Sofia Rontogianni

Keywords: GPS, Greece, Velocity field, strain, seismic hazard

Comparison of geodetic and seismic strain rates in Greece by using a uniform processing approach to campaign GPS measurements over the interval 1994-2000

A c c e p t e d M a n u s c r i p t

Introduction

The eastern Mediterranean is one of the most tectonically active regions on Earth as demonstrated by its seismicity and has been identified as an ideal natural laboratory for studying the kinematics and dynamics of plate interactions [START_REF] Mckenzie | Active tectonics of the Alpine-Himalayan Belt: the Aegean Sea and surrounding regions[END_REF][START_REF] Mckenzie | Active tectonics of the Mediterranean Region[END_REF]Jackson and McKenzie, 1988;[START_REF] Ambraseys | Seismicity and associated strain of central Greece between 1890 and 1988[END_REF][START_REF] Ambraseys | Seismicity and strain in the Gulf of Corinth (Greece) since 1694[END_REF][START_REF] Plag | Scientific objectives of current and future WEGENER activities[END_REF][START_REF] Ambraseys | Reassessment of earthquakes,1900-1999, in the Eastern Mediterranean and the Middle East[END_REF]. The region is on a convergent plate boundary comprising the subduction of the African Plate under the Eurasian plate, while the Arabian plate approaches the Eurasian plate in a northwestward motion. The latter motion leads the Anatolia plate to be displaced to the west and rotate in a counterclockwise sense relative to Eurasia [START_REF] Reilinger | Global Positioning System measurements of present-day crustal A c c e p t e d M a n u s c r i p t movements in the Arabia-Africa-Eurasia plate collision zone[END_REF][START_REF] Mcclusky | Global Positioning System Constraints on plate kinematics and dynamics in the eastren Mediterranean and Caucasus[END_REF].

In the Greek area (Figure 1) deep seismicity near Crete is related to the subduction of the northward-moving (relative to Eurasia) African oceanic lithosphere.

Shallow seismicity throughout the Aegean Sea and mainland Greece is related to the southwestward (again relative to Eurasia) extension of the Aegean continental lithosphere that is driven by a combination of gravitational spreading of the Anatolian continental lithosphere from the east and roll-back of the subducting African plate to the south (Le Pichon and [START_REF] Le Pichon | The Hellenic arc and trench system: a key to the evolution of eastern Mediterranean[END_REF]Papazachos, 1990). The transition between the active lithospheric subduction and the continental collision is located around the Ionian islands (Papazachos and Kiratzi, 1996). The tectonic evolution of the Greek region has been covered in more detail elsewhere [START_REF] Mckenzie | Active tectonics of the Mediterranean Region[END_REF][START_REF] Mckenzie | Active tectonics of the Alpine-Himalayan Belt: the Aegean Sea and surrounding regions[END_REF][START_REF] Angelier | The tectonic development of the Hellenic arc and the Sea of Crete: a synthesis[END_REF][START_REF] Armijo | Quaternary evolution of the Corinth Rift and its implications for the Late Cenozoic evolution of the Aegean[END_REF] and therefore is not elaborated upon here.

Multiple kinematic and dynamic models have been proposed as an attempt to describe the observed present-day deformation highlighting many important features of A c c e p t e d M a n u s c r i p t

Greece and surrounding areas (Anatolia, western Turkey), but there has not emerged a single, widely-accepted model. All agree upon N-S extension occurring within the Aegean Sea but the explanation of this differs from one study to another. In their majority they postulate that the complicated deformation observed in the Greek area is due to relative motion of a small number of rigid blocks or microplates, without agreeing upon a specific number of these, their boundaries or margins, and their rotation. It is not clear if there are zones of internal deformation in the otherwise rigid microplates that might define further microplates. [START_REF] Nyst | New constraints on the active tectonic deformation of the Aegean[END_REF] use published site velocities from six separate GPS networks to identify deforming regions, rigid elements, and potential microplate boundaries and build upon previous work by others to initially specify rigid elements in central Greece, the southern Aegean, Anatolia and the Sea of Marmara. One main reason for quantifying the crustal deformation field lies in the constraints it can provide on the dynamics of the system. To this extent, particular focus has to be given to the role of driving forces such as gravitational collapse (e.g. western Anatolia) [START_REF] Jones | The role of gravitational potential energy in active deformation in the southwestern United states[END_REF][START_REF] Davies | Assembling the IGS polyhedron. A densinfied weekly GPS terrestrial reference frame[END_REF][START_REF] Hatzfeld | An analogue experiment for the Aegean to describe the contribution of gravitational potential energy[END_REF][START_REF] Martinod | Continental collision, gravity spreading and kinematics of Aegean and Anatolia[END_REF], plate collision (e.g. the Adriatic block), push (e.g. the Arabian Plate) and slab retreat (roll-back in the Greek-Hellenic Arc). Debate still remains and various mechanical and numerical models have been published [START_REF] Cianetti | Reproducing the velocity and stress field in the Aegean region[END_REF][START_REF] Meijer | Present-day dynamics of the Aegean region: a model analysis of the horizontal pattern of stress and deformation[END_REF][START_REF] Lundgren | A geodynamic framework for eastern Mediterranean kinematics[END_REF][START_REF] Mantovani | Role of kinematically induced horizontal forces in Mediterranean tectonics: insight from numerical modeling[END_REF][START_REF] Martinod | Continental collision, gravity spreading and kinematics of Aegean and Anatolia[END_REF][START_REF] Cianetti | Numerical modeling of the Aegean-Anatolian region: geodynamic constraints from observed rheological heterogeneities[END_REF].

The complex tectonics and high deformation rates in the Greek area have attracted in the past decade the attention of numerous scientific groups who have organized GPS campaigns and designed networks in the region. [START_REF] Billiris | Geodetic determination of the tectonic deformation in central Greece from 1900-1988[END_REF]Stiros, 1993; Le A c c e p t e d M a n u s c r i p t [START_REF] Le Pichon | Geodetic determination of the kinematics of central Greece with respect to Europe: Implications for eastern Mediterranean tectonics[END_REF][START_REF] Davies | Assembling the IGS polyhedron. A densinfied weekly GPS terrestrial reference frame[END_REF][START_REF] Clarke | Geodetic estimate of seismic hazard in the Gulf of Korinthos[END_REF][START_REF] Reilinger | Global Positioning System measurements of present-day crustal A c c e p t e d M a n u s c r i p t movements in the Arabia-Africa-Eurasia plate collision zone[END_REF][START_REF] Clarke | Crustal strain in central Greece from repeated GPS measurements in the interval 1989-1997[END_REF][START_REF] Cocard | New constraints on the rapid crustal motion of the Aegean region: recent results inferred from GPS measurements (1993-1998) across the West Hellenic Arc, Greece[END_REF][START_REF] Cruddace | An interdisciplinary approach to studying seismic hazard throughout Greece. In Geodesy beyond 2000: the challenges of the first decade[END_REF][START_REF] Briole | Active deformation of the Corinth rift, Greece: Results from repeated Global Positioning System surveys between 1990 and 1995[END_REF][START_REF] Mcclusky | Global Positioning System Constraints on plate kinematics and dynamics in the eastren Mediterranean and Caucasus[END_REF][START_REF] Ayhan | Interseismic strain accumulation in the Marmara Sea region[END_REF]. The HELLENET and SING projects were designed to strengthen links between sites and fill in areas not previously occupied [START_REF] Cruddace | An interdisciplinary approach to studying seismic hazard throughout Greece. In Geodesy beyond 2000: the challenges of the first decade[END_REF]. Depending on each group's interests the locations differed as well as the network sampling interval, density and observation period at each monument. Subsequently GPS data analysis strategies also varied from campaign to campaign. Another parameter that enhanced the diversity of previous studies is that the methods and technology to measure crust deformation have improved greatly over the past two decades. This has resulted in an increased accuracy of space geodetic data derived from GPS. The improvement in accuracy in these measurements through the years is partly a direct result of the increased data volume from GPS sites, and partly the indirect result of improvements in the observational models, precise orbits and terrestrial reference frame with a continuous change and update. Because of the continuous improvement in the GPS system although the majority of these past campaign datasets have been analyzed and reported previously, it has not been possible to combine them rigorously into a single velocity field. They have been processed using different observation models (i.e. orbit types or even lack of that information in the early studies), satellite information (satellite clock correction files, satellite shadow event files, earth orientation parameters), constraints (i.e. fixed station solutions), and reference frames (ITRF94, ITRF96, ITRF97). Full variance-covariance information is not always available from previous solutions. In many cases, analysis has taken place in a regional reference frame with few or no sites overlapping other solutions, and few or no sites outside the Greek region with which to navigate the global velocity Strain analysis of the residual velocities to the European Euler pole is performed in order to define the type of present deformation. The geodetic strain rates are estimated by two methods: firstly that of contiguous uniformly-straining polygons, and secondly by differentiating a minimum curvature surface fitted to the velocity field. The principal horizontal strain rate tensors derived from GPS can then be compared to the seismic strain rate as calculated from three earthquake catalogues covering various parts of the region and spanning 30, 100 and 300 years respectively. This comparison is performed for the purpose of highlighting areas of potential seismic hazard. Finally, we also investigate the correspondence between the orientation of active faulting and that required to accommodate the geodetic strain field.

