present-day deformation of the pyrenees revealed by gps surveying and earthquake focal mechanisms...

Rigo et al., 2015 G. J. Int., 201, 947-964, 10.1093/gji/ggv052

Present-day deformation of the Pyrenees revealed by GPS surveying and earthquake 1

focal mechanisms until 2011 2

3

A. Rigo1 † *, P. Vernant2, K.L. Feigl3, X. Goula4, G. Khazaradze5, J. Talaya4, L. Morel6, J. 4

Nicolas6, S. Baize7, J. Chéry2, M. Sylvander1 5

6

1 Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, 7

Observatoire Midi-Pyrénées, 14 ave. Edouard Belin, F31400 Toulouse, France. 8

† Now at Laboratoire de Géologie, ENS, CNRS, PSL Research University, 24 rue Lhomond, 9

F75231 Paris Cedex 5, France. 10

* Corresponding author: tel/fax: +33561332969 / +33561332900, e-mail address: 11

[email protected]/[email protected] 12

2 Laboratoire Géosciences Montpellier, Université Montpellier II-CNRS, Pl. E. Bataillon, 13

34095 Montpellier Cedex 05, France 14

3 Dept. of Geoscience, University of Wisconsin, Madison, WI 53706, USA 15

4 Institut Cartogràfic i Geològic de Catalunya (ICGC), Barcelona 08038, Spain 16

5 Universitat de Barcelona, Barcelona 08028, Spain 17

6 Laboratoire de Géodésie et Géomatique, ESGT-CNAM, 72000 Le Mans, France 18

7 Institut de Radioprotection et de Sureté Nucléaire, 92260 Fontenay-aux-Roses, France 19

20

21

Abstract 22

The Pyrenean mountain range is a slowly deforming belt with continuous and moderate 23

seismic activity. To quantify its deformation field, we present the velocity field estimated 24

from a GPS survey of the Pyrenees spanning 18 years. The PotSis and ResPyr networks, 25


including a total of 85 GPS sites, were installed and first measured in 1992 and 1995-1997, 26

respectively, and remeasured in 2008 and 2010. We obtain a deformation field with velocities 27

less than 1 mm/yr across the range. The estimated velocities for individual stations do not 28

differ significantly from zero with 95 percent confidence. Even so, we estimate a maximum 29

extensional horizontal strain rate of 2.0 ± 1.7 nanostrain per year in a N-S direction in the 30

western part of the range. We do not interpret the vertical displacements due to their large 31

uncertainties. In order to compare the horizontal strain rates with the seismic activity, we 32

analyse a set of 194 focal mechanisms using three methods: (i) the ‘r’ factor relating their P 33

and T axes, (ii) the stress tensors obtained by fault slip inversion and (iii) the strain-rate 34

tensors. Stress and strain-rate tensors are estimated for: (i) the whole dataset, (ii) the eastern 35

and western parts of the range separately, and (iii) eight zones, which are defined based on the 36

seismicity and the tectonic patterns of the Pyrenees. Each of these analyses reveals a lateral 37

variation of the deformation style from compression and extension in the east to extension 38

and strike-slip in the west of the range. Although the horizontal components of the strain-rate 39

tensors estimated from the seismic data are slightly smaller in magnitude than those computed 40

from the GPS velocity field, they are consistent within the 2σ uncertainties. Furthermore, the 41

orientations of their principle axes agree with the mapped active faults. 42

43

Key words: (25) Space geodetic surveys, (110) Seismicity and tectonics, (142) Continental 44

deformation: extensional, (156) Dynamics: seismotectonics, (203) Europe 45

46

1 INTRODUCTION 47

The Pyrenees are a mountain belt along the French-Spanish border, where the rate of 48

deformation is low and the seismicity is continuous and moderate (Souriau & Pauchet 1998; 49

Fig. 1A). However, historical earthquakes with probable magnitudes as high as 6.0-6.5 have 50


been inferred from a seismic sequence with intensity VII - IX in 1427-1428 in the eastern 51

Pyrenees near Olot (Briais et al. 1990; Olivera et al. 2006; Perea 2009) and an earthquake 52

with intensity VIII in 1660 near Lourdes in the central part (Lambert & Levret-Albaret 1996). 53

