theory and Applications[END_REF][START_REF] Beres | Application of Ground-Penetrating-Radar Methods in Hydrogeo-logie Studies[END_REF][START_REF] Dujardin | Imagerie géoradar (GPR) en milieu hétérogène Application aux failles actives en Mongolie et aux dépôts pyroclastiques du Tungurahua (Equateur)[END_REF]Nyambayar, et al., 2018) for a review. description of the main applications).

At the beginning time of this doctoral dissertation, the innovative and advanced method of modern geophysical study, the GPR method, was not widely used in Mongolia. Therefore,

This study is only a detailed study of the main structure and substructures around the Songino fault. Within the scope of this doctoral dissertation work, the main movements of the Songino fault and the auxiliary parts of the sub fracture were identified and modeled.

Advantages and limitations

The paleo seismological GPR approach has some limitations: The first being that it can only be applied to electrically resistive materials. In addition, the acquisition is fast, but the data processing is tedious, mainly because of the separate acquisition of the GPR profiles and their location but also because of certain processing and analysis steps that cannot be fully automated. This concerns the data editing step which requires viewing and verifying each of the profiles.

The second one, pseudo-3D acquisition geometry has some drawbacks, being the inability to correct artifacts from 3D effects and especially out-of-plane reflections. Pseudo-3D acquisition, however, turns out to be a good compromise between acquisition, processing and analysis time, and spatial coverage. The use of ground penetrating radar as an imaging method also provides images of the subsoil with a high resolution compared to other existing methods and avoids the drawbacks of inversion (data smoothing and non-uniqueness of the solution).

The acquisition of several parallel profiles also helps to avoid misinterpretations of the images obtained by distinguishing noise and signal artifacts. While noise and artifacts vary from profile to profile, the signal is coherent. The interpolation between the profiles therefore makes it possible to keep only the signal.

A final limitation is that the GPR approach allows only the co-seismic displacements of past strong earthquakes to be determined, but not their ages. Knowledge of these movements is nevertheless sufficient to estimate the number and magnitudes of the strong events identified.

Good combination is the GPR result with the ages (dating) result found.

General conclusions

At the beginning time of this doctoral dissertation, the innovative and advanced method of modern geophysical study, the GPR method, was not widely used in Mongolia. Therefore, since 2010, we have been working to introduce and localize this survey methodology throughout Mongolia, in many fields. In particular, the aim is to usefully in paleoseismology surveys, and in the future to use a combination of many advanced techniques and technologies to improve the results and quality of surveys. Mots-clés : Géoradar, faille active, Mongolie, paléoséismologie

Appendixes

The purpose of this doctoral dissertation is to determine and expla active faults using modern advanced technology, such as 2D and 3D imaging techniques of GPR. was performed using 500, 250 and RTA combined with the results of satellite image analysis, quaternary geological survey, and palaeoseismological trench survey for dating.

The structure of active faults with compression active faults, the structure of tensional effect is strike slip effect is interpreted by the the 3D GPR imaging are shown by depth slices on the exa The Songino Fault is located between the A detailed GPR study of this fault movements. Songino fault have a basic left lateral strike slip movement from southeast to northwest, and this movement has been found to be caused by compression between the Hustai and Emeelt active faults and its thrust and normal structure on both sides are due The paleochannel dating samples show that the Emeelt fault is on average 10,000 years, the Hustai fault is about 900 years, and the Songino fault is about 6,000 years for last event.