Aurore Franco

Cécile Lasserre

H Lyon-Caen

Vladimir Kostoglodov

E Molina

M Guzman-Speziale

D Monterosso

V Robles

C Figueroa

W Amaya

E Barrier

L Chiquin

S Moran

O Flores

J Romero

J A Santiago

M Manea

V C Manea

Fault kinematics in northern Central America and coupling along the subduction interface of the Cocos Plate, from GPS data in Chiapas (Mexico), Guatemala and El Salvador

Keywords: Satellite Geodesy, Kinematics of crustal and mantle deformation, Seismic cycle, Subduction, Caribbean Plate, Central America

INTRODUCTION

The complex surface deformation observed in northern central America results from the interaction between the North America (NA), Cocos (CO) and Caribbean (CA) plates (figure 1). The main active structures related to this interaction are the Polochic and Motagua left lateral strike-slip faults at the NA/CA boundary, the north-striking grabens south and east of it in Guatemala and Honduras, the mid-America trench and the volcanic arc associated with the CO subduction under the CA plate (e.g., [START_REF] Plafker | Tectonic Aspect of the Guatemala Earthquake of 4 February 1976[END_REF][START_REF] Burkart | Neogene North American-Caribbean plate boundary: offset along the Polochic fault[END_REF][START_REF] Burkart | Extension and rotation of crustal blocks in northern Central America and effect on the volcanic arc[END_REF]. In the past decade, sev- Feuillet et al., 2003. PMF: Polochic-Motagua faults. EF: Enriquillo Fault. TD: Trinidad Fault. GB: Guatemala Basin. Topography and bathymetry are from Shuttle Radar Topography Mission [START_REF] Farr | Shuttle Radar Topography Mission produces a wealth of data[END_REF] and [START_REF] Smith | Global seafloor topography from satellite altimetry and ship depth soundings[END_REF], respectively. Plate velocities relative to Caribbean plate are from Nuvel1 [START_REF] Demets | Current plate motions[END_REF] for Cocos plate, [START_REF] Demets | GPS geodetic constraints on Caribbean-North America plate motion[END_REF] for North America plate and [START_REF] Weber | GPS estimate of relative motion between the Caribbean and South American plates, and geologic implications for Trinidad and Venezuela[END_REF] for South America plate. eral seismological and geodetic studies have tried to quantify the kinematics of these structures accommodating the active deformation (e.g. [START_REF] Demets | A new estimate for present-day Cocos-Caribbean plate motion: Implications for slip along the Central American volcanic arc[END_REF]Guzman-Speziale, 2001;[START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF] and to understand the different factors and the tectonic forces that control this deformation (e.g. [START_REF] Alvarez-Gomez | Constraints from finite element modeling on the active tectonics of northern Central America and the Middle America Trench[END_REF][START_REF] Rodriguez | A GPS and modelling study of deformation in northern Central America[END_REF][START_REF] Correa-Mora | Evidence for weak coupling of the Cocos plate subduction interface and strong coupling of the volcanic arc faults from modeling of GPS data: El Salvador and Nicaragua[END_REF].

From GPS measurements and modeling, [START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF] documented a 20 mm/yr rate of the NA/CA relative motion in easternmost Guatemala, mostly accommodated across the Motagua fault. This rate decreases westwards, reaching nearly 0 mm/yr near the Mexico-Guatemala border, as part of the deformation is being transferred southwards into the grabens (mainly the Guatemala city graben). [START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF] also suggest a weak coupling at the CO/CA subduction interface and a dextral slip component across the volcanic arc in Guatemala. More recent studies [START_REF] Correa-Mora | Evidence for weak coupling of the Cocos plate subduction interface and strong coupling of the volcanic arc faults from modeling of GPS data: El Salvador and Nicaragua[END_REF][START_REF] Rodriguez | A GPS and modelling study of deformation in northern Central America[END_REF][START_REF] Alvarado | Forearc motion and deformation between El Salvador and Nicaragua: GPS, seismic, structural, and paleomagnetic observations[END_REF] have used a dense GPS network in Honduras, El Salvador and Nicaragua to build a regional model of the deformation of the western part of the Caribbean plate. A main outcome is that the extension relative to the stable Caribbean plate is not limited to Guatemala but is observed in a broader area (Guatemala and western Honduras, [START_REF] Rodriguez | A GPS and modelling study of deformation in northern Central America[END_REF]. Another important result is that coupling at the CO/CA subduction interface offshore El Salvador and Nicaragua, inferred from finite element modeling ( Álvarez-Gómez et al., 2008;[START_REF] Correa-Mora | Evidence for weak coupling of the Cocos plate subduction interface and strong coupling of the volcanic arc faults from modeling of GPS data: El Salvador and Nicaragua[END_REF], is likely weak as well. These models also suggest that the volcanic arc is a rheologically weak zone. It separates the undeformed, trenchparallel moving forearc, which is pinned to the NA plate [START_REF] Correa-Mora | Evidence for weak coupling of the Cocos plate subduction interface and strong coupling of the volcanic arc faults from modeling of GPS data: El Salvador and Nicaragua[END_REF][START_REF] Alvarado | Forearc motion and deformation between El Salvador and Nicaragua: GPS, seismic, structural, and paleomagnetic observations[END_REF], and the wedge-shaped western Caribbean plate, which inner deformation is influenced by the direction of the NA/CA motion relatively to the strike of the curved Polochic-Motagua fault system ( Álvarez-Gómez et al., 2008;[START_REF] Rodriguez | A GPS and modelling study of deformation in northern Central America[END_REF].

In this paper, we densify the [START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF] data set including a third campaign of GPS measurements in Guatemala. We also extend the study area using new GPS measurements in Chiapas (southern Mexico) and El Salvador. This allows us not only to refine previous results but also to complement the regional dataset and to propose a kinematic block model in the critical area of the triple junction between the CO/CA/NA plates. In particular, we discuss the present-day GPS derived coupling along the subduction zone from southern Mexico to El Salvador.

We first present the GPS dataset, the processing strategy and a first order analysis of the GPS velocity field in terms of strain partitioning across the major volcano-tectonic structures. The GPS velocity field at the regional scale is then inverted using the DEFN-ODE model that combines relative block rotations and elastic deformation due to coupling at the block boundaries [START_REF] Mccaffrey | Crustal block rotations and plate coupling[END_REF]. Finally we discuss the implication of this new dataset and modeling for the understanding of the complex regional tectonics.

DATA AND PROCESSING

GPS sites and data acquisition

We use GPS data from 34 campaign sites (figure 2, Table 1) : 23 sites in Guatemala, 3 sites in El Salvador, and 8 sites in Chiapas (southern Mexico). Data from 4 regional permanent stations belonging to the Servicio Sismologia Nacional (SNN) mexican network (site TPCH) or to the International GPS Service (IGS) network (sites ELEN, HUEH, SSIA) complement this campaign data set (figure 2).

The first two campaigns of measurements in Guatemala were carried out in February 1999 and 2003 and are described in [START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF]. Remeasurements were done in January 2006, (including 6 new sites first measured in 2003) using 9 Ashtech ZXtrem receivers with Geodetic IV antennas, and 3 Trimble 5700 receivers with Zephyr Geodetic antennas. All sites were occupied for at least two sessions of 12h to 24h, with two sites that were measured continuously during six and ten days (PIN and COB, respectively, figure 2), as in 1999 and 2003.

The three sites in El Salvador were measured in February 2003 during the campaign in Guatemala, with 48h of occupation at each site, using Ashtech Z12 receivers and Geodetic III and IIA antennas. They were remeasured in march 2006 using Z-Max Thales receivers and antennas, together with the ZAC guatemalan site (figure 2), during four consecutive, 10h-long daily sessions.

Measurements in Chiapas, conducted by UNAM (Universidad Nacional Autónoma de Mexico), began in 2002 and were repeated each year until 2005. Leica SR520 receivers were used with Leica A504 Dorne Margelin antennas. Each site was measured during two to five 24h sessions. Table A1 in Supporting Information summarizes the sites occupation.

Processing strategy

We use the GAMIT software [START_REF] King | Documentation for the GAMIT GPS Analysis software[END_REF] to process data from the campaign and permanent stations mentioned above. The GAMIT unconstrained solutions of daily station coordinates and their associated covariances are combined with selected SOPAC (Scripps Orbital and Permanent Array Center) solutions, using GLOBK [START_REF] Herring | GLOBK, Global Kalman filter VLBI and GPS analysis program[END_REF], to obtain stations positions and velocities in the ITRF2000 reference frame. Velocity uncertainties are estimated using a Markovian error model during the GLOBK daily solution combination [START_REF] Herring | GLOBK, Global Kalman filter VLBI and GPS analysis program[END_REF]. We authorize for each station a random walk of 2 mm/ √ yr around their position. Velocity vectors are determined without introducing a Markovian noise. Several earthquakes occurred within our regional network between 1999 and 2006. We select all earthquakes with an hypocentral depth shallower than 30 km, and the Mw≥6 deeper earthquakes that are located within 500 km from the center of our study area (15.1 • N, 269.7 • E). We estimate their rupture parameters (slip and rupture size), based on the CMT catalog and scaling laws [START_REF] Wells | New Empirical Relationships among Magnitude, Rupture Length, Rupture Width, Rupture Area, and Surface Displacement[END_REF]. For the 2001, Mw=7.7 earthquake in El Salvador in particular, these parameters are given by [START_REF] Bommer | The El Salvador Earthquakes of January and February 2001: context, characteristics and implications for seismic risk[END_REF] and [START_REF] Vallée | The 13 January 2001 El Salvador earthquake: A multidata analysis[END_REF]. We use an elastic half-space model [START_REF] Okada | Surface deformation due to shear and tensile faults in a half-space[END_REF] to estimate the cumulative coseismic displacements associated with the selected earthquakes at each GPS site (Tables A2 andA3), and take them into account during the GLOBK combination process. This changes velocities by up to 1.8 mm/yr for site CON in eastern Guatemala (figure 2) but no more than 1mm/yr for the other sites.

Due to the short overlapping in time between measurements made within the Guatemala/El Salvador sub-network, and those made within the Chiapas sub-network (Table A1), we first compute two independent GLOBK velocity solutions for these two subnetworks, following the procedure described above. While both referenced to ITRF2000 with comparable residuals (∼ 5 mm on positions, 1.3 mm/yr on velocities for Guatemala/El Salvador, ∼ 4.5 mm on positions, 2.5 mm/yr on velocities for Chiapas), the two resulting velocity fields are not fully consistent, with common sites such as TPCH showing different velocity vectors (figure A1a). To make both velocity fields consistent, we use the more robust Guatemala/El Salvador solution as a reference, and we estimate the angular velocity that best adjusts the Chiapas velocity vectors with the Guatemala/El Salvador ones, at the common regional site TPCH and at the IGS stations used by GLOBK for the stabilization in ITRF2000 (figure A1a). We use the same strategy to adjust the resulting Guatemala-El Salvador-Chiapas velocity field to that of [START_REF] Demets | Present motion and deformation of the Caribbean plate: constraints from new GPS geodetic measurements from Honduras and Nicaragua[END_REF] in ITRF2000 (figure A1b). Angular velocities used to obtain the ITRF2000 velocities (Table 1) are listed in Table A4.