In order to characterize and to quantify the present-day Pyrenean deformation, we present a 54

velocity field obtained from two GPS networks encompassing the whole Pyrenean range that 55

were surveyed several times between 1992 and 2010. 56

In addition to quantifying the deformation field in the Pyrenees, these two GPS 57

networks may provide answers to questions that have emerged since their installation 20 years 58

ago especially about N-S extension suggested by recent studies (e.g., Nocquet & Calais 2004; 59

de Vicente et al. 2008; Chevrot et al. 2011; Asensio et al. 2012): Is this extension uniform 60

throughout the range? Is this extension mainly revealed by normal fault plane solutions, the 61

expression of horizontal movements or, maybe, principally of vertical movements as 62

suggested by Lacan & Ortuño (2012) and Vernant et al. (2013)? Is it now possible to obtain a 63

coherent stress field in the Pyrenees when others had failed until now, leaving it an open 64

question (Nicolas et al. 1990; Delouis et al. 1993; Souriau et al. 2001; de Vicente et al. 2008; 65

Stich et al. 2010)? 66

Our estimated GPS velocity field is compared with the tectonics and seismicity of the 67

range. In particular, we focus on the stress and strain-rate tensors determined from an updated 68

compilation of the focal mechanisms. 69

70

2 SEISMOTECTONIC SETTING 71

2.1 Pyrenean tectonics 72

The Pyrenees result from the convergence of the Iberian plate and the Eurasian plate. 73

The convergence started 65 Ma ago, following a period of extension (-115 to -80 Ma) related 74

to the opening of the Bay of Biscay to the west (e.g., Jammes et al. 2010). The eastern part 75


has been affected by Mediterranean tectonics and the opening of the Gulf of Lion 30 Ma ago 76

as reviewed by Choukroune (1992) and Vissers & Meijer (2012). From north to south, the 77

main tectonic structures running E-W along the mountain range include: (1) the North 78

Pyrenean Frontal Thrust (NPFT) overlapping the sedimentary Aquitaine Basin to the north, 79

(2) the North Pyrenean Fault (NPF), separating the Iberian and Eurasian plates, and (3) the 80

South Pyrenean Frontal Thrust (SPFT) overlapping the Ebro Basin to the south (Fig. 1). The 81

NPF is the northern limit of the axial zone constituting a Palaeozoic domain (grey area in Fig. 82

1), and coincides at depth to a step in the Moho of 15-20 km, with the thicker crust below the 83

Iberian part of the range (Hirn et al. 1980). The Moho step progressively decreases in 84

magnitude from west to east and vanishes below the Mediterranean Sea. The structure of the 85

Pyrenees has been analysed by several methods, including: refraction and reflection seismic 86

experiments (e.g., Gallart et al. 1981; Daignières et al. 1994), gravimetric studies and 87

modelling (Torné et al. 1989; Vacher & Souriau 2001), lithospheric tomography with 88

seismological methods (Souriau & Granet 1995; Souriau et al. 2008; Chevrot et al. 2014), and 89

geoelectrical and magnetotelluric methods (Pous et al. 1995; Ledo et al. 2000; Campanyà et 90

al. 2011, 2012). Although the Pyrenean structure is relatively well known at the scale of the 91

range, the characterisation of the structures that are tectonically active at the present time is 92

still a mater of debate. The main E-W faults described above (NPF, NPFT and SPFT) show 93

no evidence of recent activity in the field. The faults identified as active by Lacan & Ortuño 94

(2012) are shown in red in Fig. 1. Both the activity and the mode of deformation of these 95

faults are controversial. For example, in the easternmost part of the Pyrenees, the Têt and 96

Tech faults are considered as normal faults by some authors (e.g., Briais et al. 1990) or as 97

reverse, strike-slip or inactive by others (e.g., Philip et al. 1992; Calvet 1999; Goula et al. 98

1999; Carozza & Delcaillau 2000). Geomorphological investigations identified E-W 99

extensional active faults in the axial zone (Fig. 1B), especially in the Maladeta massif and 100


near the city of Lourdes (Alasset & Meghraoui 2005; Dubos-Sallée et al. 2007; Ortuño et al. 101

2008; Lacan & Ortuño 2012). To the west of Lourdes, thrust faulting prevails, indicating a 102

compressive regime (Lacan et al. 2012; Fig. 1). According to Lacan & Ortuño (2012), 103

deformation at the present time in the Pyrenees is governed by strike-slip in their western 104

part, by north-south compression with reverse faulting in the northern part of the central 105

Pyrenees. In the axial zone (i.e. the part of the Pyrenees where the topographic elevation 106

reaches 3000 m), deformation occurs on E-W striking en echelon normal faults, indicating N-107

S extension. This extension is also clearly expressed by the focal mechanisms of earthquakes 108

(Fig. 4; Chevrot et al. 2011) and the stress tensors determined by de Vicente et al. (2008). 109

110

2.2. Seismicity 111

Figure 1A shows the seismicity between 1989 and 2011, as compiled by the 112

Observatoire Midi-Pyrénées (OMP) and the Institut Cartogràfic i Geològic de Catalunya 113