GPS data processing

For the processing of the 18 campaigns (Figure 2, Table 1) we aimed to produce the best possible consistency of reference frame definition, thus the frame of each campaign was navigated from as many as 43 IGS [START_REF] Dow | The International GPS Service: Celebrating the 10th anniversary and looking to the next decade[END_REF] and EUREF [START_REF] Bruyninx | Regional Densification of the IGS in Europe Using the EUREF Permanent GPS Network (EPN)[END_REF] sites (Figure 3). These sites were chosen to have a global distribution and to have been in The raw data were imported to the software via the RINEX [START_REF] Gurtner | RINEX: The Receiver Independent Exchange Format[END_REF] format.

Where necessary, transformation from raw (in this case Trimble or Ashtech) format took place using UNAVCO' s TEQC program (Estay and Meertens, 1999; UNAVCO). The information given in the RINEX header was corrected in order to agree with the station log files. All antenna heights were corrected to agree with the standard ARP (antenna reference point) adopted by the IGS. Because the daily networks typically had a mixture of antenna types, we used elevation-dependent relative phase centre offsets as provided by NOAA (http://www.ngs.noaa.gov/ANTCAL).

For the majority of data processing we used the Bernese version 4.2 GPS software, (http://www.ipa.nw.ru/PAGE/DEPFUND/GEO/nap/publ/docu42_1.pdf, [START_REF] Beutler | Bernese GPS software Version 4[END_REF]. As a reference frame for the whole solution we used ITRF2000, realized by IGS precise orbits, and we applied troposphere, ionosphere, and ocean tide loading corrections as described in Table 2. We corrected any detected cycle slips both automatically and manually, and attempted to resolve all carrier phase ambiguities to integers as part of each UT day's network solution, typically with more than 70% success [START_REF] Rontogianni | Strain rates in Greece using GPS measurements from 1994-2000[END_REF]. The three Egion campaigns were processed with the GAMIT version 10.2 software using a similar strategy but with 24-hour sessions aligned to the times of the local campaign observations rather than the UT day (Michael Floyd, personal communication, 2007). For each campaign, the normal equations of daily sessions were combined into a campaign solution as a check on processing. Daily Solution Independent

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Exchange (SINEX) files were saved in order to be used in velocity analysis. For the IGS sites, the a priori velocities were given in ITRF2000 and free network conditions were set up for their coordinates.

Velocity field results

Blewitt and [START_REF] Blewitt | Effect of annual signals on geodetic velocities[END_REF] while working with GPS data sets proposed that below 2.5 years of continuous time series, the velocity bias can become unacceptably large. They recommended that 2.5 years be adopted as a standard minimum data span for velocity solutions intended for tectonic interpretation or reference frame production. In our analysis we are working with campaign data where only the sites DION and CG54 were present in all the campaigns during the 6 years of processing and have the maximum number of observations from the local sites (Figure 4). Errors in the analysis reflect the limited number of observations, a fundamental limitation of campaign data collected by different teams, and narrow span of the time series. Site velocities have been estimated

from the combination of 221 loosely-constrained daily SINEX files created by the Bernese 4.2 software, using the TANYA software developed and used at Newcastle University for IGS Global Network Associate Analysis (GNAAC) combinations [START_REF] Davies | Assembling the IGS polyhedron. A densinfied weekly GPS terrestrial reference frame[END_REF][START_REF] Davies | Methodology for global geodetic time series estimation: A new tool for geodynamics[END_REF][START_REF] Lavallée | Tectonic plate motions from global GPS measurements[END_REF]. Station daily coordinates are rigorously combined using a free network approach [START_REF] Davies | Assembling the IGS polyhedron. A densinfied weekly GPS terrestrial reference frame[END_REF][START_REF] Davies | Methodology for global geodetic time series estimation: A new tool for geodynamics[END_REF]. In our case the solution from Bernese 4.2 is a loosely constrained solution, and any constraints on the geometry arise from the fixed IGS orbits. The core_IGS00 SINEX file (the IGS realization of ITRF2000 Core site coordinates) was used [START_REF] Altamimi | ITRF2000: A new release of the International Terrestrial Reference Frame for earth science applications[END_REF] to navigate the solution to the ITRF2000 reference frame. Antenna height corrections and A c c e p t e d M a n u s c r i p t a three dimensional data-snooping outlier rejection method are also applied if needed at this stage.

The sites were introduced in the solution starting with ones that had more than 50 daily observations (the vast majority of the IGS sites). All sites that were present in more than one campaign and had two or more daily observations were used in the final solution. A conservative limit of four standard deviations was used iteratively to exclude outliers in each station coordinate residual time series. We chose that rejection outlier boundary after considering the fact that we are dealing with small samples of campaign data and so we need to be cautious in automatically removing outliers to avoid removing data points. The resulting free network solution is aligned to ITRF2000 by applying a 12parameter Helmert transformation (translation and rotation with their time derivatives, but no scale or scale rate). From the time series of the daily solutions we estimate a kinematic solution which for each station includes a reference epoch position at 1999.00 and a constant velocity.

For the sites whose time series were edited, no more than 10% of observations were removed. Excluded from the velocity solution are some of the IGS sites that had very noisy time series. The majority of excluded IGS sites are located in the southern hemisphere (i.e. very long baselines, with not enough overlap of visible satellites between the two stations forming the baseline). The excluded sites are YAR1, USUD, SUWN, KOKB, PIN2, KERG, HARK, and IRKT. Data for the station ANKR are used until 1999, i.e. before the devastating 1999 Izmit earthquake (17 August, Mw 7.4) that took place in this area. No other sites suffered from discernible co-seismic offsets. The output results in ITRF2000 are given in Table 3.

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All errors have been scaled by the unit variance [START_REF] Cross | Working Paper 6: Advanced least squares applied to position fixing, 2nd Edition[END_REF][START_REF] Lavallée | Tectonic plate motions from global GPS measurements[END_REF][START_REF] Rontogianni | Strain rates in Greece using GPS measurements from 1994-2000[END_REF]. The sites that appear with larger velocity errors are the ones with small numbers of observations, and also sites with short time series (Figure 4). As shown in Figure 5 and Figure 6 larger velocity errors have the sites with less than 16 months data span between the first and last measurement. In this category we can place the sites from the EGION campaign. The vertical velocities are not reported since the uncertainties are too large to allow reliable interpretation.

For campaign GPS networks, scaling the errors with the unit variance leads to a very conservative error bounds. To verify that the scaling is appropriate for continuous GPS sites we also apply a chi-square test (χ2/DOF, Equation 1) to the horizontal velocity field where our velocity solution is compared with the published SOPAC solution (http://sopac.ucsd.edu/cgi-bin/refinedModelVelocities.cgi). errors have been scaled by the unit variance; the error ellipses represent 39% confidence intervals (Table 3 and Figure 6). The sites around the Chalkidiki and Grevena area show, A c c e p t e d M a n u s c r i p t in their majority, a very small residual motion of around 10 mm/yr. Further south there is an obvious SW motion of the sites around Attica, Evia, east and south Peloponnisos as well as the Aegean islands. This motion increases from ~25 mm/yr for the Aegean islands to around 30 mm/yr near Attica, Evia and the eastern Peloponnisos coast, and reaches ~40 mm/yr for the sites in southern Peloponnisos. The rapid displacement that is concentrated in the small area of the Corinth and Patras Gulfs is very prominent. The Patras area shows south-westerly motion of ~30-35 mm/yr. This kinematic field for Greece has also been suggested previously e.g. [START_REF] Cocard | New constraints on the rapid crustal motion of the Aegean region: recent results inferred from GPS measurements (1993-1998) across the West Hellenic Arc, Greece[END_REF] and [START_REF] Peter | Establishment of continuous GPS network across Kephalonia Fault Zone, Ionian islands, Greece[END_REF] and more recently by [START_REF] Hollenstein | Crustal motion and deformation in Greece from a decade of GPS measurements, 1993-2003[END_REF]. Hollenstein et al. while combining campaign and continuous GPS data suggest that the rates increase from about 20-26 mm/yr in the northeastern Aegean up to 30-36 mm/yr in the southwest. They continue suggesting that towards Attica the velocities reach 31.1 ± 0.1 mm/yr. There is a difference in the velocity direction for the island of Astypalea probably caused by its location within the eastern volcanic arc. [START_REF] Bohnhoff | Microseismic activity in the Hellenic Volcanic Arc, Greece, with emphasis on the seismotectonic setting of the Santorini-Amorgos zone[END_REF] after analyzing 2175 microseismic events in the Greek-Hellenic Volcanic Arc define the Santorini-Amorgos zone as a major structural boundary of the volcanic arc subdividing it into a seismically and volcanically quite western and an active eastern part. In the same study for the Amorgos-Astypalea area they indicate a vertical structure possibly related to migration of fluids or degassing processes SE of Amorgos.