This procedure allows us to use the CA/ITRF2000 and NA/ITRF2000 angular velocities estimated by [START_REF] Demets | Present motion and deformation of the Caribbean plate: constraints from new GPS geodetic measurements from Honduras and Nicaragua[END_REF] (Table A5) to reference our final regional velocity field in the CA and NA reference frames (figure 3). Note that we do not propagate the uncertainties of the angular velocities into the uncertainties of our sites velocities in these two frames. We estimate them to be on the order of 1 mm/yr at our regional stations, by comparison with velocity fields obtained using different values of the CA/ITRF2000 and NA/ITRF2000 angular velocity within their error bars. We neglect them in the following.

GPS VELOCITY FIELD AND ANALYSIS AT FAULT SCALE

Figure 3 illustrates the complexity of the velocity field in the CO/CA/NA triple junction area. We first describe its main features in the NA and CA reference frames before analyzing the slip partitioning among the different faults, the volcanic arc and the subduction zone.

Overall description

In the North America reference frame, the three sites ELEN, RUB and MOD, to the north, form a consistent group with small residual velocities (Figure 3a), comparable to those of sites CHET, VILL and CAMP on the Yucatan peninsula. On the first order, they can be considered as part of the stable North America plate, as the Yucatan sites (Marquez-Azua & DeMets, 2009).

In the Caribbean reference frame, the three sites CON, TEXW and TEJU show small residual velocities and can be considered at the first order as representing the stable Caribbean plate (Figure 3b). In the western, wedge-shaped part of the plate in between the Motagua fault and the volcanic arc, an east-west internal extension is observed across the grabens, confirming results from [START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF].

Figure 3 also shows the overall left-lateral motion between the NA and CA plates and the associated velocity gradient across the Polochic and Motagua faults. In the NA frame, all stations in Chiapas have a consistent motion towards north-east, roughly perpendicular to the trench, suggesting coupling at the CO/NA slab interface (Figure 3a). In contrast, in the CA frame, the velocities of the coastal sites south of the volcanic arc are mostly trenchparallel (Figure 3b). This could result from a low coupling at the CO/CA slab interface, as already proposed by [START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF]. However, the velocities in El Salvador also indicate a rightlateral motion across the volcanic arc relatively to the stable CA plate (Figure 3b). The velocity field in southern Guatemala thus reflects the combined effects of coupling at the CO/CA interface and motion across the arc.

Strain partitioning across the major tectonic structures

Assuming first that the effect of the CO subduction on the velocity field is low in Guatemala and El Salvador [START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF], we quantify the slip rates across the Polochic-Motagua faults (PMF), the grabens south of them and the volcanic arc, and refine the previous analysis by [START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF], taking advantage of the network densification and remeasurement. We project the ITRF2000 velocities along 3 north-south trending profiles (East E, Central C, and West W) roughly perpendicular to the Polochic and Motagua faults (Figures 3b,4a,4d,4f). We also project the horizontal CA-fixed velocities along a southern, eastwest trending profile (South S) perpendicular to the grabens (Figure 3b and5).

The Polochic-Motagua fault zone

We first use a one fault, half-space elastic model (e.g. [START_REF] Savage | Geodetic determination of relative plate motion in Central California[END_REF] as in [START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF]. We invert for the interseismic velocity, the locking depth and the location of the fault trace along profiles E and C (Figures 4a to e). While strain accumulation is clearly concentrated on the northern trace of the Motagua fault on profile E, it seems offset by ∼15 km north of it on profile C. Based on the χ 2 =1 contour, the far-field velocity and locking depth ranges are 18-22 mm/yr (best fit 20 mm/yr) and 14-28 km (best-fit 20 km) for profile E (best constrained model), and 14-26 mm/yr (best fit 20 mm/yr) and 12-66 km (best-fit 39 km) for profile C. The 20 km best-fit locking depth for profile E is consistent with the maximum depth of the seismogenic zone across the Polochic-Motagua fault system, derived from the present crustal seismicity distribution [START_REF] Franco | Seismicity and Crustal Structure of the Polochic-Motagua Faults System Area (Guatemala)[END_REF]. We thus assume a constant 20 km locking depth along the entire fault system, and favor a model with a velocity of 20 mm/yr across profile E, decreasing to 16 mm/yr across profile C (figures 4a to 4e). At least 4 mm/yr seems to be accommodated by the PMF across profile W (velocity difference between SOL and QUE, figure 4f), although the limited length of the profile and its small number of sites do not allow any elastic modeling.

The new data set and analysis thus confirm the decrease of the far-field velocity across the PMF from eastern to western Guatemala, probably tending towards zero in the triple junction area. It also confirms the dominant role of the Motagua fault with respect to the Polochic fault in the accommodation of the deformation associated with the NA and CA relative motion [START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF]). An homogeneous half-space elastic model including the two faults (with a fixed location) shows that at least 88% of the total strain is accommodated by the Motagua fault in the eastern part of Guatemala (Figure 4c). This fault is generally considered as the geological boundary between the NA and the CA plates (e.g. [START_REF] Carfantan | Du domaine cordilliérain Nord-Américain au domaine Caraïbe : étude géologique du Mexique Méridional[END_REF]Donelly et al., 1990[START_REF] Martens | High-pressure Belts of Central Guatemala: The Motagua Suture and the Chuacs Complex[END_REF]. However the area in between the Polochic and Motagua faults is a wide complex metamorphic zone. Furthermore there is no doubt on the Holocene activity of the Polochic fault as attested by the similarities of its morphology, Holocene slip rate, historical and present-day seismicity with that of the Motagua fault [START_REF] Carr | Underthrusting and Quaternary faulting in northern Central America[END_REF][START_REF] Burkart | Offset across the Polochic fault of Guatemala and Chiapas[END_REF][START_REF] Schwartz | Quaternary faulting along the Caribbean North American plate boundary in Central America[END_REF][START_REF] Burkart | Neogene North American-Caribbean plate boundary: offset along the Polochic fault[END_REF][START_REF] White | The Guatemala earthquake of 1816 on the Chixoy-Polochic fault[END_REF][START_REF] White | Destructive upper-crustal earthquakes of Central America since 1900[END_REF], Ambraseys and Adams, 2001[START_REF] Kovach | Early Earthquakes of the Americas[END_REF][START_REF] Franco | Seismicity and Crustal Structure of the Polochic-Motagua Faults System Area (Guatemala)[END_REF][START_REF] Suski | Localization and characterization of an active fault in an urbanized area in central Guatemala by means of geoelectrical imaging[END_REF]). The lack of strain accumulation across the Polochic fault remains a puzzling result. We investigate below the possible influence of rigidity contrasts across the Polochic-Motagua fault zone and of post-seismic effects following the 1976 earthquake on the present-day velocity field.

A recent Receiver Function study suggests variations of the Moho depth (resp. Vp/Vs ratio) across the Polochic and Motagua faults [START_REF] Franco | Seismicity and Crustal Structure of the Polochic-Motagua Faults System Area (Guatemala)[END_REF], with a Moho depth thinner by 4-6 km (resp. a Vp/Vs decreased by 6-7%) in between the two faults. Such variations, that are likely related to the geological history of this region, could result in asymmetric velocity profiles across faults, as- sociated with contrasts in elastic parameters or elastic thicknesses on both sides of the faults [START_REF] Le Pichon | Asymmetry inelastic properties and the evolution of large continental strike-slip faults[END_REF][START_REF] Schmalzle | Strain accumulation across the Carrizo segment of the San Andreas fault, California: impact of laterally varying crustal properties[END_REF][START_REF] Chéry | Geodetic strain across the San Andreas Fault reflects elastic plate thickness variations (rather than fault slip rate)[END_REF][START_REF] Jolivet | Thin-plate modeling of interseismic deformation and asymetry across the Altyn Tagh fault zone[END_REF]. Our tests on velocities of profile E, using a modified half-space elastic model taking into account asymmetry (formulation of Le [START_REF] Le Pichon | Asymmetry inelastic properties and the evolution of large continental strike-slip faults[END_REF], still show that the Polochic fault does not accommodate more than ∼15% of the total deformation (figure 4c).

To estimate the contribution to the present-day velocity field of postseismic relaxation related to the 1976 earthquake, we use a 3D-viscoelastic model developed by [START_REF] Yu | Surface deformation due to a strike slip fault in an elastic gravitational layer overlying a viscoelastic gravitational half space[END_REF]. We consider an elastic layer, with a thickness H=30 km and a shear modulus µ = 3.10 10 N.m, above an homogeneous viscous half-space, characterized by a viscosity η ranging from 10 18 to 10 21 Pa.s. The 1976 rupture on the Motagua fault is modeled using an infinitely-long vertical fault extending from the surface to a depth of 15 km, with an homogeneous coseismic slip of 1.5 m consistent with field observations by [START_REF] Plafker | Tectonic Aspect of the Guatemala Earthquake of 4 February 1976[END_REF]. Figure A2 shows the modeled post-seismic displacements as a function of time normalized by the relaxation time ( t τ , with τ = 2 η µ ), for various distances y from the fault. Maximum velocities related to postseismic relaxation are on the order of 0.6 mm/yr: 0.4 cm between the 1999 and 2006 GPS campaigns, at a distance of 40 km from the Motagua fault trace and for a viscosity of 10 19 Pa.s. In any case, we conclude that postseismic effects can not explain the apparent lack of strain accumulation across the Polochic fault.

East-West Extension

The southern profile S shows an extension rate of ∼9 mm/yr across the Caribbean graben series north of the volcanic arc, between sites TEJU to the east and TPCH in south-western Chiapas (figure 5). This gives only a first order estimate as we do not take into account the rotation of microblocks separating the active grabens. Most of the extension is concentrated across the Guatemala city grabens (rate of 5 ± 2 mm/yr), which confirms previous estimation by [START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF]. The remaining extension, given our GPS network geometry and error bars, is not clearly localized on specific structures. Seismicity and fault plane solutions (Guzman-Speziale, 2001[START_REF] Caceres | Crustal deformation in northern Central America[END_REF][START_REF] Franco | Seismicity and Crustal Structure of the Polochic-Motagua Faults System Area (Guatemala)[END_REF], as well as complementary GPS observations [START_REF] Rodriguez | A GPS and modelling study of deformation in northern Central America[END_REF]; note that data are referenced to ITRF2005 instead of ITRF2000 in this study) and finite element modeling [START_REF] Alvarez-Gomez | Constraints from finite element modeling on the active tectonics of northern Central America and the Middle America Trench[END_REF] in northern central America indicate that some extension is accommodated eastward from Guatemala city, up to north-eastern Honduras, across the Ipala graben and the depression of Honduras in particular.