(ICGC). The seismic activity is continuous with more than 15,000 moderate events with 114

magnitudes less than 5.5. Most of the events are located in the upper crust with depths less 115

than 15 km. The deepest events with depth 23-25 km occur in the western part of the belt. 116

Two important clusters are not considered in the following because they were induced 117

by human activity. The cluster located to the west of the city of Pau is related to production of 118

natural gas in the Lacq gas field (Bardainne et al. 2008). The E-W elongated cluster (between 119

1°W and 1°24’W) east to the city of Pamplona (Spain) is mainly but not entirely related to the 120

water impounded behind the Itoiz dam beginning in 2004 (Ruiz et al. 2006a; Durá-Gómez & 121

Talwani 2010). 122

The pattern of the seismicity shows a dichotomy between the eastern and the western 123

parts of the range, as suggested earlier (Souriau & Pauchet 1998). In the western part, the 124

seismicity is concentrated along a linear band of at least 30 km width. On N-S cross-sections, 125


the seismicity dips to the north at 55-60° down to 25 km depth (Gagnepain-Beyneix 1987; 126

Rigo et al. 2005). This seismicity is not clearly associated with crustal faults mapped in the 127

belt and the NPF seems to be seismically inactive. The most significant events were the M 128

5.3-5.7 Arette earthquake in 1967 (Hoang Trong & Rouland 1971), the Ml 5.1 Arudy 129

earthquake in 1980 (Gagnepain-Beyneix et al. 1982) and Ml 5.0 Argelès-Gazost earthquake in 130

2006 (Sylvander et al. 2008). 131

In the eastern part of the range, the seismicity is more diffuse and less prominent than 132

in the west. In the Maladeta area, the seismicity is fairly dense and active normal faults have 133

been identified in the field. The most recent significant earthquake occurred in 1996 near the 134

city of St-Paul-de-Fenouillet with a magnitude Ml 5.2 (Rigo et al. 1997; Pauchet et al. 1999; 135

Rigo & Massonnet 1999; Rigo 2010). 136

137

2.3 Previous geodetic studies 138

In 1992, the PotSis GPS network was installed in the easternmost part of the Pyrenees 139

in order to survey the area affected by the 1427 - 1428 seismic sequence (Briais et al. 1990; 140

Goula et al. 1996; Lambert & Levret 1996; Olivera et al. 2006: Lacan & Ortuño 2012). In 141

1995 and 1997, we installed the ResPyr network covering the entire Pyrenees range in order 142

to characterise and quantify its internal deformation. At that time, the deformation rate of 143

Pyrenees had not been measured directly. The deformation pattern expected was N-S 144

compression, according to geological and geodynamical considerations over time scales of 10 145

Myr. Surprisingly, the first GPS measurement of deformation across the Pyrenees yielded 146

extension at a rate of 0.5 ± 0.5 mm/yr (Nocquet & Calais 2004). The large uncertainty was 147

due to the inclusion of only a single continuous GPS site on the Spanish side of the range. 148

Nocquet (2012) updated this rate to a value less than 0.2 mm/yr. However, studying the 149

kinematics of the Iberia-Maghreb plate boundary, Stich et al. (2006) reported southward 150


velocities at 1.0 ± 0.6 mm/yr with respect to Europe of continuous GPS stations LLIV and 151

ESCO (fig. 8) on the Spanish side of the Pyrenees. Another recent study (Asensio et al. 2012), 152

based on continuous GPS observations over 3.5 years at 35 sites, shows that the stations south 153

of the western and central Pyrenees move away from the stable part of western Europe with a 154

velocity of 0.5 to 1.5 mm/yr. A NNE-SSW profile across the western Pyrenees indicates 155

extension at 2.5 ± 0.5 nstrain/yr. If we assume the width of the Pyrenees to be 100 - 150 km, 156

then this strain rate would be equivalent to 0.3 -0.4 mm/yr of extension. 157

158

3 GPS DATA AND ANALYSIS 159

3.1 GPS campaigns 160

Two GPS networks were installed in the Pyrenees in three phases during the 1990’s 161

(Fig. 1B). The PotSis network, covering the eastern part of the range with a 20 km average 162

distance between sites, was installed and measured for the first time in 1992 (Talaya et al. 163

1999). Each of the 24 sites was surveyed four times: in 1992, 1994, 1999, and 2006. Some 164

stations were also surveyed in 1996 following the Ml 5.2 earthquake of St-Paul-de-Fenouillet 165