χ 2 /DOF                    N N N N N E E E E v v v v N 2 2 2 2 2 2 ) ( ) ( 2 1     (

Geodetic strain analysis

We use the Europe-fixed GPS velocity field to determine the crustal strain rate field and rotation rates in the Greek area. We calculated the principal strain rate components using A c c e p t e d M a n u s c r i p t two different methods. First we applied the polygon method which allows us to define the regions in each of which uniform strain is to be computed (e.g. based on geological criteria). The second method of deriving strain rates calculates the strain by applying continuous curvature gridding to the entire horizontal velocity dataset, and thus is independent of geological preferences and allows interpolation between points.

Polygon strain analysis

In this method the area is divided up into polygonal regions bounded by geodetic monuments. The strain rate is assumed to be spatially uniform within each polygonal region and constant with respect to time. No assumptions are made regarding the continuity of velocity or strain rate between regions [START_REF] Frank | Deduction of earth strains from survey data[END_REF][START_REF] Prescott | An extension of Frank's method for obtaining crustal shear strains from survey data[END_REF]Welsch, 1983). The choice of polygons is very important, but somewhat arbitrary although reasonable criteria can be adopted [START_REF] Rontogianni | Strain rates in Greece using GPS measurements from 1994-2000[END_REF]. The area is quite complicated because it contains many active faults, most of which appear to have segments of around 15-20 km in length. Primarily we created the smallest possible polygons according to major tectonic zones as defined by previous researchers (Le [START_REF] Le Pichon | The Hellenic arc and trench system: a key to the evolution of eastern Mediterranean[END_REF][START_REF] Le Pichon | Geodetic determination of the kinematics of central Greece with respect to Europe: Implications for eastern Mediterranean tectonics[END_REF][START_REF] Kahle | The strain rate field in the eastern Mediterranean region estimated by repeated GPS measurements[END_REF]Papazachos et al., 1999). Following this, neighboring polygons with similar strain rates are merged. We tried to ensure that each final polygon bounds the whole of either a main fault zone or a trough zone. This was not always possible, because the faults can be quite small or concentrated in a narrow area (e.g. Corinth Gulf, Evia Gulf). In the final polygons, the uniform strain is a good fit (in terms of the root mean square weighted residual) to all the stations that pertain to it. In total, 22 polygons were formed. The principal horizontal A c c e p t e d M a n u s c r i p t strain rates and their formal errors, for each of the chosen polygons, are computed using the program polystr2 [START_REF] Clarke | Tectonic motion and tectonic deformation in Greece from GPS measurements[END_REF]. Table 4 andFigure 7 summarize the results for each polygon, and Figure 8 presents the geodetic rigid body rotation for each of the 22 polygons. In some cases there is a large uncertainty in the azimuth of the principal strain rates, although their magnitudes are relatively certain. This occurs if strains are small compared with their formal errors, or if the principal strain rate components are roughly equal (in the limit of exact equality the azimuth is undefined). However, if the two principal strain rate components differ considerably, with one being much larger than the other, then the azimuth will be well determined. Clockwise rotation is observed around the North Eastern Aegean, Gulf of Corinth, and Gulf of Evia, with the rates decreasing around the Grevena and Chalkidiki areas. The sense of rotation changes in the South Aegean and Cyclades area where anti-clockwise rotation is present.

In the North Eastern Aegean region the results show a dominant extensional deformation in the ENE-WSW direction of around 0.2±0.06 ppm/yr (polygon 22). This extensional deformation turns to a NNW-SSE direction further to the west and the Chalkidiki area (polygon 21). This extension is suggested by [START_REF] Kahle | The strain rate field in the eastern Mediterranean region estimated by repeated GPS measurements[END_REF]1999;[START_REF] Lavallée | Tectonic plate motions from global GPS measurements[END_REF], but for their case the deformation is even stronger, reaching 0.205 ppm/yr at the western end of the North Anatolian Fault (NAF) and 0.15 ppm/yr at the North Aegean Trough. Straub et al. (1997) use GPS data to determine the strain rate pattern at the western end of the NAF, and conclude on an average value of 0.11 ppm/yr for the whole Marmara region with a maximum value of 0.31 ppm/yr.

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The Grevena area (polygon 20) is previously thought as aseismic region but here it appears to extend in a NNW-SSE direction (~0.07 ± 0.02 ppm/yr) with a small degree of shortening. Theodulidis (1998) suggests after observing macroseismic intensities and strong motion data following the 1995 Kozani-Grevena earthquake that in this area there are seismic faults that may be unknown or considered as non-active but still capable of generating destructive earthquakes. This is an area where the lack of contemporary or historical seismic activity can indicate 'hazard' rather than safety [START_REF] Pavlides | The 13 May 1995 western Macedonia, Greece (Kozani-Grevena) earthquake; preliminary results[END_REF][START_REF] Meyer | The 1995 Grevena (Northern Greece) earthquake: fault model A c c e p t e d M a n u s c r i p t constrained with tectonic observations and SAR interferometry[END_REF]Pantosti et al., 1996;[START_REF] Chatzipetros | Understanding the 13 May 1995 western Macedonian earthquake: a paleoseismological approach[END_REF][START_REF] Pavlides | The 1995 Kozani-Grevena earthquake (N. Greece): an introduction[END_REF]Pavlides and Caputo, 2004). Polygon 7 is centered on the Gulf of Patras and western Corinth Gulf, east-west compressional deformation combines with north-south extension to give right lateral transpression. The east-west compression increases towards the entrance of the Gulf of Patras from 0.13±0.08 ppm/yr (polygon 6) to 0.24±0.11 ppm/yr (polygon 7). Shortening has been pointed out by [START_REF] Kahle | The strain field in northwestern Greece and the Ionian islands: results inferred from GPS measurements[END_REF] who suggest a pure compressional tectonic stress dominant in the Gulf of Patras in E-W direction. [START_REF] Davies | Geodetic strain in Greece in the interval 1892-1992[END_REF] suggest a predominant contraction in the Pindos mountains and Epirus region to the west.

The Gulf of Corinth and the Gulf of Evia are major grabens bounded by faults that are segmented and not continuous for more than 15-20 km. Both gulfs have been tectonically active since the Quaternary and have experienced intense earthquake activity in the last century [START_REF] Ambraseys | Seismicity and associated strain of central Greece between 1890 and 1988[END_REF], although earthquake activity in the Gulf of Evia has been lower in the last 30 years. In the Corinth Gulf, most historical activity has occurred within a seismogenic belt that is 30 km wide by 130 km long between Corinth and the Patras area [START_REF] Roberts | Structural and seismological segmentation of the Gulf of Corinth fault system: implications for models of fault growth[END_REF]. Here, WSW-A c c e p t e d M a n u s c r i p t trending normal faults are accommodating most of the N-S extension [START_REF] Doutsos | Listric faulting, sedimentation and morphological evolution of the Quaternary eastern Corinth rift, Greece. First stages of continental rifting[END_REF][START_REF] Jackson | Active Tectonics of the Aegean Region[END_REF]. Studies in the area of the Gulf of Corinth, have been investigating the tectonic evolution as well as the mechanism related to the seismicity resulted by these onshore and offshore faults. Two different tectonic phases seem to be responsible for extreme complexity of the structure of the region. The first one is represented by the Cenozoic E-W compressional phase, which gave rise to the Hellenic orogenic belt. The second tectonic phase is represented by the extension of the Aegean region, which started in Miocene time. This phase was initially characterized by a predominant NE-SW direction in the Corinth region. During the Quaternary, The Aegean extension was accelerated by the southwestward propagation of the NAF, which has reactivated features from the first tectonic period approximately 1Ma. (e.g [START_REF] Armijo | Quaternary evolution of the Corinth Rift and its implications for the Late Cenozoic evolution of the Aegean[END_REF][START_REF] Rigo | A microseismic study in the Gulf of Corinth (Greece): Implications for large-scale normal faulting mechanisms[END_REF][START_REF] Bernard | Aigion earthquake (Greece): evidence for low angle normal faulting[END_REF][START_REF] Clarke | Geodetic estimate of seismic hazard in the Gulf of Korinthos[END_REF][START_REF] Briole | Active deformation of the Corinth rift, Greece: Results from repeated Global Positioning System surveys between 1990 and 1995[END_REF][START_REF] Moretti | The Gulf of Corinth : an active half graben?[END_REF][START_REF] Latorre | A new seismic tomography of Aigion area (Gulf of Corinth, Greece) from the 1991 data set[END_REF][START_REF] Lyon-Caen | First results of the CRLN seismic network in the western Corinth Rift: evidence for old-fault reactivation[END_REF]. The results here show a dominant extension rate present around the Corinth Gulf (polygon 8) 0.24±0.22 ppm/yr in an almost N-S direction also pointed out by [START_REF] Hollenstein | Crustal motion and deformation in Greece from a decade of GPS measurements, 1993-2003[END_REF]. The extension rates are lower around the Gulf of Evia, 0.11±0.4 ppm/yr in an E-W direction. Around the Saronic Gulf (polygon 11) the extension rates turn in an NE-SW direction at 0.1±0.1 ppm/yr. In Attiki (polygon 12) extension and compression are both present giving shear, while in the Saronic islands (polygon 1) we have pure extension of 0.28±0.16 ppm/yr in a NNE-SSW direction. The fact that the maximum extensional strain is perpendicular to the strike of the Corinth Gulf is in agreement with the strain shown in the focal mechanisms of earthquakes [START_REF] Ambraseys | Seismicity and associated strain of central Greece between 1890 and 1988[END_REF]. According to [START_REF] Clarke | Crustal strain in central Greece from repeated GPS measurements in the interval 1989-1997[END_REF] there may be fewer active fault segments taking up the strain across the gulf of Evia and it is A c c e p t e d M a n u s c r i p t possible that earthquake recurrence times on individual fault segments in the Gulf of Corinth are shorter to accommodate higher strain rates in the underlying lithosphere.