Volcanic arc

In the Caribbean plate reference (figure 3b), GPS sites along the coast in Guatemala and El Salvador (TPCH, MAZ, CHL, SIGN, SSIA) show velocity vectors parallel to the Mid-America Trench (MAT), consistent with dextral motion across the volcanic arc at a rate of up to 14mm/yr in El Salvador (see TEJU-SSIA-SIGN velocity gradient in figure 4a). Such dextral shear is in agreement with previous GPS measurements in Guatemala by [START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF]. Similar rates are also observed by GPS in El Salvador and Nicaragua (≃15 mm/yr; [START_REF] Turner | Kinematics of the Nicaraguan forarc from GPS geodesy[END_REF][START_REF] Correa-Mora | Evidence for weak coupling of the Cocos plate subduction interface and strong coupling of the volcanic arc faults from modeling of GPS data: El Salvador and Nicaragua[END_REF][START_REF] Alvarado | Forearc motion and deformation between El Salvador and Nicaragua: GPS, seismic, structural, and paleomagnetic observations[END_REF] and Costa-Rica [START_REF] Norabuena | Geodetic and seismic contraints on some seismogenic zone processes in Costa Rica[END_REF] or attested by dextral mechanisms of recent crustal earthquakes (M6.6, 02/13/2001 earthquake in El Salvador in particular, [START_REF] Canora | Geological and Seismological Analysis of the 13 February 2001 Mw 6.6 El Salvador Earthquake: Evidence for Surface Rupture and Implications for Seismic Hazard[END_REF]. It can be interpreted as dextral slip on a northwest-striking, intra-arc, sub-vertical fault, bounding to the north an independent forearc sliver. Such dextral fault system within the volcanic arc has been evidenced in the field in El Salvador (Martinez-Diaz et al.,2004[START_REF] Corti | Active strikeslip faulting in El Salvador, Central America[END_REF]. Satellite images and topography analysis suggest that it may continue westwards under the volcanic deposits in Guatemala [START_REF] Carr | Underthrusting and Quaternary faulting in northern Central America[END_REF]. In Nicaragua, [START_REF] Lafemina | Bookshelf faulting in Nicaragua[END_REF] suggest that the dextral shear is rather accommodated by book-shelf faulting involving northeaststriking left-lateral faults perpendicular to the trench. Slip partitioning related to the obliquity of the convergence of the Cocos plate has been proposed to explain the observed dextral shear across the arc [START_REF] Demets | A new estimate for present-day Cocos-Caribbean plate motion: Implications for slip along the Central American volcanic arc[END_REF]. However, it would require strong coupling along the subduction interface, in contradiction with the most recent studies [START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF], Turner et al., 2007, Correa-Mora et al., 2009, this paper). Instead, the trench-parallel, northwestwards, forearc motion may be related to the indentation of the Cocos ridge on the Caribbean plate, offshore Costa Rica [START_REF] Lafemina | Fore-arc motion and Cocos ridge collision in Central America[END_REF][START_REF] Alvarado | Forearc motion and deformation between El Salvador and Nicaragua: GPS, seismic, structural, and paleomagnetic observations[END_REF]).

REGIONAL MODELING OF THE VELOCITY FIELD

To refine the proposed first-order interpretation of the crustal deformation, we model the GPS velocity field using the 3D-inversion method DEFNODE developed by [START_REF] Mccaffrey | Crustal block rotations and plate coupling[END_REF]. GPS velocities are considered as resulting from the combination of relative block rotations and elastic deformation due to coupling at the block boundaries. The relative block motions are defined by spherical Earth angular velocity vectors (Euler rotation poles and rates) while the interseismic deformation is modeled as backslip on the faults that separate blocks [START_REF] Okada | Surface deformation due to shear and tensile faults in a half-space[END_REF][START_REF] Savage | A dislocation model of strain accumulation and release at a subduction zone[END_REF]. The faults at the boundary of the finite blocks are defined in 3D, by a series of nodes along the fault planes (forming an irregular grid of points along strike and down dip). Fault locking is parameterized at each node by a coupling factor φ, which represents the fractional part of the relative block motion that is not accommodated by steady, aseismic slip. φ ranges between 0 (no coupling) and 1 (full coupling). Block angular velocities and coupling factors φ can be inverted by minimizing the misfit between observations (e.g. GPS velocities and slip vectors) and predicted data, using a simulated annealing method.

Input data, model geometry

We constrain our models using the horizontal GPS velocities and their associated uncertainties listed in Table 1. However, given the poor density of points that defines the east-west extension in the westernmost part of the Caribbean plate, we can not model this extension and do not take into account velocities at sites QUE, CML, GUAT, PIN (Figure 3). We also use slip vectors of subduction earthquakes of Mw≥5.9 (Table 2) from the complete CMT catalog, to provide constraints on the slip direction along the subduction plane.

We define two sets of model geometries : (1) a 3 blocks (North America, NA, Caribbean, CA, and Cocos, CO) and 2 faults (Motagua fault, MF, and Middle-America Trench, MAT) model, called 3B model hereafter, and (2) a 4 blocks (NA, CA, CO, and the forearc microplate, AR) and 3 faults model (MF, MAT, and the Volcanic Arc Fault, VAF), called the 4B model (figure 6). The MF that marks the NA/CA boundary follows the Motagua fault surface trace to the east. To the west, we extrapolate this fault trace under the volcanic deposits, and connect it to the Polochic fault near the Mexican border, then to the MAT (Figures 2 and6). The MF is considered as a vertical fault. The VAF, defined as a vertical fault as well, is the continuation to the west, below the volcanic arc, of the well known trace of the dextral fault that runs from Costa-Rica to northern Salvador (e.g. [START_REF] Corti | Active strikeslip faulting in El Salvador, Central America[END_REF]. We use the bathymetry and the microseismicity distribution relocated by Engdhal & Villasenor (2002) to delineate the depth contours of the Cocos plate slab (Figure 6).

Model parameters

For both the 3B and 4B models, the CO and NA angular velocity vectors relative to the CA block are from DeMets et al. ( 2007) and DeMets et al. (1990) (Nuvel1-A), respectively. There is no clear evidence neither onland, nor offshore, nor in the seismic activity, that the western continuation of the MF west of its junction with the volcanic arc (named MFw hereafter, Figure 6), exists and is active.

As we need to materialize all block limits between the CO/NA/CA plates in the DEFNODE model, we assign to MFw a uniform full coupling, from the surface down to 250 km. This is equivalent in the model to considering that the AR block is pinned to the NA block [START_REF] Alvarez-Gomez | Constraints from finite element modeling on the active tectonics of northern Central America and the Middle America Trench[END_REF][START_REF] Rodriguez | A GPS and modelling study of deformation in northern Central America[END_REF]. In any case, the geodetic data onland would not allow to test other hypothesis.

We have tested that our main conclusions are not sensitive to this modeling choice.

In the 3B model, we invert for the coupling along the MF (east of MFw, Figure 6) and the MAT. In the 4B model, we fix the coupling along the MF according to the results from the 3B model and invert for the coupling along the VAF, MAT and for the AR/CA angular velocity (Figure 6). We discuss the trade-off between the inverted parameters.

Along the MAT subduction interface, we assume that coupling can occur down to 25 km depth (Marquez-Azua & DeMets, 2003[START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF]. The locking depth along the MF is initially fixed to 20 km, as deduced from the analysis in section 3.2.1, and to 15 km along the VAF (same order of magnitude as found eastward in El Salvador, [START_REF] Correa-Mora | Evidence for weak coupling of the Cocos plate subduction interface and strong coupling of the volcanic arc faults from modeling of GPS data: El Salvador and Nicaragua[END_REF].

A series of resolution tests (Figure A3) indicate that our data distribution allows to constrain along-strike variations of coupling on the subduction interface while along-dip variations can not be resolved.

Results

3B models

Given the complex internal deformation (extension in the western part, dextral slip across the volcanic arc) within the CA block as defined in the 3B model, this model obviously can not account for the observed GPS velocities on the CA block. We thus only consider the GPS vectors on the NA block for this first set of models.

We start from a simple parametrization of the MF and MAT (same coupling factor at all nodes for each fault) and progressively allow for potential along-strike variations in the coupling. The analysis of the residual velocities of this series of test models shows that, given our data distribution, a model with two sections along the MF (MFe, MFc for the east and central sections, respectively) as well as along MAT (MAT ch and MATgs under Chiapas and Guatemala/El Salvador respectively), is a good compromise to account for the observed GPS velocities (Figure 7). Assuming a constant locking depth of 20 km all along the MF (see discussion in section 3.2.1), the resulting 3B model shows a decreasing coupling on the fault from eastern (φ=0.9 along MFe) to central (φ=0.38 along MFc) Guatemala (Figure 7). We interpret these coupling lateral variations as resulting from the westwards velocity decrease on the MF. The coupling values correspond to average velocities of 18 mm/yr and 8.5 mm/yr along the MFe and MFc respectively. These velocities that represent average values on fault section are consistent with that deduced from the elastic half-space modeling on local profiles in section 3.2.1. Figure 7 also shows the resulting coupling along the subduction zone below Chiapas that is fairly high (φ=0.79 along MAT ch ) compared to the coupling below Guatemala and El Salvador (φ=0.34 along MATgs). This apparent contrast of coupling is better constrained and discussed from the 4B models below.

4B models

All GPS vectors are now inverted after fixing the coupling along the MFe and MFc to the values estimated in the previous section. Coupling along the VAF is considered uniform given our data distribution, although we would expect it to decrease westwards, as along the MF, due to the CA plate internal extension.

Assuming first that the VAF is fully locked on the upper 15 km (φV AF =1, corresponding to a velocity of 15 mm/yr ), the best fit model (Figure 8) also shows lateral variations of coupling along the subduction zone as in model 3B, with a CO/NA high coupling offshore Chiapas (φ=0.61 along MAT ch ), and a CO/AR low coupling south of the volcanic arc offshore Guatemala and El Salvador (φ=0.25 along MATgs). However there is a trade-off in the inversion of the forearc velocity field between the contributions of the AR/CA rotation and the coupling along the VAF and MAT in Guatemala and El Salvador. In order to evaluate these relative contributions, we fix the CO/NA coupling (φ=0.61 along MAT ch , figure 9) and run a series of inversion of the AR/CA angular velocity for different sets of coupling along the VAF and the MATgs. Note that in El Salvador, [START_REF] Correa-Mora | Evidence for weak coupling of the Cocos plate subduction interface and strong coupling of the volcanic arc faults from modeling of GPS data: El Salvador and Nicaragua[END_REF] suggest that φV AF is larger than 0.85.

All best fit models indicate a CO/AR coupling (φMAT gs ) around 0.25±0.1, independently of the φV AF value (Figure 9), confirming the contrast with the CO/NA coupling (φMAT ch ). Pacheco et al., 1993 estimated a low coupling value along the CO subduction interface (0.2), from the analysis of the cumulative seismic moment during the XXth century. However, this was an "averaged" value from Chiapas to Costa Rica. A GPS-derived, low coupling value offshore El Salvador was obtained more recently by Correa-Mora et al. ( 2009), consistent with our results.

SUMMARY AND DISCUSSION

The new GPS measurements presented herein represent the first from the Chiapas region of Mexico, complement previous work from Guatemala and El Salvador, and enable the refinement of regional kinematic models previously proposed for the Cocos-North America-Caribbean plate triple junction [START_REF] Lyon-Caen | Kinematics of the North America-Caribbean-Cocos plates in Central America from new GPS measurements across the Polochic-Motagua fault system[END_REF][START_REF] Plafker | Tectonic Aspect of the Guatemala Earthquake of 4 February 1976[END_REF]. The joint analysis of our results from the elastic half space modeling (section 3) and the DEFNODE block modeling (section 4) of the GPS velocity field brings new constraints on the kinematics of the active structures as well as on the coupling along the subduction zone, as summarized in figure 10. Our results are in overall agreement with recent models based on GPS data in El Salvador, Honduras and Nicaragua and on geological and strain rate data [START_REF] Alvarez-Gomez | Constraints from finite element modeling on the active tectonics of northern Central America and the Middle America Trench[END_REF][START_REF] Correa-Mora | Evidence for weak coupling of the Cocos plate subduction interface and strong coupling of the volcanic arc faults from modeling of GPS data: El Salvador and Nicaragua[END_REF][START_REF] Rodriguez | A GPS and modelling study of deformation in northern Central America[END_REF].

Regional fault kinematics

The Motagua fault concentrates the present day strain accumulation due to the NA/CA relative motion. The absence of resolvable strain accumulation across the active Polochic fault cannot be explained in the modeling by postseismic relaxation or rheological lateral variations. This suggests that slip on the Polochic and Motagua faults may vary with time as a result of mechanical interactions within this strike slip fault system. Transient slip rate and activity switch between faults have already been observed from geodesy (east California shear zone, [START_REF] Peltzer | Transient strain accumulation and fault interaction in the Eastern California shear zone[END_REF], or from historical seismicity analysis and modeling (north and east Anatolian fault, [START_REF] Hubert-Ferrari | Long-term elasticity in the continental lithosphere; modelling the Aden Ridge propagation and the Anatolian extrusion process[END_REF].

The NA/CA motion decreases from 18-22 mm/yr in eastern Guatemala to 14-20 mm/yr in Central Guatemala assuming a uniform locking depth of 14-28 km (best contrained by profile E, Figure 4) and to ∼4 mm/yr in western Guatemala. West of the Mexican border, the Motagua fault likely connects with the Polochic fault but does not accommodate any significant deformation. The east-west extension across the grabens in Guatemala at a rate of ∼ 9 mm/yr is mostly localized on the Guatemala city graben ( ∼5 mm/yr), while the remaining part is not clearly localized on specific grabens. A more complete discussion of the extension accommodation from western Guatemala to Honduras would require a joint analysis of all regional GPS data in a common reference frame. Up to 15 mm/yr of dextral motion could be accommodated across the volcanic arc in El Salvador and southeastern Guatemala, consistent with estimations by [START_REF] Alvarado | Forearc motion and deformation between El Salvador and Nicaragua: GPS, seismic, structural, and paleomagnetic observations[END_REF] in El Salvador and Nicaragua.

Kinematically, the extension in the western wedge of the Caribbean plate requires a westward decrease of both the Polochic-Motagua fault slip-rate and the volcanic arc fault slip-rate (figure 10). This decrease can only be partly modeled herein given the limited spatial sampling of our velocity field.

Lateral coupling variation along the Mid-America Trench

The coupling along the Cocos subduction zone varies along the Mid-America Trench with a fairly high coupling (∼0.6) offshore Chiapas and a low coupling offshore Guatemala (∼0.25). Lateral variations of coupling along subduction zones have been evidenced by cumulative seismic moment and geodetic studies in many areas (e.g. [START_REF] Pacheco Javier | Nature of Seismic Coupling Along Simple Plate Boundaries of the Subduction Type[END_REF]. In recent years, the increasing space and time density of GPS data, in particular, have allowed to obtain maps of interseismic coupling along subduction interfaces, that are generally interpreted in terms of seismic hazard assessment, as in Sumatra [START_REF] Prawirodirdjo | Geodetic observations of intersismic strain segmentation at the Sumatra subduction zone[END_REF][START_REF] Chlieh | Heterogeneous coupling of the Sumatra megathrust constrained by geodetic and paleogeodetic measurements[END_REF], Kamchatka [START_REF] Bürgmann | Interseismic coupling and asperity distribution along the Kamchatka subduction zone[END_REF], the Aleutian Islands [START_REF] Cross | Plate coupling variation and block translation in the Andreanof segment of the Aleutian arc determined by subduction zone modeling using GPS data[END_REF] or South America (e.g. [START_REF] Pritchard | An aseismic fault slip pulse in northern Chile and along-strike variations in seismogenic behavior[END_REF][START_REF] Ruegg | Interseismic strain accumulation measured by GPS in the seismic gap between Constitución and Concepción in Chile[END_REF][START_REF] Perfettini | Seismic and aseismic slip on the Central Peru megathrust[END_REF]. Segments with high coupling in the seismogenic zone are considered as the loci of large (M > 8) mega-thrust earthquakes, while segments with low coupling are associated with aseismic slip and moderate seismicity. Such along-strike variations of coupling are likely representative of heterogeneities in the mechanical properties at the interface. They are assumed to be rather stable in space and time through successive seismic cycles, although it may depend on their origin. A correlation between the degree of coupling and the age of the subducting lithosphere or the convergence rate has been suggested by Ruff & Kanamori (1983), in contradiction with more recent studies [START_REF] Heuret | Plate motions, slab dynamics and back-arc deformation[END_REF] and the occurrence of mega-thrust earthquakes in Sumatra (2004 Sumatra-Andaman) and Japan (2011 Tohoku-oki). High coupling areas along subduction zones may be spatially correlated with forearc basins or thick sediments in the trench, and associated with negative free air gravity anomalies (e.g [START_REF] Ruff | Do trench sediments affect great earthquake occurence in subduction zones?[END_REF][START_REF] Wells | Basin-centered asperities in great subduction zone earthquakes : A link between slip subsidence, and subduction erosion[END_REF][START_REF] Song | Large trench-parallel gravity variations pedict seismogenic behaviour in subduction zones[END_REF][START_REF] Bürgmann | Interseismic coupling and asperity distribution along the Kamchatka subduction zone[END_REF][START_REF] Loveless | Testing mechanisms of subduction zone segmentation and seismogenesis with slip distributions from recent Andrean earthquakes[END_REF]. Low coupling areas have also been correlated with the location of subducting ridges or seamounts (e.g. [START_REF] Cloos | Thrust type subduction-zone earthquakes and seamount asperities : A physical model for seismic rupture[END_REF].

The analysis of free air gravity anomalies and marine seismic profiles offshore Mexico and Central America allows to derive a map of sediment thickness in this region [START_REF] Divins | Thickness of sedimentary cover in the Eastern Pacific Ocean[END_REF]. Lateral variations of both Bouguer gravity anomalies and sediment thickness are observed offshore Chiapas and Guatemala (Figure 11). The observed gradients, although rather low, would be consistent with, and may explain the coupling decrease at the subduction interface that we model (figure 8) from Chiapas to Guatemala and El Salvador [START_REF] Song | Large trench-parallel gravity variations pedict seismogenic behaviour in subduction zones[END_REF]. However, the historical seismicity, although poorly documented [START_REF] White | Seismic history of the Middle America subduction zone along El Salvador, Guatemala and Chiapas[END_REF], does not reveal any clear lateral variations of the seismic behavior along the subduction zone. Several M 7.5 -8.1 subduction earthquakes are reported, that seem to release less than 50% of the accumulated slip [START_REF] White | Seismic history of the Middle America subduction zone along El Salvador, Guatemala and Chiapas[END_REF]. This would be consistent with the overall low coupling values (0.25-0.6) that we model. The significance of such low values and of their lateral variations, in terms of seismic hazard for the study area, as well as their permanent or transient feature, thus remain open questions.

Figure 11. Maps of (a) bathymetry, (b) marine sediment thickness [START_REF] Divins | Thickness of sedimentary cover in the Eastern Pacific Ocean[END_REF]; http://www.ngdc.noaa.gov/ngg/sedthick/sedthick.html) and (c) Bouguer gravity anomalies (calculated from raw data given by the Bureau Gravimétrique International (BGI)). GB in (a) is for Guatemala Basin. See discussion in text.

Figure 1 .

 1 Figure 1. Tectonic setting of the Caribbean plate. Gray rectangle shows study area of Figure 2. Faults are mostly from Feuillet et al., 2003. PMF: Polochic-Motagua faults. EF: Enriquillo Fault. TD: Trinidad Fault. GB: Guatemala Basin. Topography and bathymetry are from Shuttle Radar Topography Mission[START_REF] Farr | Shuttle Radar Topography Mission produces a wealth of data[END_REF] and[START_REF] Smith | Global seafloor topography from satellite altimetry and ship depth soundings[END_REF], respectively. Plate velocities relative to Caribbean plate are from Nuvel1[START_REF] Demets | Current plate motions[END_REF] for Cocos plate,[START_REF] Demets | GPS geodetic constraints on Caribbean-North America plate motion[END_REF] for North America plate and[START_REF] Weber | GPS estimate of relative motion between the Caribbean and South American plates, and geologic implications for Trinidad and Venezuela[END_REF] for South America plate.

Figure 2 .

 2 Figure 2. GPS network: campaign sites in Guatemala/El Salvador (black triangles), Chiapas and Mexico (open triangles) and permanent sites (white triangles with names in bold). MAT: Mid-America Trench, PF: Polochic Fault, MF: Motagua Fault, JF : Jocotan Fault, GG: Guatemala-city Graben, IG: Ipala Graben, HD: Honduras Depression, SF: Swan Fault.

Figure 3 .

 3 Figure 3. GPS velocity field from table 1 in (a) North America and (b) Caribbean plates reference frames. Euler NA/ITRF2000 and CA/ITRF2000 angular velocities as well as velocity vectors of sites GUAT and TEGU (white arrows) are from DeMets et al. (2007). Dark gray bold lines outline active faults (names as in Figure 2). Light gray lines indicate location of profiles shown in Figure 4. Dotted line follows volcanic arc.

Figure 4 .