According to [START_REF] Kahle | The strain rate field in the eastern Mediterranean region estimated by repeated GPS measurements[END_REF]1999;[START_REF] Lavallée | Tectonic plate motions from global GPS measurements[END_REF], in central Greece the maximum extensional components of the strain rate tensor are centered around the Gulf of Corinth in an almost N-S direction.

The central Aegean undergoes a NE-SW extensional deformation of 0.08±0.05ppm/yr. The Cyclades area is almost strain free. The strain free Cyclades area has been suggested by other researchers in the past again based on GPS measurements. Kahle at al., 1998 while estimating the strain rate field in the eastern Mediterranean region with repeated GPS measurements, conclude that there is a relative strain free region in the south-central Aegean, between the volcanic and non-volcanic Greek-Hellenic arc. [START_REF] Kahle | The GPS strain rate field in the Aegean Sea and western Anatolia[END_REF] return to the subject and suggest that the SW Aegean Sea and central Anatolia are almost strain-free adding that these strain free regions are nearly aseismic. They conclude by saying that the Cyclades islands are almost strain-free and show low seismicity. [START_REF] Jenny | Earthquake recurrence parameters from seismic and geodetic strain rates in the eastern Mediterranean[END_REF] refer to the southern Aegean region as a low straining region where deformation occurs mostly aseismically. Very thin lithosphere due to extension and hydration by the subduction slab probably limits the seismogenic thickness. The Sea of Crete in its eastern part (polygon 18) extends in a NNE-SSW direction while its western part (polygon 17) extends NE-SW. These two polygons rotate differently. This agrees with what has been suggested by [START_REF] Kahle | The strain rate field in the eastern Mediterranean region estimated by repeated GPS measurements[END_REF]1999;[START_REF] Lavallée | Tectonic plate motions from global GPS measurements[END_REF], that the area of the South Aegean Sea (around the Dodecanese islands with principal extension value of around 0.09 ppm/yr) is probably related to tectonic processes in the SW Anatolia graben system accompanied by recent volcanic activity. Recently Kokkalas The deformation seems to ease out in Peloponnisos (polygons 2, 3, 4). In polygon 3 low levels of extension and compression are present while in polygon 2 there is only pure NNE-SSW extension of around 0.04±0.1 ppm/yr. In the Messinia Gulf (south Peloponnisos, polygon 4) low levels of extension are present, while strong compression is shown (0.3±0.27 ppm/yr) between the Laconia Gulf and the island of Kythira (polygon 19). Both of these gulfs are bounded by active faults.

Minimum curvature strain analysis

This second method of deriving horizontal geodetic strain rates is independent of geological preferences, depending only on the velocity field data. We fitted a continuous minimum-curvature surface to the east and north site velocity datasets in turn, allowing interpolation between points. The method has been analyzed in detail by [START_REF] Smith | Gridding with continuous curvature spines in tension[END_REF] and has been applied in different parts of the world to describe deformation patterns (e.g., [START_REF] Chang | Strain and stress field in Taiwan oblique convergence system: constraints from GPS observations and tectonic data[END_REF]. This interpolation process puts strong emphasis on the nearby measurement point as compared with distant ones, which allows better characterization of local variations in the rate of deformation field. Three different grid spacing values were tested (20, 30, 40 minutes). To optimize grid spacing without over-fitting the data, we computed a chi-square statistic for each fit (mean square weighted residual). The 20 minute grid gave a result of 0.23, the 30 minute grid gave a result of 0.50, and the 40 minute grid gave a result of 0.54. Despite its lower statistic, the A c c e p t e d M a n u s c r i p t 20 minute grid did not show any regions of visibly different strain patterns when compared with the 30 minute grid, whereas the 40 minute grid masked detail that was observed with the 30 minute grid. Therefore the 30 minute grid spacing was chosen. The continuous east and north velocity fields were then differentiated to give the velocity gradient tensor, from which principal strain and rotation rates were computed (Figure 9).

The comparison between the two strain determination methods showed very good agreement. In Figure 10 the average strain rate for each polygon as determined from the polygon method is compared with the point values taken at the centroid of each polygon as derived by the minimum curvature method.

Grevena, northern Greece, central Aegean, and Sea of Crete areas show agreement in both the direction and magnitude of deformation. The same applies for western Greece, the Patras area and western Corinth Gulf, the central Corinth Gulf and SW Peloponnisos (polygons 3, 4). Cyclades (polygon 16) appear strain free in both methods. Polygon 19 (the Gulf of Laconia) deforms differently for both methods. An obvious difference in the direction of deformation is observed around the northern Gulf of Evia where the type of deformation is extension ~ 0.11ppm/yr. In the polygon method (see polygon 10) the direction of deformation appears as E-W while in the gridding method the direction of deformation appears as NNW-SSE. A difference in direction also exists for polygon 11 (Saronic Gulf). Compression is observed for polygon 12 (Attiki) in both methods but for the gridding method there is no extension for this area. These differences can be explained because in the polygon method the azimuth often appears with large uncertainty, particularly if the polygon has a large aspect ratio. In the gridding method data from the wider region will have an influence, and so small differences in The SE Aegean region rotates counterclockwise and moves towards the Greek-Hellenic trench (i.e., toward the SE). They suggest that much of the Aegean is deforming by distributed extension that is widespread in Greece and in the northern Aegean Sea. For the Greek area they included data from observations in the Aegean region (1988-south, 1989-north, 1992-all, and 1996-all) and also data from the Western Greek Arc as these are described in [START_REF] Kahle | Trajectories of crustal deformation of western Greece from GPS observations 1989-1994[END_REF]. Coverage in Central Greece and other sensitive areas such as gulf of Argos, Grevena, Evia that need special attention was poor. Armijo et al.

Seismic hazard estimation

Methodology

The next step of our analysis is to compare the strain rate values obtained through geodetic measurements with those derived from recorded seismic events that took place in the Greek area. Various studies have been conducted in different parts of the world performing this comparison between seismicity and geodesy (Ward, 1998a(Ward, , 1998b;;[START_REF] Shen-Tu | Contemporary kinematics of the Western United states determined from earthquake moment tensors, very long base interferometry, and GPS observations[END_REF][START_REF] Kreemer | Active deformation in eastern Indonesia and the Philippines from GPS and seismic data[END_REF][START_REF] Masson | Seismic versus aseismic deformation in Iran inferred from earthquakes and geodetic data[END_REF][START_REF] Pancha | Comparison of Seismic and Geodetic Scalar Moment Rates across the Basin and Range Province[END_REF]. The dense geodetic data distribution, relatively long historical catalogues and high activity rates make this region ideal for constructing independent measurements of seismic and geodetic strain rates. We map seismic strain rates in the same grid as used for the geodetic field. A key question is what duration of earthquake catalogue is most suitable for relating seismic to geodetic strain, at the spatial scales that we have observed the latter and if the bulk of the tectonic strain in this region is released by seismic slip on faults.

Seismic moment rates that are low compared with tectonic strain rates may indicate that A c c e p t e d M a n u s c r i p t deformation occurs in part aseismically, which has been suggested previously (Jackson and McKenzie, 1988;[START_REF] Briole | Active deformation of the Corinth rift, Greece: Results from repeated Global Positioning System surveys between 1990 and 1995[END_REF][START_REF] Jenny | Earthquake recurrence parameters from seismic and geodetic strain rates in the eastern Mediterranean[END_REF].

To derive the seismic strain rate we used three earthquake catalogues (Figure 11).

In general, the smaller the area under investigation the longer the sample time must be for a representative seismic moment rate to be derived [START_REF] Ambraseys | Seismicity and strain in the Gulf of Corinth (Greece) since 1694[END_REF]. We used the Global Centroid Moment Tensor (GCMT) catalogue (http://www.globalcmt.org, former Harvard CMT) that covers the whole of the Greek area for magnitudes greater than ~4.5 and goes back to 1976; the Ambraseys and Jackson 100-year catalogue ) complete for M S >5.8 for central Greece [START_REF] Ambraseys | Seismicity and associated strain of central Greece between 1890 and 1988[END_REF]; and finally the Ambraseys and Jackson 300-year catalogue (1694-1995) that covers the Corinth area for M S >6.0 [START_REF] Ambraseys | Seismicity and strain in the Gulf of Corinth (Greece) since 1694[END_REF]. The [START_REF] Ambraseys | Seismicity and strain in the Gulf of Corinth (Greece) since 1694[END_REF]1990) studies even though they extensively covered the seismicity surrounding prominent, active geological structures, have not been (as far as we know) compared to geodetic results.

In order to determine if there is any imbalance between geodetic and seismic strain rates we assign the seismic events from each of the three catalogues to the polygons as defined in Figure 7. All three catalogues refer to crustal earthquakes (depth ≤ 40 km). For our area of research the GCMT catalogue includes events with surface wave magnitude M S ≥4. The 100-year catalogue includes events with M S ≥5.8 and the 300-year catalogue includes events M S ≥6.0. Events with large M S are the ones which are actually of interest here, because these release the majority of the strain and consequently are responsible for much of the deformation. To estimate seismic strain reliably, accurate knowledge about the rates of recurrence of moderate size events (Mw= 4.5-6.5) is A c c e p t e d M a n u s c r i p t needed. For b ≥ 1, these events can accommodate 60 per cent of the strain [START_REF] Jenny | Earthquake recurrence parameters from seismic and geodetic strain rates in the eastern Mediterranean[END_REF]. In order to estimate the strain accounted as seismicity during the three different periods covered by the catalogues we used the technique of [START_REF] Kostrov | Seismic moment and energy of earthquakes and seismic flow of rock[END_REF]. The seismic moment tensors are constructed from the fault plane solution (strike, dip, and rake) and scalar seismic moment M 0, for each of the events. We used a seismogenic layer thickness of 10-15 km as is commonly assumed as a reasonable estimate of the depth range over which the bulk of the seismic strain is accommodated in continental areas (Taymaz et al., 1991;[START_REF] Rigo | A microseismic study in the Gulf of Corinth (Greece): Implications for large-scale normal faulting mechanisms[END_REF][START_REF] Jenny | Earthquake recurrence parameters from seismic and geodetic strain rates in the eastern Mediterranean[END_REF]. The Cartesian components of M (moment tensor) for a double-couple event described by the strike s  , the rake  , dip  and moment 0  are obtained as described in Aki and Richards (Page 112, Box 4.4) (2002). For the GCMT catalogue the moment magnitude Mw has already been computed (using the formula of [START_REF] Kanamori | The energy release in great earthquakes[END_REF]). For the other two catalogues the seismic moment M 0 values were calculated from the M S values using the most recent formulas given for the eastern Mediterranean by [START_REF] Ambraseys | Reassessment of earthquakes,1900-1999, in the Eastern Mediterranean and the Middle East[END_REF]. Table 5 presents the seismic strain rates results for the three catalogues, using seismogenic layer thicknesses of 15 km (for the 10 km thickness these values need to be multiplied by 1.5; [START_REF] Rontogianni | Strain rates in Greece using GPS measurements from 1994-2000[END_REF].

Application and results

For the 30-year catalogue (Figure 12 a) the seismic strain rate in polygons 20 (Grevena) and 9 (western central Greece) is large, and especially for polygon 9 exceeds that of the geodetic strain. This is not observed in the two longer catalogues (Figure 12 b, and Figure 12 c). When seismic strain is larger than the geodetic strain it implies that the observed earthquakes release strain accumulated over longer periods than the seismic A c c e p t e d M a n u s c r i p t strain rates are calculated (i.e. the period covered by each catalogue). Another explanation may be that the area covered by a polygon may be small but has a large seismic moment rate caused by a single very strong earthquake. That is exactly what happens for polygon 9, which is quite small but includes the July 1980 earthquake with M S =6.4. Polygon 20 surrounds the area most influenced by the May 1995 Kozani-Grevena earthquake with M S =6.6, this area was previously considered to be of low seismic risk (Papazachos, 1990). This is strongly suggested by the surrounding polygons where there is negligible amount of recent seismic strain release (e.g. polygon 6).

Polygon 11 also has a large amount of seismic strain caused by the 24 February (M S =6.7) and 25 February (M S =6.4) 1981 Alkyonides earthquakes [START_REF] Leeder | Normal faulting and crustal deformation, Alkyonides Gulf and Perachora peninsula, eastern Gulf of Corinth rift, Greece[END_REF].

Polygon 11, which surrounds the Saronic Gulf and east Corinth area, appears to have a significant rate of seismic strain release for all of the 3 catalogues. Polygon 8 (central Gulf of Corinth) has a high level of seismic strain which is not surprising since it includes the 18 November 1992 earthquake with M S =5.9 and the 15 June 1995 Egion earthquake with M S =6.2. Both these events are, unfortunately, not included in any of the other two, older, catalogues. From this we can infer that a 30 years period is not capable to describe fully the seismic strain rate at the scale of chosen polygons and it is highly affected by the magnitude of the events that struck the area during the 30-year interval.

The 100-year catalogue reaches 1988 which means that it does not include all of the above-mentioned earthquakes. In this catalogue the seismic deformation is distributed amongst the majority of the polygons with only the Saronic -east Corinth area (polygon 11) having a considerably higher amount of seismic strain release. Polygon 12 appears to have released a great amount of seismic strain. In this polygon we have placed the 1955, M S =5.9 earthquake. Unfortunately this is an event that is placed directly on the margins between polygon 11 and polygon 12 which means that it could equally well be included in polygon 11. It is not an event that exists in any of the other catalogues since the 300-year catalogue includes M S ≥6 and the 30-year catalogue goes back only to 1977.

Even the 100-year catalogue is insufficient to reveal all the major active tectonic structures, and any assessment of seismic risk should take a longer perspective. This suggestion is also supported by [START_REF] Ambraseys | Seismicity and associated strain of central Greece between 1890 and 1988[END_REF]. For the same catalogue, [START_REF] Davies | Geodetic strain in Greece in the interval 1892-1992[END_REF] suggested that whereas seismic strain in the eastern Gulf of Corinth is commensurate with the geodetic strain, there is a rapid extension across the western Gulf of Corinth with negligible seismic strain. They concluded that this is consistent with the deformation that would be expected of a sheet of fluid moving towards a low-pressure boundary at the Greek-Hellenic Trench.

For the 300-year catalogue it can be suggested that there is an amount of seismic activity present in polygons 7, 8, and 11, but nonetheless lower than the geodetic strain.

Polygon 7 appears free of seismic strain in both the 100 and 30-year catalogues but appears with a considerable amount in the 300-year catalogue. Polygon 8 (central Gulf of Corinth) has increased its seismic strain release rate from the 100-year catalogue but has decreased with respect to the 30-year catalogue. Finally, the relatively large polygon 11, which lies in a highly active region, has similar seismic strain release rates for all three catalogues. However, for the other, generally smaller or less straining polygons, it can be suggested that the 300-year catalogue best describes the area at the scale of these polygons and is less influenced by individual events or polygon sizes. Ward (1998 a, b)

A c c e p t e d M a n u s c r i p t while working in data sets from Europe and the United states suggested that short catalogues are more likely to underestimate than overestimate long-term moment rates.

The above observations indicate where unreleased strain is accumulating in an area, but cannot predict when or how this strain will be released. It is fortunate that for the Corinth area, macroseismic evidence exists for events of the pre-instrumental period to assess seismicity and compare with more recent catalogues of events (100 years and 30 years). It is difficult to assign a given earthquake to a fault even with modern data.