 4 Figure 4. (a) Topography (top) and ITRF2000 GPS velocities projected onto fault-parallel components along eastern profile, E (bottom), with fit model for a locking depth of 20 km. Main active fault traces are indicated by dotted lines. Volcanic arc area is shaded. (b) χ 2 = 1 contour line for locking depth and rate estimated using half-space elastic modeling (see section 3 in text). Cross shows model for a 20 km locking depth (best-fit). (c) χ 2 = 1 contour line for a two fault model, showing the relative contribution of the Polochic fault to the NA/CA motion, as a function of the asymmetry coefficient K across fault (Le Pichon et al., 2005). Case K=0 corresponds to an homogeneous half-space model. (d) and (e): same as (a) and (b), respectively, for central profile, C. Shaded area on (e) is that consistent with the locking depth range estimated from eastern profile in (b). (f): same as (a) and (d) for western profile, W. See Figure 3 for the location of profiles.

Figure 5 .

 5 Figure 5. Topography and GPS vectors in CA plate reference frame projected perpendicularly to the mean grabens orientation along southern profile, S. Sites names are indicated at bottom. See Figure 3 for the location of profiles.

Figure 6 .

 6 Figure 6. 4 blocks (4B) model geometry. The NA, CA, CO and AR blocks are delimited by 3 faults: the Mid-America Trench (MAT), the Volcanic Arc Fault (VAF) and the Motagua Fault (MF, noted MFw west of its junction with VAF). The 3 blocks (3B) model follows the same geometry, with AR and CA blocks combined into a single CA block.

Figure 7 .

 7 Figure 7. Inverted coupling coefficients along the MFe, MFc, MAT ch and MATgs, and residual velocities for best-fit 3B model.

Figure 8 .

 8 Figure 8. Same as Figure 7 for best-fit 4B model, with coupling along VAF fixed to 1.

Figure 9 .

 9 Figure 9. Reduced χ 2 as a function of φ M ATgs , for φ V AF fixed to 0, 0.5 and 1. φ M AT ch is fixed to 0.61. Only the Euler AR/CA rotation parameters are inverted.

Figure 10 .

 10 Figure 10. Proposed model of faults kinematics and coupling along the Cocos slab interface, revised from Lyon-Caen et al. (2006). Numbers are velocities relative to CA plate in mm/yr. Focal mechanisms are for crustal earthquakes (depth ≤30 km) since 1976, from CMT Harvard catalog.

Figure A2 .

 A2 Figure A2. Cumulative post-seismic displacements, estimated along a profile perpendicular to the Motagua fault, for each GPS campaign in 1999, 2003, 2006, corresponding to 23, 27 and 30 years after the 1976 Guatemala earthquake, respectively, and for viscosity of 10 18 , 10 19 , 10 20 and 10 21 Pa.s.

Figure A3 .

 A3 Figure A3. Resolution tests for 3B models. (a): Forward model imposing along-strike and along-dip coupling variations along the subduction zone. (b): Coupling inverted from (a) and residual velocities. The synthetic GPS vectors obtained in (a) are associated with uncertainties from Table 1, and inverted for coupling along the MAT (all other parameters are fixed). Along-dip variations cannot be retreived. (c): Same as (a) with along-strike variations of coupling only. (d): Same as (b). Lateral variations are well retrieved by inversion.

Table 1 .

 1 GPS sites information. Lon, Lat, Ve, Vn, σe, σn, σ are longitude, latitude, east and north velocities referenced to ITRF2000, east and north standard errors and form factor, respectively. Sites in bold are permanent sites, * indicates velocities calculated by[START_REF] Demets | Present motion and deformation of the Caribbean plate: constraints from new GPS geodetic measurements from Honduras and Nicaragua[END_REF]

	Lon ( • E) Lat ( • N)	Ve	Vn	σe	σn	σ	Site
				Chiapas
	266.063	16.225	-5.46	1.53	1.27 1.23 -0.027	AZTE
	267.307	16.125	-2.61	5.67	1.30 1.23 -0.017	CONC
	267.927	15.696	-1.25	6.86	1.28 1.23 -0.021	ESPI
	266.396	15.935	-4.98	4.63	1.27 1.23 -0.024	ESPO
	267.143	16.419	-4.84	1.71	1.56 1.50	0.002	GRUT
	267.106	15.459	-2.00	4.33	1.27 1.23 -0.023	MAPA
	267.611	15.281	-2.07	9.92	1.28 1.23 -0.041	SELE
	267.481	16.675	-4.64	4.12	1.30 1.23 -0.014	SOLE
	267.704	14.883	-0.96	6.72	1.53 1.49 -0.026	TPCH
				Guatemala
	270.182	15.605	-3.25	0.44	1.29 1.16	0.028	CAH
	270.941	15.394	-0.3	-0.66	1.38 1.21	0.075	CAM
	270.348	14.779	9.25	4.71	1.25 1.14	0.053	CHI
	269.618	14.075	5.45	4.79	1.36 1.18	0.026	CHL
	269.196	14.638	2.33	0.87	1.25 1.14	0.035	CML
	269.611	15.464	-2.53	0.24	1.20 1.10	0.087	COB
	270.548	14.517	12.73	4.84	1.26 1.15	0.040	CON
	270.760	14.854	5.09	3.22	1.87 1.23	0.052	CPJ
	270.132	16.916	-6.78	-0.32	1.21 1.19	0.024	ELEN
	269.478	14.590	3.20	2.80	1.71 2.40 -0.118	GUAT*
	270.385	15.030	1.89	0.74	1.31 1.19	0.026	HON
	268.531	15.282	-1.24	1.03	1.31 1.17	0.052	HUE
	268.497	15.318	-2.65	3.76	1.22 1.20	0.063	HUEH
	269.174	15.011	-3.22	0.36	1.27 1.21	0.003	JOY
	268.450	14.537	2.55	3.16	1.66 1.47 -0.200	MAZ
	270.329	15.084	1.42	1.41	1.26 1.15	0.039	MIN
	270.766	15.930	-5.63	0.36	1.31 1.21	0.040	MOD
	269.370	15.458	-2.42	4.7	1.23 1.20	0.043	PAM
	269.620	14.551	7.62	2.39	1.20 1.10	0.098	PIN
	268.486	14.871	0.53	-2.51	1.28 1.14	0.051	QUE
	269.553	15.990	-5.52	0.73	1.26 1.15	0.046	RUB
	269.719	15.075	0.09	1.38	1.29 1.16	0.038	SAL
	269.751	14.816	6.02	1.41	1.26 1.15	0.054	SAN
	268.506	15.571	-2.61	1.37	1.29 1.16	0.044	SOL
	269.130	15.348	-5.08	-1.92	1.24 1.20	0.061	USP
	270.499	14.981	5.45	4.92	1.47 1.30	0.012	ZAC
				El Salvador
	270.680	13.495	-0.69	13.05 1.39 1.29	0.010	SIGN
	270.883	13.697	4.48	8.60	1.40 1.00	0.500	SSIA
	270.895	14.175	10.37	5.39	1.38 1.29	0.010	TEJU
	270.500	14.116	12.67	5.17	1.38 1.29	0.010	TEXW
				Honduras
	272.794	14.090	8.90	5.70	1.60 1.20	0.000	TEGU*

Table 2 .

 2 Selected earthquakes from CMT-Harvard catalog with slip vector azimuths to constrain slip direction along the subduction (see section 4 in text).

	Long.	Lat.	Azimuth Depth Magnitude Reference
	( • E)	( • N)	( • )	(km)	CMT Mw	CMT
	272.84	12.7700	38	40.3	6.5	053178A
	268.95	13.8800	33	25.0	6.3	103078A
	270.36	13.1500	31	29.4	6.1	120678B
	269.12	13.8300	27	29,8	6.8	102779A
	269.27	13.7800	26	27.6	6.8	102779B
	267.89	14.2700	36	42.6	5.9	040682A
	268.06	14.0500	27	31.0	7.0	120283A
	273.44	11.5900	34	54.6	6.1	082384A
	273.44	11.9300	40	50.0	6.1	041985A

Table A1 .

 A1 Occupation history of campaign GPS sites

		Measurement duration (days)
	Site	1999 2002	2003	2004 2005	2006
			Chiapas
	AZTE	3.2	3	2.5	2.6
	CONC	3	2.7	2.5	3
	ESPI	4	2.3
	ESPO	3.2	2	2.5	2.6
	GRUT			2	2
	MAPA		2	2.6	3
	SELE	3.6	2.5	3	2.8
	SOLE	3	3.3	3.2	3
			Guatemala
	CAH	2	2			2
	CAM		1.4			2
	CHI	2	4			4
	CHL	2	2			4
	CML	2	1.8			4
	COB	8	7.3			8
	CON	2	2.5			2+2
	CPJ		1.8			2
	HON	2	2.7+0.5			2
	HUE	2	2			2
	JOY		0.6			2
	MAZ	2	2			2
	MIN	2	4			3
	MOD		1.3			2
	PAM		2			2
	PIN	8	9			5
	QUE	2	3			2
	RUB	3	3			4
	SAL	2	2			2
	SAN	2	1			2
	SOL	2	2			2
	USP		2			2
	ZAC		2			4
			El Salvador
	SIGN		2			4*0.5
	TEJU		2			4*0.5
	TEXW		2			4*0.5

Figure A1. (a) ITRF2000 velocity field for the Guatemala/El Salvador GPS sub-network (black arrows), Chiapas sub-network (gray arrows) and DeMets et al. (

2007

) solution (white arrows), before adjustment to a common reference frame (see processing strategy in text and Table

A5

). Inset shows common IGS permanent sites used in GLOBK (squares) and for reference frame adjustment (circles) for both Guatemala/El Salvador and Chiapas solutions. (b) ITRF2000 velocity field after adjustment (see Table

1

Table A2 .

 A2 Cumulative coseismic displacement modeled at each station, for all events that occurred between1999-2006 or 2003-2006. No estimate was done for sites ELEN, GUAT and SSIA (we use velocities computed by[START_REF] Demets | Present motion and deformation of the Caribbean plate: constraints from new GPS geodetic measurements from Honduras and Nicaragua[END_REF].

			Cumulated displacement (mm)
	Site	1999-2003	2003-2006	1999-2006
		East	North	East	North	East	North
	CAH	2.06	4.51	0.01	-0.28	2.07	4.23
	CAM	-	-	-0.14	-0.36	-0.14	-0.36
	CHI	7.95	0.37	0.46	-1.25	8.41	-1.62
	CHL	3.78	-7.07	0.14	-0.75	3.92	-7.82
	CML	3.43	-3.62	0.05	-0.31	3.48	-3.93
	COB	2.07	0.47	0.08	-0.2	2.15	0.67
	CON	12.65	-3.45	-0.31	-1.62	12.34	5.07
	CPJ	-	-	-0.33	-0.66	-0.33	-0.66
	HUE	1.49	-1.02	0.04	-0.05	1.53	-1.07
	HUEH	-	-	0.04	-0.05	0.04	-0.05
	HON	5.99	3.99	-0.07	-0.63	5.92	3.36
	JOY	-	-	0.1	-0.14	0.1	-0.14
	MAZ	0.42	-2.10	0.22	-0.23	0.64	-2.33
	MIN	5.52	3.77	-0.03	-0.61	5.49	3.16
	MOD	-	-	-0.04	-0.21	-0.04	-0.21
	PAM	-	-	0.08	-0.13	0.08	-0.13
	PIN	5.37	-4.65	0.14	-0.61	5.51	-5.26
	QUE	1.35	-1.78	0.09	-0.13	1.44	-1.91
	RUB	0.43	1.82	0.04	-0.11	2.25	-0.07
	SAL	4.09	-0.87	0.15	-0.37	4.24	1.24
	SAN	5.28	-2.50	0.20	-0.54	4.74	-2.30
	SOL	1.20	-0.52	0.02	-0.03	1.22	-0.55
	USP	-	-	0.08	-0.09	0.08	-0.09
	ZAC	-	-	-0.14	-0.52	-0.14	-0.52
	SIGN	-	-	-0.68	-0.26	-0.68	-0.26
	TEJU	-	-	-1.04	-1.27	-1.04	-1.27
	TEXW	-	-	-1.27	-2.17	-1.27	-2.17
	TPCH	-	-	0.04	-0.03	0.04	-0.03

Table A3 .