According to Pavlides and Caputo (2004) it is possible due to large recurrence intervals the fault or the fault system responsible for earthquakes to be unidentified and the potential danger underestimated. [START_REF] Ambraseys | Seismicity and associated strain of central Greece between 1890 and 1988[END_REF] suggest that the typical time interval between occurrences of major earthquakes on a fault in this region certainly exceeds 100 years and possibly even 1000 years in some of the more slowly straining parts of the region. That is supported by palaeoseismological studies in Greece that have shown the average recurrence interval of specific active faults to be commonly longer than 500 years and usually some thousands years (Pantosti et al., 1996;[START_REF] Chatzipetros | Understanding the 13 May 1995 western Macedonian earthquake: a paleoseismological approach[END_REF]. For the Kaparelli, Eliki and Egion faults, all located with the Gulf of Corinth, the mean recurrence intervals that have been suggested are 2520 years, 900-400 years (or 270-1200 years according to [START_REF] Koukouvelas | A palaeoseismological and geoarchaeological investigation of the Eliki fault, Gulf of Corinth, Greece[END_REF]) and 360 years respectively, for the Paleohori-Sarakina fault (near Kozani area) this interval is 30.000 years [START_REF] Pantosti | Palaeoseismological investigations of the Aigion Fault (Gulf of Corinth, Greece)[END_REF][START_REF] Chatzipetros | Palaeoseismic data and their implication for active deformation in Greece[END_REF][START_REF] Kokkalas | Paleoseismicity of the Kaparelli fault (eastern Corinth Gulf): evidence for earthquake recurrence and fault behavior[END_REF].

There are several parameter values that have to be assumed in order to derive the seismic strain, which contributes to the uncertainty in the above results. For the seismic rates estimated from the historical earthquake catalogues to be valid, the average for regions the size of an urban area, as desired for detailed seismic-hazard analysis. [START_REF] Pancha | Comparison of Seismic and Geodetic Scalar Moment Rates across the Basin and Range Province[END_REF] support the hypothesis that even a few years of detailed geodetic monitoring can provide a good constrain on earthquake occurrence rate estimates for large enough regions. Ward (1998 a) suggests that median catalogues of 200-300 years duration are adequate for regions straining at 10 -7 yr -1 , for regions straining at 10 -9 yr -1 more than 20.000 years of earthquake data are needed. For geodesy to give reliable estimates two conditions are necessary. First, the geodetic measurements should sample a large enough spatial scale so that they are not affected by non-linear strain accumulation during the earthquake cycle on individual faults. Second, they should sample a long enough time interval that measurement uncertainties have a minimum effect on the estimated velocities. Our analysis takes the two steps under consideration. [START_REF] Pancha | Comparison of Seismic and Geodetic Scalar Moment Rates across the Basin and Range Province[END_REF] propose that if the deformation rate is fast enough then there can be a meaningful comparison of average strain rates. The distributed seismicity of the region is assumed to be caused by a sufficient number of faults at different stages of the earthquake cycle to compensate for the long recurrence interval of individual faults. According to these suggestions the comparison between the two strain rates can be applied with meaningful results for Greece. Geodetic and seismic measurements sample different aspects of the A c c e p t e d M a n u s c r i p t deformation field. Seismic and geological estimates serve only as a record of brittle deformation, whereas geodesy encompasses both seismic and aseismic strain accumulation. Geodetic rates cannot uniquely determine slip at depth and may only give a measure of the instantaneous strain transients, which may not be preserved throughout the earthquake cycle [START_REF] Savage | Viscoelastic coupling model of the San Andreas Fault along the big bend, southern California[END_REF]Shen-Tu et al., 1999). The final uncertainty arises in converting strain rates in seismic moment rates. [START_REF] Field | A mutually consistent seismic-hazard source model for Southern California[END_REF] also analyze in detail factors that contribute to uncertainties in estimates of long term seismic moment rate from historical catalogues. The greatest uncertainty comes from: a) the M 0 -M S relation used to obtain the seismic moment, b) the assumption of the effective seismogenic layer thickness; 10 km and 15 km represent reasonable lower and upper bounds on this, c) in the 100-year and 300-year catalogues, only the larger events that took place in the Greek area are included; it has been suggested (Ambraseys and [START_REF] Ambraseys | Seismicity and associated strain of central Greece between 1890 and 1988[END_REF]) that the smaller events (with M S ≤ 5.8) may contribute to and increase the displacement in the central Greece area by up to 50 per cent.

For the majority of the polygons in this study the geodetic strain is larger than the seismic strain, which implies that a strain deficit may be released by earthquake activity at some point. That difference between geodetic and seismic strain rates has been suggested also by Ward (1998b) when working with data from Europe. According to the geodetic measurements there is a high rate of deformation in the Gulf of Patras-western Corinth Gulf (polygon 7) and central Gulf of Corinth (polygon 8), of which less than half has been released by seismic strain. However, this strain need not necessarily be released by very large earthquakes, even though the Corinth area has given M S = 6.7 (24 February 1981) earthquake in the past, but could be released by many small earthquakes (Clarke et , 1997). Equally, we cannot be certain that aseismic deformation does not happen, since aseismic creep has been observed in other parts of the Mediterranean region and the Greek-Hellenic Trench [START_REF] Jackson | Active Tectonics of the Aegean Region[END_REF]McKenzie, 1988, 1988a). A continuous aseismic deformation in the uppermost crust of the Corinth rift has been suggested by [START_REF] Briole | Active deformation of the Corinth rift, Greece: Results from repeated Global Positioning System surveys between 1990 and 1995[END_REF]. [START_REF] Jenny | Earthquake recurrence parameters from seismic and geodetic strain rates in the eastern Mediterranean[END_REF] suggested that the major strike-slip zones in the region, the Northern Anatolian Fault and the Kephalonia Fault, experience little to negligible aseismic deformation. In the remaining eastern Mediterranean up to 10-30 per cent of the total deformation is aseismic. The Hellenic Trench is largely uncoupled, with at least 50 per cent and up to 90 per cent of the compressive strain released aseismically.

Consistency of velocity field with orientation of faulting

Faults in the upper crust are shear fractures and represent therefore, lines parallel to which there is no local change of length. We examine the orientation of faulting which is capable of accommodating the observed distribution of geodetic strain rates and whether the calculated orientation of faulting is consistent with the input fault orientation as given in seismic catalogues. [START_REF] Holt | Velocity fields in deforming Asia from the inversion of earthquake-released strains[END_REF] suggested that, if there are no variations in the magnitude of fault slip along the direction parallel to strike, then the only orientations of faulting that can reproduce the horizontal components of the strain tensor correspond to the directions of no length change in the velocity field. These directions are equivalent to the strikes of the nodal planes from any fault plane solution compatible with the local horizontal strain tensor [START_REF] Bourne | Distributed deformation of the South island of New Zealand[END_REF]. Figure 13 presents the predicted strikes of faulting calculated from the continuous-curvature velocity field (black symbols)

together with the fault plane solutions (red symbols) from the combination of the three Where the events overlap we kept the ones from the more recent catalogue. For the GCMT catalogue we used only the events with M S ≥ 5.5. We see close agreement between the orientations of the larger faults as calculated from the strain rate field and lines of no length change (Figure 13). These areas are the North Aegean (extension of North Anatolia Fault Zone), Skyros island, Gulf of Evia, Epirus area, Saronic Gulf and Gulf of Argos. We observe problems in matching orientations in the area surrounding the Gulf of Corinth. The main reasons of misalignment have been analyzed by [START_REF] England | The field of crustal velocity in Asia calculated from Quaternary rates of slip on faults[END_REF] and [START_REF] Holt | Velocity fields in deforming Asia from the inversion of earthquake-released strains[END_REF] while applying the technique in Asian data. According to these authors the misalignment can result either from the local orientations of faults being controlled by pre-existing weaknesses of the crust or because in these areas the strain field changes rapidly over short distances. Both of these explanations can relate to the Gulf of Corinth, [START_REF] Armijo | Quaternary evolution of the Corinth Rift and its implications for the Late Cenozoic evolution of the Aegean[END_REF] suggested two different phases of deformation with the later (1 Myr) reactivating a pre-existing earlier phase (15 Myr ago or earlier) his suggestion is supported by more recent geomorphologic, stratigraphic and microseismicity studies [START_REF] Goldsworthy | Migration of activity within normal fault systems: examples from the Quaternary of mainland Greece[END_REF][START_REF] Lyon-Caen | First results of the CRLN seismic network in the western Corinth Rift: evidence for old-fault reactivation[END_REF]. From our analysis we also showed that the strain pattern does actually change rapidly as we move from the east to the west of the Corinth Gulf in a 120 km long and up to 30 km wide area. We can also add that the catalogues include only relatively few earthquakes, which may not be entirely representative of the longerterm seismic character (indeed, the discrepancy between geodetic and seismic strain also suggests this).

A c c e p t e d M a n u s c r i p t

In order to investigate the level of agreement, we derived a quantitative estimate of the difference between the geodetic directions of no length change and the fault plane

azimuths  (   90 0   
) using:

g s g s      cos cos sin sin cos 1    (2)
where s  is the strike as given in our combined seismic catalogue and g  is the strike calculated from the continuous velocity field.

For the older catalogues only one of these fault planes is given and must be matched with the appropriate no-length-change direction; for the GCMT catalogue, where both conjugate fault planes are given, we use the better-fitting of the two directions. For the combined 300 and 100 year catalogues, the mean absolute difference in orientation is 32°, and for the GCMT catalogue it is 35°.

In view of the close agreement between the orientations of the larger faults and of lines of no length change we conclude that the comparison is possible and the orientation of active faulting compares well with the configuration required to accommodate the present-day distribution of strain rates. The fact that the active faults accommodate the present-day regional strain rates increases the likelihood that the geodetic-seismic strain deficit will be released by future earthquake activity in this area.

Correlation of results with seismicity from 2000-2008

Our conclusions derived from the reprocessing and analysis of the old campaign GPS data, can be justified by the seismicity as this developed from 2000 till 2008 (Figure 14).

The events plotted are from the GCMT and have Mw≥5.0. From Figure 14 is shown that in the polygon 7 we have 4 events during this 8 year period. Three events occurred in The forth event occurred in June 2008 with Mw 6.4 located at 22 km depth [START_REF] Konstantinou | Rapture process and aftreshocks relocation of the 8 June 2008 (Mw 6.4) earthquake in NW Peloponnese, western Greece[END_REF]. Konstantinou et al. mention a possibility of reactivation of a fault structure inherited by previous tectonic phases based on the fault depth and strike. This earthquake activity justifies our conclusion according to which there is a high rate of deformation in the Gulf of Patras (polygon 7) and central Gulf of Corinth (polygon 8), of which less than half has been released by seismic strain. For polygon 7 seismic strain rates are observed in the 300-year catalogue that are not observed in the 100 and 30-year catalogues. Large events are also located in central Aegean (polygon 14). In July 2001 an event with Mw 6.4 occurred in the North Aegean Sea, this event mainly affected the area around the Skyros island. This area is only covered by the 30 and 100 year catalogues, nevertheless polygon 14 is characterized by strong extension and in both catalogues the geodetic strain rates are larger than seismic strain rates. We also show that the North Aegean (extension of North Anatolia Fault Zone) and Skyros island are areas where there is a close agreement between the orientations of the larger faults as calculated from the strain rate field and lines of no length change so that deficit can be expressed by seismicity.

Papadimitriou and Sykes (2001) while evaluating the evolution of the stress field in the northern Aegean Sea, extrapolate their results to the year 2020 suggesting that the region north to Skyros island as one of the three possible candidate areas where large events are more likely to occur.

Conclusion

A c c e p t e d M a n u s c r i p t

In this work a complete processing of 6 years of campaign-type GPS data was performed.

 18 GPS campaigns were successfully combined including areas not fully covered previously. Through the whole study a consistent processing approach was followed and for better reference frame definition the local sites were combined with IGS and EUREF sites.  For the seismic hazard determination the geodetic and seismic strain rates were compared. From the three earthquake catalogues used, the 300-year catalogue is less influenced by individual events or polygon sizes. The Saronic Gulf-eastern

Corinth area has a consistent, low degree of seismic strain release in all three catalogues. The Gulf of Patras and western Gulf of Corinth (polygon 7) appear to be undergoing seismic deformation for the 300-year catalogue at a higher rate than observed in the 100-year catalogue. For the majority of Greek region the geodetic strain rates are larger than the seismic strain rates. This suggests that accumulated strain has not been released yet by earthquake activity.

 Close agreement is observed between the orientations of the larger faults as calculated from the strain rate field and lines of no length change. These areas are the North Aegean (extension of North Anatolia Fault Zone), Skyros island, Gulf of Evia, Epirus area, Saronic Gulf and Gulf of Argos. Orientation mismatching around the Gulf of Corinth is explained by pre-existing weaknesses of the crust and the rapid change of the strain field over short distances. The fact that the active faults accommodate the present-day regional strain rates enhances the likelihood that the geodetic-seismic strain deficit will be released by future earthquake activity in this area.

 Our analysis conclusions were verified by the earthquake activity that followed 3.

  made to remove or make compatible existing constraints have not been successful, leading each group to compute their own individual solutions. This work aims 1) to reprocess 18 previous GPS campaigns, spanning from 1994-2000 and including 280 local sites, combine them rigorously to obtain a unified solution for the area. Our data fill gaps in potential hazardous areas such as Chalkidiki, Grevena, Argos/Saronic gulfs, Egion; (2) assess how the area can be divided into zones of different deformation style and rate; (3) what amount of geodetic strain is released by seismicity.

  sopac.ucsd.edu/dataArchive.

  A c c e p t e d M a n u s c r i p t and[START_REF] Kokkalas | Strain-dependent stress field and plate motions in the south-east Aegean region[END_REF] suggest that the area from eastern Crete to the Dodecanese islands (an area further south than our grid cover) display an arc-parallel extension combined with sinistral shear due to oblique convergence in the area.

  A c c e p t e d M a n u s c r i p t velocity between closely-spaced sites are less likely to cause large errors. The geodetic rigid-body rotation rates show a similar pattern for both the methods applied. Strong clockwise rotation is observed in the Peloponnisos area, central Greece and the northern Aegean Sea, while strong counter-clockwise rotation is observed in the central Aegean, Cyclades, and Sea of Crete. These results agree well with previous studies that took place in the eastern Mediterranean region. Taymaz et al. (1991) noted the predominance of strike-slip faulting in the northern Aegean and extension in central Greece and proposed a brokenslat model where central Greece rotates clockwise and the southern Marmara slats rotate counter-clockwise. This admittedly simplistic configuration of nine broken-slat microplates model is now recognized not to be quantitatively comparable with the full velocity field. Le Pichon et al. (1979; 1995) proposed a two block model where central Greece (north of the Gulf of Corinth) rotates clockwise and the south Aegean and Anatolia rotate counter-clockwise. Extension is observed across the Gulf of Corinth and a wide zone of distributed strike-slip deformation is present in the northern Aegean. McClusky et al. (2000) define rigid blocks in central Anatolia and the southern Aegean.

  propose that lower rates of extension occur across a small number of discrete zones in the central Aegean and SW Anatolia. Most of the Aegean, Greek Arc and Peloponnisos are extruded in SW direction. Propagation of the southern branch of the NAF across the Aegean may have progressively reactivated the Skyros island, the northern Evia basin, the Evia graben, and finally the Corinth rift. Accelerated clockwise rotation may occur within Peloponnisos and central Greece. The kinematic model that they proposed emphasized the extensional end effects of the right-lateral termination of the NAF in the NW Aegean and central Greece.

  required to be shorter than the historical record. For an individual fault a complete earthquake cycle is required. Alternatively, for a region containing multiple faults, the historical seismicity record is required to be long enough to capture a statistical sample of all phases of the seismic cycle, including of course earthquakes, but different parts of the cycle can be represented by different faults. With this constraint, the catalogue duration is almost always too short to give a reliable occurrence rate estimate

  April 2007 with Mw 5.0, Mw 5.1 and Mw 5.2 at 15 km depth[START_REF] Evangelidis | Waveform relocation and focal mechanism analysis of an earthquake swarm in Trichonis Lake, Western Greece[END_REF].



  In a Eurasian reference frame sites around the Chalkidiki and Grevena areas show a small residual motion of around 10 mm/yr. Sites around Attica, Evia, and the eastern and southern Peloponnisos, as well as the Aegean islands, have a SW motion which increases from ~25 mm/yr for the Aegean islands to around 30 mm/yr around Attica, Evia and the eastern Peloponnisos coast and ~40 mm/yr for sites in the southern Peloponnisos. The Patras area shows a residual motion of around 30-35mm/yr.  The central and south Aegean are extending but the Cyclades area is almost strain free. The Grevena area previously thought as aseismic appears to extend in a NNW-SSE direction. The Gulf of Patras and western Corinth Gulf have east-west compressional deformation combined with north-south extension. The compression increases towards the entrance to the Patras Gulf and western Greece. North-south extension is dominant in the Corinth Gulf area. The Evia Gulf appears with a lower degree of extension when compared with the Corinth Gulf. For the Saronic gulf there is pure extension in a NNE-SSW direction. Clockwise rotation is observed in central Greece around the Gulfs of Corinth and A c c e p t e d M a n u s c r i p t Evia, turning to anticlockwise rotation in the south Aegean and southern Peloponnisos.

  the study from 2000-2008 where in polygon 7 four events occurred with Mw 5.0, Mw 5.1, Mw 5.2 and Mw 6.4.