 A3 Coseismic displacement (mm) modeled at each station, for each event that occurred between1999-2006 or 2003-2006.

	Date (YYMMDD), Mw Date (YYMMDD), Mw 030121, 6.4 041213, 5.9	030825, 5.9 050518, 5.3	041120, 6.3
	Site		Site	East	North East North East	North East North East	North
	Date (YYMMDD), Mw Site CAH CAH CAM CAM	990508, 6.0 East North -0.24 -0.55 -0.01 ---0.02	990711, 6.7 East North 0.08 0.07 0.01 0.00 0.04 0.04 0.01 0.02	010113, 7.7 East -0.06 -0.32 -0.03 North -0.18 -0.42 0.01
	CAH CHI	CHI	-0.04 -0.57	-0.04 -0.74 -0.03		0.05 0.07 0.11	1.46 0.08 0.78	1.89 -0.36 -0.79	4.27 -0.65
	CHI CHL		CHL	-0.05 -2.18	-0.04 -2.98 0.07	1.25 -0.19 -0.01	1.99 0.01 0.07	5.86 0.19 -0.05	1.12 -0.70
	CHL CML	CML	0.00 0.0	-0.01 -1.46 -0.01		0.48 0.05 0.01	0.55 0.04 -0.04	5.12 0.05 0.00	-3.22 -0.36
	CML COB	COB	-0.12 -0.18	-0.09 -0.82 -0.01		0.32 0.10 0.01	0.45 0.11 0.00	2.75 -0.01 -0.01	-1.25 -0.31
	COB CON	CON	-0.07 -0.49	-0.08 -0.49 -0.01		-0.05 0.07 0.31	0.75 0.02 0.13	2.03 -0.50 -0.29	1.51 -1.66
	CON CPJ	CPJ	-0.01 -	-0.01 -0.01		1.12 0.07 0.12	1.94 0.06 0.13	9.37 -0.54 0.17	-1.66 -1.01
	HON HUE	HUE	-0.03 0.23	-0.02 -0.49 0.01		1.59 0.02 0.01	2.40 0.03 -0.01	3.79 0.02 0.00	3.50 -0.09
	HUE HUEH HUEH	-0.20 -	-0.31 -0.01		0.05 0.02 0.01	0.28 0.03 -0.01	0.94 0.02 0.00	-0.21 -0.09
	MAZ HON	HON	-0.75 -0.62	-0.56 -0.61 -0.02		0.13 0.11 0.09	0.41 0.07 0.02	0.51 -0.18 -0.10	-0.81 -0.69
	MIN JOY		JOY	-0.03 -	-0.03 -0.01		1.49 0.07 -0.02	2.25 0.20 -0.02	3.53 0.04 -0.01	3.44 -0.31
	PIN MAZ MAZ	-0.05 0.26	-0.04 -0.39 0.01		0.55 0.21 0.00	0.85 -0.23 -0.01	4.87 0.02 0.00	-1.75 0.00
	QUE MIN	MIN	-0.36 -0.41	-0.55 -0.67 -0.02		0.06 0.10 0.07	0.44 0.05 0.00	1.22 -0.11 -0.05	-0.49 -0.68
	RUB MOD MOD	-0.05 -	-0.08 -0.00		-0.96 0.04 0.02	0.68 0.05 0.01	1.22 -0.09 0.01	2.26 -0.29
	SAL PAM	PAM	-0.06 -	-0.06 --0.01		0.46 0.10 0.01	0.94 0.18 -0.02	3.26 0.01 0.00	0.54 -0.32
	SAN PIN		PIN	-0.05 -0.93	-0.04 -2.38 -0.03		0.53 0.11 0.04	1.00 0.12 -0.07	4.46 0.13 -0.01	-0.57 -0.76
	SOL QUE	QUE	-0.08 0.11	-0.22 -0.68 0.00		-0.23 0.11 0.00	0.31 -0.10 -0.01	1.16 -0.01 0.00	0.27 -0.03
	Date (YYMMDD), Mw RUB RUB Site SAL SAL SAN SAN	010213, 6.5 -0.08 -0.50 -0.01 East North -0.38 -1.19 -0.02 -0.61 -1.54 -0.03	010625, 5.2 0.06 0.08 0.01 0.00 East North 0.14 0.10 0.03 0.04 0.25 0.10 0.04 -0.09	-0.01 0.00 021109, 6.0 -0.20 East -0.01 -0.51 0.01 North 0.07 -0.68 0.00
	CAH SOL CHI USP CHL ZAC	SOL USP ZAC	-0.03 0.09 -0.01 -0.01 -	-0.02 -0.47 0.00 -0.04 --0.01 -0.01 --0.01		0.52 0.03 0.00 1.57 0.08 0.01 0.59 0.08 0.09	-0.48 0.07 -0.01 -1.81 0.04 -0.02 -1.10 0.08 0.09	-0.09 0.00 0.00 -0.10 0.03 0.00 -0.24 -0.30 0.09	-0.10 -0.13 -0.14 -0.11 -0.30 -0.78
	CML COB SIGN CON TEJU HON TEXW	0.03 -0.18 0.01 0.01	0.01 -0.16 0.06 0.01 -0.18 0.01 -0.07		0.67 0.58 0.02 2.70 0.40 1.41 0.39	-0.66 -0.61 0.00 -3.19 0.02 -1.21 0.00	-0.22 -0.06 0.00 -0.05 -0.01 -0.16 0.01	-0.62 -0.12 -0.05 -0.08
	HUE AZTE	0.01	0.03 0.00		0.45 0.00	-0.22 0.00	0.01 0.00	-0.10
	MAZ ESPO	0.02	0.02 0.00		0.19 0.00	-0.39 0.00	0.06 0.00	-0.38
	MIN MAPA	0.01	-0.02 0.00		0.99 0.00	-1.12 0.00	-0.06 0.00	-0.08
	PIN	SELE	0.03	-0.04 0.00		1.11 0.00	-1.01 -0.01	-0.21 0.00	-0.28
	QUE	ESPI	0.04	0.03 0.00		0.24 0.00	-0.31 -0.01	0.04 0.00	-0.22
	RUB SOLE	-0.04	-0.04 0.00		0.37 0.00	-0.40 0.00	-0.03 0.00	-0.10
	SAL GRUT	0.10	-0.09 0.00		0.81 0.00	-0.83 0.00	-0.10 0.00	-0.18
	SAN CONC	0.06	-0.06 0.00		1.02 0.00	-1.09 0.00	-0.13 0.00	-0.20
	SOL TPCH	-0.03	0.03 0.00		0.32 0.00	-0.30 -0.01	-0.03 0.00	-0.14

Table A4 .

 A4 Angular velocities used in this study. Ref 1: DeMets et al. (2007), Ref 2: DeMets et al. (1990) and DeMets et al. (1994).

				Eulerian poles			Errors
	Blocks	Reference	Latitude Longitude	Rotation rate	σxx	σyy	σzz	σxy	σxz	σyz
			• N	• E	• .Myr -1
	CA-ITRF2000	Ref 1	36.3	-98.5	0.255	0.	0.831	0.175	-0.262	0.000	-0.184
	NA-ITRF2000	Ref 1	-7.64	-86.21	0.196	0.011	0.182	0.107	0.015	-0.011 -0.125
	NA/CA	Ref 1	75.9	191.5	-0.182	0.143	1.013	0.282	-0.247 -0.011 -0.309
	blocks	Reference	Latitude Longitude	rotation rate(w)	σmax	σ min	σw
			• N	• E	• .Myr -1	• N	• E	• .Myr -1
	CO/CA	Ref 2	24.1	-119.4	1.31	2.5	1.2	0.05	-	-	-

Table A5 .

 A5 Rotation parameters used to adjust the Guatemala/El Salvador,Chiapas, and DeMets et al. (2007) solutions in the same reference frame.

			Eulerien poles
	Solutions	Latitude	Longitude	Rotation rate
		• N	• E	• .Myr -1
	Chiapas and Guatemala/El Salvador	-44.261	88.285	0.0085
	Guatemala/El Salvador and DeMets et al. (2007)	-7.706	29.69	0.0058

ACKNOWLEDGMENTS

This work was partly funded by INSU-CNRS DyETI program. We thank the guatemalan institutions INSIVUMEH, CONRED, IGN, CUNOR and CESEM from USAC, as well as the IGN in El Salvador, for their unvaluable logistic support. The french embassy in Guatemala provided help during field surveys. We thank Christophe Vigny for his help with GAMIT/GLOBK software and discussions concerning reference frames. We thank Christel Tiberi for help with the gravity data and Eric Calais for discussions concerning coupling on slab interface. We thank Charles DeMets and an anonymous reviewer for their constructive comments.


C Beauval

Regional differences in subduction ground motions

Keywords: GMPEs, subduction events, testing against observations, regional difference

) LLH method prove to be efficient in providing one number quantifying the overall fit, but additional analysis on the between-event and within-event variabilities are mandatory, to control if median prediction per event and/or variability within an event is within the scatter predicted by the model.

INTRODUCTION

To carry out a probabilistic seismic hazard calculation for a region, an empirical ground-motion prediction equation (GMPE) is needed to predict ground motions generated by earthquakes. The GMPE selected might have been developed from local data, but more often due to the lack of data a model including data from different regions is used. Few regions in the world have a strong-motion database complete enough to be used alone for deriving a prediction equation. The present study focuses on empirical GMPEs developed for predicting the ground motions produced by subduction interface and inslab events. Inslab events produce on average larger near-source amplitudes than interface events, but these amplitudes attenuate much faster, causing damaging ground motions over a much smaller area. All the data considered here (except for Greece) is extracted from the generating dataset of the global model developed by Abrahamson et al. (submitted), gathering data from different subduction zones (Japan, Taiwan, Central and South America, ..). We test candidate GMPEs developed for subduction zones at a global or local scale against observations. The "global" GMPEs have been developed based on worldwide data assuming that there is no major regional difference in the attenuation of ground motions between subduction zones (Youngs et al. 1997, Atkinson andBoore 2003, Abrahamson et al. submitted). Other models are regional, established using mainly local data, and aimed at predicting accelerations for a specific subduction region (e.g. [START_REF] Zhao | Attenuation Relations of Strong Ground Motion in Japan Using Site Classification Based on Predominant Period[END_REF][START_REF] Lin | Ground-Motion Attenuation Relationships for Subduction-Zone Earthquakes in Northeastern Taiwan[END_REF][START_REF] Arroyo | Strong ground-motion relations for Mexican interplate earthquakes[END_REF]. The aim of the present study is to understand the performance of global and local models with respect to the data of the different seismic zones. The databases used for the regression of the GMPEs may differ or partially overlap the dataset considered here, and thus we do not expect a perfect adequacy between models and data. To quantify the fit, the LLH method proposed by [START_REF] Scherbaum | Model Selection in Seismic Hazard Analysis: An Information-Theoretic Perspective[END_REF] is used. The LLH evaluates the overall performance of the model with respect to the data. Complementary to the LLH values, event terms and within-event dispersions are calculated for the earthquakes considered. We will focus on the earthquakes contributing the most to the seismic hazard in these regions, i.e. moment magnitudes higher or equal to 6. Only recordings for which spectral accelerations can be calculated up to at least 25 Hz are selected. [START_REF] Scherbaum | Model Selection in Seismic Hazard Analysis: An Information-Theoretic Perspective[END_REF] provides a ranking criterion based on information theory (see the original paper for a detailed description of the theory behind). This technique is based on the probability for an observed ground motion to be realized under the hypothesis that a model is true. It provides one value, the negative average log-likelihood LLH (Delavaud et al. 2012), that reflects the fit between data and model:

METHOD

with N the number of observations x i , and g the probability density function predicted by the GMPE (normal distribution, with standard deviation the total sigma of the model). The ranking of models according to their fit to the data is then straightforward. A small LLH indicates that the candidate model is close to the model that has generated the data, while a large LLH corresponds to a model that is less likely of having generated the data. Taking advantage of synthetic data, the LLH value can be easily interpreted in terms of residual distributions (Fig. 1, see Beauval et al. in press). Synthetic datasets are generated from an original Gaussian distribution, and distributions with modified characteristics (in terms of mean and sigma) are tested against these synthetic datasets. The final LLH characterizing the fit between the data and the model is simply the mean of all individual LLH. If the tested distribution is equal to the original distribution, the LLH value is close to 1.4-1.5. If the tested Gaussian is different from the original distribution, the LLH increases. Besides LLH values, we calculate event terms and within-event variabilities, and compare them to the between-event and within-event sigmas of the GMPEs. The between-event dispersion integrates the variability on the characteristics of the source, whereas the within-event dispersion integrates roughly the variability in the wave propagating path and in the site condition. The total residual R i between an observed acceleration and the acceleration predicted by a particular GMPE is obtained as follows:

R i = ln(A i ) OBS -ln(A i ) GMPE
where ' OBS ' refers to the observed acceleration from recording i, and ' GMPE ' refers to the median acceleration value, for location i predicted by the GMPE, considering the earthquake magnitude, appropriate source distance and site condition (5% damped spectral acceleration). The event term of an event is simply the mean value of the residuals, therefore calculated over all locations available for the event. The event terms can then be compared to the between-event sigma () of the GMPE, which has been evaluated empirically from recorded earthquakes during the development of the GMPE. If the event terms is small and fall within one standard deviation, this suggests that the model predicts the records rather well. The within-event variabilities are also calculated, they correspond to the standard deviation of the residuals of each earthquake. They can be compared to the within-event sigma of the model ().

COMPARISON OF PREDICTIONS AND OBSERVATIONS

Ground-motions from interface and inslab events in Japan

The earthquakes considered here are included in the generating dataset of Zhao et al. 2006 model, which is constituted of all available records from earthquakes with moment magnitudes higher or equal to 5.0 since 1968 in Japan.

INTERFACE: We consider 7 interface earthquakes that occurred in the two subduction zones in Japan (Table 1 andFig The event term and the within-event dispersion for each earthquake are calculated relatively to the Abrahamson et al. ( 2010) GMPE, and compare to the corresponding sigmas of the model (Fig. 3, see e.g. [START_REF] Stewart | Ground Motion Recordings from the Mw 6.3 2009 L'Aquila Earthquake in Italy and their Engineering Implications[END_REF]. It is interesting to observe that the event terms of the two earthquakes contributing the most to the LLH value (Table 1, 222 and 319 recordings each) are yielding opposite trends. For one earthquake, predictions are under-predicting the observations and event terms range from 0.3 to 0.6 (natural log units), while event terms range from -0.7 to -0.4 for the other event.

Considering all earthquakes, event terms are roughly within the scatter of event terms from the earthquakes used in the development of the BCHydro equation. The within-event variabilities are superimposed to the within-event sigma of the model (Fig. 3 right). Depending on the event, the dispersion is equal, slightly higher or lower than the within-event sigma.

The same analysis is performed for understanding the results obtained for the Zhao et al. ( 2006) model (Fig. 4), which is yielding LLH values slightly higher than the BCHydro model. The event terms for the 6 earthquakes range between +1 and -2, indicating that the ground motion depends greatly on the specific earthquake, and that accelerations are on average lower than predicted by the model. On the other side, the model is rather well predicting the within-event variability (Fig. 6, right).

Figure 3. Data: Japan interface events (Table 1), GMPE: BChydro (Abrahamson et al. submitted). Left: event terms for each event, calculated for 5 periods: 0.01 (PGA), 0.2, 0.5, 1, 2 seconds. The size of the symbol is proportional to the number of recordings per event. Red solid lines: ± (sigma between-event of the GMPE); red dashed lines: ±2. Right: dispersion within each event (sigma of the residuals). Red line: (sigma within-event of the GMPE); red dashed line: +2. INTRASLAB: We consider 7 intraslab earthquakes that occurred in the two subduction zones in Japan (Table 1 andFig

Ground-motions from interface and inslab events in Taiwan

INTERFACE: Three the six interface earthquakes considered are included in the dataset used in the regression of the Lin and Lee (2008) GMPE for northeastern Taiwan (events with M7 are all foreign events). The six interface earthquakes are located on the same subduction zone, at the boundary between the Philippine Sea plate and the Eurasian plate. Results are displayed in Fig. 6. Above 2 Hz, the models yielding the best fit to the data are the BCHydro and Lin and Lee (2008) models. The models [START_REF] Zhao | Attenuation Relations of Strong Ground Motion in Japan Using Site Classification Based on Predominant Period[END_REF], [START_REF] Atkinson | Empirical Ground-Motion Relations for Subduction-Zone Earthquakes and Their Application to Cascadia and Other Regions[END_REF] and [START_REF] Kanno | A New Attenuation Relation for Strong Ground Motion in Japan Based on Recorded Data[END_REF] yield slightly higher LLH. Youngs et al. and McVerry et al. are predicting accelerations which differ significantly from the observations (2<LLH<3.3). Below 2 Hz, none of the model is providing a good fit. This poor fit for long periods will need to be understood.

Calculating the event terms relative to the Lin and Lee model show the same trend for all earthquakes: the median of the residuals for one earthquake is in the scatter predicted by the model for periods lower than 0.5s (event terms between -and 0, Fig. not shown). Note that in [START_REF] Lin | Ground-Motion Attenuation Relationships for Subduction-Zone Earthquakes in Northeastern Taiwan[END_REF], only the total sigma () is provided. For higher periods, the event terms takes increasingly positive values.

As for the dispersion quantified within each earthquake, it is close to the variability predicted by the model (and thus close to the variability of the generating dataset of Lin and Lee (2008) model). [START_REF] Lin | Ground-Motion Attenuation Relationships for Subduction-Zone Earthquakes in Northeastern Taiwan[END_REF] GMPE developed for northeastern Taiwan (event in 1995). In the generating dataset, events with M>6 are all foreign events. For most models (Fig. 7), the resulting LLH values are stable according to the frequency. The bestfitting models are the [START_REF] Zhao | Attenuation Relations of Strong Ground Motion in Japan Using Site Classification Based on Predominant Period[END_REF] Japanese model, the [START_REF] Lin | Ground-Motion Attenuation Relationships for Subduction-Zone Earthquakes in Northeastern Taiwan[END_REF] model developed for Taiwan, and the BCHydro model (1.5<LLH< 2.5). [START_REF] Mcverry | New Zealand acceleration response spectrum attenuation relations for crustal and subduction zone earthquakes[END_REF], [START_REF] Youngs | Strong Ground Motion Attenuation Relationships for Subduction Zone Earthquakes[END_REF] and [START_REF] Atkinson | Empirical Ground-Motion Relations for Subduction-Zone Earthquakes and Their Application to Cascadia and Other Regions[END_REF] models yield comparable LLH values above 5 Hz, but higher values for lower frequencies. The event terms are estimated relatively to the [START_REF] Zhao | Attenuation Relations of Strong Ground Motion in Japan Using Site Classification Based on Predominant Period[END_REF] model, and compared to its between-event sigma (Fig. 8, left). Event terms vary according to the earthquake, negative for some, positive for others, but contained in the scatter predicted by the model (withinand +2). The variability of the ground motion within each earthquake is very close to the withinevent sigma of the model (Fig. 8, right).

Ground-motions from interface events in Central and South America

From the global database, 15 interface events have been extracted, with moment magnitude higher or equal to 6, and spectral accelerations extending at least up to 25 Hz, providing in total 113 recordings (rupture distance up to 300km). We consider as a whole central and South America, although we might be mixing different interface geometries. As shown in Fig. 9, the model ranked first and yielding lowest LLH values is the Arroyo et al. ( 2010) GMPE, designed from and for Mexican interface events (on rock sites). This result must be taken with caution, as the number of recordings is divided by two because only records at rock are considered. Models by [START_REF] Atkinson | Empirical Ground-Motion Relations for Subduction-Zone Earthquakes and Their Application to Cascadia and Other Regions[END_REF], Zhao et al. (2006), BCHydro, and[START_REF] Youngs | Strong Ground Motion Attenuation Relationships for Subduction Zone Earthquakes[END_REF] appear to well predict the data. They yield close LLH values (in-between 1.5 and 1.8) stable with frequency. The results are quite coherent with the study of [START_REF] Arango | An Evaluation of the Applicability of Current Ground-Motion Models to the South and Central American Subduction Zones[END_REF], which identify the same 4 bestfitting models from a larger dataset of interface events in the Peru-Chile region. In the same study, they find that [START_REF] Lin | Ground-Motion Attenuation Relationships for Subduction-Zone Earthquakes in Northeastern Taiwan[END_REF] model is well predicting data of interface events in Central America, which is not the case on our dataset.