  ., Collier, R., D' Addezio, G., Masana, E., Sakellariou, D., 1996. Direct geological evidence for prior earthquakes on the 1981 Corinth fault (Central Greece). Geophys. Res. Lett. 23, 3795-3798. Papadimitriou, E. E., Sykes, L. R., 2001. Evolution of the stress field in the northern Aegean Sea (Greece). Geophys. J. Int. 146, 747-759. Papazachos, B.C., 1990, Seismicity of the Aegean and the surrounding area. Tectonophysics 178, 287-308. Papazachos, B.C., Papaioannou, C., Papazachos, C.B., Savvaidis, S.A., 1999. Rupture zones in the Aegean region. Tectonophysics 308, 205-221. Papazachos, B.C., and Papazachou (1997). The earthquakes of Greece, Ziti Editions, Thessaloniki. Papazachos, C. B., Kiratzi, A., 1996. A detailed study of the active crustal deformation in the Aegean and surrounding area. Tectonophysics 253, 129-153.Pavlides, S.B., Caputo, R., 2004. Magnitude versus faults' surface parameters: .N., 1998b. On the consistency of earthquake moment release and space geodetic strain rates: Europe. Geophys. J.Int. 135, 1011-1018. Welsch, W.M., 1983. Finite element analysis of strain patterns from geodetic observations across a plate margin. Tectonophysics 97, 57-71.

Figure 1 .

 1 Figure 1. Major tectonic faults and location map of Greece (Papazachos and Papazachou, 1997).

Figure 2 .

 2 Figure 2. GPS networks in Greece used in this study.

Figure 3 .

 3 Figure 3. The 43 Global GPS sites used in the processing stage.

Figure 4 .

 4 Figure 4. Local sites histogram showing the interval in months between the first, and the last

Figure 5 .

 5 Figure 5. Scatter plot showing the east, and north velocity errors in mm/yr relative to the data

Figure 6 .

 6 Figure 6. Velocities for the Greek sites in a Eurasian Fixed reference frame from the period

Figure 7 .

 7 Figure 7. Principal strain rates computed for the 22 polygonal regions. Extensional axes are

Figure 8 .

 8 Figure 8. Geodetic rigid-body rotation rates computed from the polygonal regions

Figure 9 .

 9 Figure 9. Principal strain rates (left) and rotation rates (right) computed from the minimum

Figure 10 .

 10 Figure 10. Comparison of principal geodetic strain rates computed via the two methods

Figure 11 .

 11 Figure 11. (1) Locations of earthquakes taken from the 30 year Harvard CMT catalogue (open

Figure 12 .

 12 Figure 12. Seismic strain rate (white arrows) results from seismic catalogues compared to

Figure 13 .

 13 Figure 13. Calculated directions of no length change (nlc) from the continuous velocity field

  Figure1

Figure3A

  Figure3

Figure4A

  Figure4

Figure5A

  Figure5

Figure6A

  Figure6

Figure7A

  Figure7

Figure8A

  Figure8

Figure10A

  Figure10

Figure11A

  Figure11

Figure12A

  Figure12

Figure13A

  Figure13

Figure14A

  Figure14

Table 2 .

 2 Processing strategy followed during the project. CODE global ionosphere map used to aid ambiguity resolution in wide-lane solutions. Troposphere model Saastamoinen model for dry zenith delay using standard atmospheric conditions; wet zenith delay estimated at 2-hour intervals. Niell mapping function (Niell, 1996) (wet and dry). Tidal loading IERS 1996 Conventions for solid Earth tides incorporated within software. Ocean tide loading specified a priori for 11 major constituents using the CSR4.0 ocean tide model, computed using http://www.oso.chalmers.se/~loading/

	Session choice	Strict 24-hour window
	Baseline choice	Minimise total baseline length and antenna mismatches, whilst
		maximising common observation times.
	Orbits	IGS precise orbits, held fixed.
	Data windowing	10° elevation mask (unless higher in raw data).
	Ionosphere model Ionosphere free observable used; Ambiguity Double-difference ambiguities resolved on per-baseline basis
	resolution	within daily network solutions, using either "Quasi Ionosphere
		Free" model (campaigns Grevena 96, Patras 98, Argos 98) or
		sigma-dependent rounding in wide-lane followed by
		ionosphere-free solutions (all other campaigns).

Table 3 .

 3 Site velocities in ITRF2000, and in a Eurasian reference frame and their formal errors (1-sigma) from TANYA, in mm/yr. The errors have been scaled by the unit variance. The "span" column indicates the interval in months between first and last occupations of the site.

	N v , E v ,

Table 5 .

 5 Seismic strain rates in ppm/yr for the three seismic catalogues and a 15 km thickness of the seismogenic layer. Note that not all polygons are covered by every catalogue.

				a) 30-year catalogue
	Polygon	Shear 1 ( 1 )	Shear 2 ( 2 )	Dilatation (Δ)	Total shear (Γ)	Eps1	Eps2	Azim2 (deg)
		-0.0014	-0.0001	0.0013	0.0014	0.0014	0.0000	87.1
		0.0056	-0.0040	0.0043	0.0068	0.0056 -0.0013	17.9
		-0.0002	0.0035	0.0043	0.0035	0.0039	0.0004	-46.4
		-0.0059	0.0028	0.0032	0.0065	0.0049 -0.0017	-77.1
		-0.0004	-0.0002	0.0004	0.0004	0.0004	0.0000	77.0
		-0.0056	0.0018	0.0023	0.0059	0.0041 -0.0018	-81.0
		-0.0678	0.0108	0.0663	0.0687	0.0675 -0.0012	-85.5
		-0.2379	-0.0773	0.1738	0.2501	0.2120 -0.0382	81.0
		-0.0043	0.0005	0.0026	0.0043	0.0034 -0.0009	-86.5
		-0.0654	-0.0312	0.0664	0.0724	0.0694 -0.0030	77.2
		-0.0081	0.0069	0.0105	0.0107	0.0106 -0.0001	-69.8
		-0.0649	0.0002	-0.0018	0.0649	0.0316 -0.0333	-89.9
		-0.0009	0.0002	0.0006	0.0009	0.0007 -0.0001	-83.2
		0.0000	-0.0001	0.0001	0.0001	0.0001	0.0000	35.6
		-0.0009	0.0003	0.0008	0.0010	0.0009 -0.0001	-80.3
		0.0049	-0.0057	0.0069	0.0075	0.0072 -0.0003	24.8
		-0.0203	-0.0221	0.0296	0.0300	0.0298 -0.0002	66.3
		-0.0392	0.0107	0.0019	0.0407	0.0213 -0.0194	-82.4
		-0.0001	0.0001	0.0001	0.0002	0.0002	0.0000	-70.0
				b) 100-year catalogue
	Polygon	Shear 1 ( 1 )	Shear 2 ( 2 )	Dilatation (Δ)	Total shear (Γ)	Eps1	Eps2	Azim2 (deg)
		-0.0050	0.0000	0.0020	0.0050	0.0035 -0.0015	89.9
		0.0098	0.0063	0.0098	0.0117	0.0108 -0.0009	-16.3
		0.0112	0.0011	0.0066	0.0113	0.0089 -0.0023	-2.9
		-0.0017	0.0261	0.0017	0.0261	0.0139 -0.0122	-46.9
		-0.0178	0.0110	0.0177	0.0209	0.0193 -0.0016	-74.2
		-0.0467	-0.0059	0.0427	0.0471	0.0449 -0.0022	86.4
		-0.0408	0.0000	0.0432	0.0408	0.0420	0.0012	90.0
		-0.0028	0.0203	0.0028	0.0205	0.0117 -0.0089	-48.9
		-0.0240	0.0041	0.0345	0.0244	0.0294	0.0051	-85.1
		-0.0120	-0.0277	0.0042	0.0302	0.0172 -0.0130	56.7
		-0.0160	0.0034	0.0320	0.0163	0.0242	0.0078	-84.0
		0.0040	0.0020	0.0053	0.0045	0.0049	0.0004	-13.0

Acknowledgements

We thank staff and students of numerous institutions who have taken part in the field GPS campaigns on which this analysis is based. Michael Floyd (Oxford University) is thanked for his contribution in processing the Egion campaigns using GAMIT. P.J. Clarke and M.A. King from School of Civil Engineering and Geosciences, Newcastle University, UK, are thanked for their suggestions made during preparation of the manuscript. D.A. Lavallée, Faculty of Aerospace Engineering, Delft University of Technology, for his assistance in using TANYA and commenting on earlier drafts. We acknowledge the very useful comments and suggestions from K.I. Konstantinou, Institute of Geophysics, National Central University, Taiwan, S. Kokkalas and an anonymous reviewer. Support of the author by the Taiwanese National Science Council is greatly appreciated.