Ground-motions from inslab events in Greece

Figure 10. Greece, intraslab events: map of stations and epicenters, and LLH values versus frequency, calculated for the GMPEs: BCHydro, Zhao et al., Lin and Lee, Kanno et al., Youngs et al., Atkinson and Boore, McVerry et al. The Greek subduction zone is the most active subduction zone within Europe. Intermediate-depth earthquakes along the Hellenic arc occur along a well-defined Wadati-Benioff zone, which have more or less amphitheatrical shape, at depths ranging from about 60 to 170km. The Wadati-Benioff zone is due to the subduction of the Eastern Mediterranean lithosphere under the Aegean microplate. Intermediate-depth events are strike-slip events with a significant thrust component. The acceleration dataset was compiled using data recorded by the acceleration-sensor networks operated by the Institute of Engineering Seismology and Earthquake Engineering (ITSAK). Source-site distances range from 70 to 300km. Currently, there is no local ground-motion prediction model fulfilling all the requirements for being used in a PSHA study. In the SHARE project (Delavaud et al. 2012), GMPEs needed to be selected to predict the ground motions produced by interface and intraslab earthquakes. The choice of the GMPEs was guided both by expert opinion and using the results of the testing on the data available. The data available is restricted (Table 3, 7 events and 68 recordings), but still it is worth evaluating the fit with respect to the local and global models. Three models are providing a relatively good fit to the data for frequencies higher than 2Hz: BCHydro, [START_REF] Zhao | Attenuation Relations of Strong Ground Motion in Japan Using Site Classification Based on Predominant Period[END_REF] and [START_REF] Lin | Ground-Motion Attenuation Relationships for Subduction-Zone Earthquakes in Northeastern Taiwan[END_REF]. For the same frequency range, [START_REF] Kanno | A New Attenuation Relation for Strong Ground Motion in Japan Based on Recorded Data[END_REF][START_REF] Youngs | Strong Ground Motion Attenuation Relationships for Subduction Zone Earthquakes[END_REF][START_REF] Mcverry | New Zealand acceleration response spectrum attenuation relations for crustal and subduction zone earthquakes[END_REF] are providing higher LLH (between 1.6 and 2.8), while [START_REF] Atkinson | Empirical Ground-Motion Relations for Subduction-Zone Earthquakes and Their Application to Cascadia and Other Regions[END_REF]

DISCUSSION AND CONCLUSIONS

The aim of the study was to test systematically global and local models against the same regional datasets (M6, most relevant to hazard calculation), and obtain a hierarchy of their fit to the data. Among the global models, the BCHydro equation (Abrahamson et al., submitted) appears to be the model which is providing the best fit to interface and intraslab datasets, in the different regions tested (Japan, Taiwan, Greece, Central and South America, Greece). The good performance of this model, with respect to the local models, does not highlight strong regional differences, suggesting that it can be applied safely for predicting ground motions in all subduction regions. Among the local models, the Zhao et al. model (2006, Japan) shows to predict quite well the data, in all regions studied. The Lin and Lee ( 2008) model (Taiwan) is predicting well the ground motions produced by earthquakes in Taiwan and in Greece, but not elsewhere. [START_REF] Atkinson | Empirical Ground-Motion Relations for Subduction-Zone Earthquakes and Their Application to Cascadia and Other Regions[END_REF] global base model is used here (the correction factors for Japan are not accounted for). It provides rather good fit to the data, except in the case of interface events in Japan and inslab events in the Greek subduction zone.

The between-event and within-event variabilities, playing a role in the LLH estimate, must be partitioned. Analysing the event terms and within-event variabilities relative to a GMPE indicates if the event terms for specific regions are consistent with event-to-event scatter predicted by the models (observed from past earthquakes). The within-event variabilities of the earthquakes can be compared to the within-event sigma of the models. We have analysed the residuals with respect to all models, but only a few examples were displayed in the paper. They show that a small LLH usually correspond to within-event variabilities compatible with the within-event sigma of the model, and to event terms which are mostly contained in the scatter predicted by the model (negative and positive, and resulting in an average close to 0). More work is required to have a more comprehensive picture of the adequacy between models and data, which has been evaluated here in a broad sense, in particular in terms of attenuation of amplitudes with distance. More work is also needed on analysing the regional variability of the ground motions on a finer scale and its dependence with frequency.

Figure 1 .

 1 Figure 1. LLH calculation: synthetic ground motion datasets generated from an original Gaussian distribution (black), are tested against distributions with modified characteristics (dashed blue curve). The final LLH is the mean of the 200 LLH values calculate from the 200 synthetic accelerations.

  . 2, in total 800 recordings with source distances up to 300km). The LLH values, reflecting the fit between the predictions of the model and the observations, varies with the GMPE considered, but are quite stable according to frequency for a given GMPE. The GMPEs predicting accelerations which are the most in accordance with the recordings are 1) the global model BCHydro(Abrahamson et al. submitted), 2)[START_REF] Zhao | Attenuation Relations of Strong Ground Motion in Japan Using Site Classification Based on Predominant Period[END_REF] established from a Japanese database (a few overseas events at close distance), 3)[START_REF] Kanno | A New Attenuation Relation for Strong Ground Motion in Japan Based on Recorded Data[END_REF] established from Japanese data (behavior for f<2.5Hz needs to be understood). The Lin and Lee model (2008), established for Taiwan, from both local data (up to M w =6) and foreign data (M>6), also provides reasonable LLH values, although slightly higher.[START_REF] Youngs | Strong Ground Motion Attenuation Relationships for Subduction Zone Earthquakes[END_REF] and[START_REF] Atkinson | Empirical Ground-Motion Relations for Subduction-Zone Earthquakes and Their Application to Cascadia and Other Regions[END_REF] models are yielding higher LLH (2.2-3.2).

Figure 2 .

 2 Figure 2. Japan, interface events considered: map of stations (red points) and epicenters (blue points), and LLH values versus frequency, calculated for the GMPEs: BCHydro, Zhao et al., Lin and Lee, Kanno et al., Youngs et al., Atkinson and Boore.

Figure 4 .

 4 Figure 4. Data: Japan interface, GMPE: Zhao et al. (2006). See legend of Fig. 3. The size of the symbol is proportional to the number of recordings per event.

  . 5, in total 844 recordings). The obtained LLH values rank first the local model[START_REF] Zhao | Attenuation Relations of Strong Ground Motion in Japan Using Site Classification Based on Predominant Period[END_REF], then comes[START_REF] Kanno | A New Attenuation Relation for Strong Ground Motion in Japan Based on Recorded Data[END_REF], as well as the global modelsAbrahamson et al. (2010) and[START_REF] Atkinson | Empirical Ground-Motion Relations for Subduction-Zone Earthquakes and Their Application to Cascadia and Other Regions[END_REF]. Note that, unlike[START_REF] Atkinson | Empirical Ground-Motion Relations for Subduction-Zone Earthquakes and Their Application to Cascadia and Other Regions[END_REF] who discarded distances beyond 200km for the Japanese recordings, here sources distances up to 300 km are taken into account.[START_REF] Youngs | Strong Ground Motion Attenuation Relationships for Subduction Zone Earthquakes[END_REF] model is yielding higher LLH values (2-2.5), whereas Lin and Lee (2008) and McVerry et al. (2006) are predicting accelerations which do not fit the dataset. The event terms relative to the best fitting equation, Zhao et al., are roughly included in the typical range of past Japanese earthquakes (varying between -1 and +2, Fig. not shown), whereas the within-event standard deviation of the 7 earthquakes are very close to the within-event sigma of the model.

Figure 5 .

 5 Figure 5. Japan, intraslab events considered: map of stations and epicenters, and LLH values versus frequency, calculated for the GMPEs: BCHydro, Zhao et al., Lin and Lee, Kanno et al., Youngs et al., Atkinson and Boore, McVerry et al.

Figure 6 .

 6 Figure 6. Taiwan, interface events: map of stations and epicenters, and LLH values versus frequency, calculated for the models: BCHydro, Zhao et al., Lin and Lee, Kanno et al., Youngs et al., Atkinson and Boore, McVerry et al.

Figure 7 .

 7 Figure 7. Taiwan, intraslab events considered: map of stations and epicenters, and LLH versus frequency, for the GMPEs: BCHydro, Zhao et al., Lin and Lee, Kanno et al., Youngs et al., Atkinson and Boore, McVerry et al.

Figure 8 .

 8 Figure 8. Data: Taiwan intraslab events, GMPE: Zhao et al. (2006). See legend of Fig. 3. The size of the symbol is proportional to the number of recordings per event.

Figure 9 .

 9 Figure 9. Central and South America, interface events: map of stations and epicenters, and LLH versus frequency calculated for the GMPEs: BCHydro, Zhao et al., Lin and Lee, Kanno et al., Youngs et al., Atkinson and Boore, McVerry et al, Arroyo et al. (only the 54 recordings on rock sites have been used in this case).

Table 1 .

 1 Japan: interface and inslab events used in the testing (parameters fromAbrahamson et al. (submitted))

	Date	Magnitude Mw	Depth (km)	# of recording sites	Source type
	1996/10/19	6.7	18	47	Interface
	1996/12/03	6.7	29	33	Interface
	2000/06/25	6.	10	25	Interface
	2003/09/26	8.3	23	319	Interface
	2003/09/26	7.4	50	222	Interface
	2003/09/25	6.5	41	86	Interface
	2003/10/08	6.7	41	68	Interface

Table 2 .

 2 Taiwan: interface and inslab events used in the testing (parameters fromAbrahamson et al. (submitted))

	Date	Magnitude Mw	Depth (km)	# of recording sites	Source type
	1992/09/28	6.4	38	10	Interface
	1994/05/23	6.2	32	23	Interface
	1996/03/05	6.3	44	207	Interface
	2002/03/31	7.1	32	381	Interface
	2001/12/18	6.8	28	308	Interface
	2004/11/08	6.3	30	271	Interface
	1995/06/25	6.	57	275	Intraslab
	2004/10/15	6.6	112	434	Intraslab
	2005/10/15	6.4	200	176	Intraslab
	2006/12/16	7.	44	309	Intraslab
	2006/12/26	6.7	50	475	Intraslab
	INTRASLAB: One of the five intraslab earthquakes considered are included in the generating dataset
	of the

  yield large LLH (up to 3.6). Below 2 Hz, LLH values of all models increase. Lin and Lee and Youngs et al. models are best predicting the observations, with LLH around 2.0.

Table 3 .

 3 Greece: inslab events used in the testing

	Date	Magnitude Mw	Depth (km)	# of recording sites
	1994/05/23	6.1	90	4
	1996/04/26	5.8	64	2
	1999/06/11	5.2	91	6
	2006/01/08	6.7	66	19
	2008/01/06	6.2	80	22
	2008/07/15	6.4	40	7
	2011/04/01	6.1	60	8

AKCNOWLEDGEMENT

This work has been done mainly with the support of the SHARE European program FP7 (contract number 226967, http://www.share-eu.org/).