(Rigo et al. 1997). 166

The ResPyr network, covering the whole Pyrenees and the northern and southern 167

forelands, was installed and first measured in 1995 from the Mediterranean Sea to the centre 168

of the range (at the city of Bagnères-de-Bigorre, site BGRN), and in 1997 from the centre to 169

the Atlantic Ocean and for the forelands (Fig. 1B; Fleta et al. 1996). The ResPyr network is 170

composed of 70 stations from which 9 are common to the PotSis network, with an average 171

spacing of 50 km. In 1995 and 1997, the satellites’ signals were recorded at each site during 172

three evening sessions of at least 8 hours. 173

Both networks were re-surveyed in 2008 and 2010 with sessions of 36 - 72 hours 174

(Table 1). Given the long time span between the first and the last surveys, several different 175


types of receivers (Trimble 4000SST, SSE and NetRS; Ashtech XII, Z-XII, Z-X, L-XII, LM-176

XII and UZ-XII; Rogue SNR-8100) and antennas (Trimble models 14532.00, 14532.10, 177

22020-00, 29659.00, 41249.00; Ashtech models 700228D, 700936A, 701975.01A, 700228A, 178

701945-01, and a Rogue antenna with choke rings) were used. The 74 sites observed are 179

given in Table 1. The sites with names starting with "0" use a constrained centering technique 180

on pillars. The others were surveyed using tripods. The tripod setup can be subject to 181

centering errors of up to ~2 mm, inducing velocity errors of less than 0.2 mm/yr over time 182

spans of more than 10 years. Anomalously high velocities relative to the surrounding points 183

were obtained due to local instabilities or due to identified mistakes by the operators. These 184

aberrant measurements are not shown or discussed in the following. 185

186

3.2 Data analysis 187

We use the GAMIT/GLOBK software package (http://www-gpsg.mit.edu) to compute 188

the coordinates and velocities of the surveyed GPS sites using a three-step strategy (Dong et 189

al. 1998; Feigl et al. 1993). To tie our local network to the ITRF reference frame, we include 190

GPS data from 18 International GNSS Service (IGS) stations, when available (BELL, CAGL, 191

CANT, CREU, EBRE, ESCO, GRAS, LLIV, MADR, MARS, MTPL, POTS, SFER, TLSE, 192

VILL, WSRT, WETT, ZIMM). No IGS sites were available in 1992, but in 1994, nine sites 193

were already available (BELL, CREU, EBRE, ESCO, LLIV, MADR, TLSE, WETT and 194

ZIMM). The two continuous GNSS sites close to our network were also added to the analysis 195

(LACA and FJCP). Following Reilinger et al. (2006), we account for temporally correlated 196

noise in each continuous GPS time series by using the first-order Gauss-Markov extrapolation 197

(FOGMEX) algorithm proposed by Herring (2003) to determine a random-walk noise term, 198

which we then incorporated into the Kalman filter used to estimate the velocities. For the 199

episodically measured sites, we apply a random-walk of 1 mm/sqrt(yr), equal to the average 200


of values obtained for 320 continuous GPS stations globally distributed with time series spans 201

ranging from 2.5 to 17 years. Velocities and their 1σ uncertainties were estimated in the 202

ITRF2008 reference frame and then transformed into the Eurasian reference frame by 203

minimizing the horizontal velocities of the survey sites and the IGS stations located on the 204

Eurasian plate (BELL, CAGL, CANT, CREU, EBRE, ESCO, GRAS, LLIV, MADR, MARS, 205

MTPL, POTS, TLSE, VILL, WSRT, WETT, ZIMM). The velocities in the Eurasia reference 206

frame are given in Table 2. The WRMS value for the horizontal velocity residuals of these 93 207

sites is 0.11 mm/yr. The velocities with 95% confidence ellipses are shown in Fig. 2. 208

209

4 VELOCITY FIELD 210

We finally obtain a set of 80 velocity vectors (74 campaign stations plus 6 continuous 211

stations) covering the whole Pyrenean range and the forelands. We do not interpret the 212

vertical velocities because their uncertainties are large. The time spans vary from 18 years for 213

PotSis (the easternmost and densest network), to 15 years and 13 years for the eastern and 214

western parts of ResPyr, respectively. Most of the velocities are smaller than their 215

uncertainties. Their directions are variable, making it difficult to characterise a general trend 216

of the deformation in the Pyrenees. Averaged over the entire network, the velocity 217

components VE = -0.3 ± 0.3 mm/yr (± 1σ) and VN = 0.0 ± 0.3 mm/yr (± 1σ) seem to suggest a 218

westward motion of the Pyrenees with respect to Eurasia. However, this motion is 219

concentrated in the centre part of the network (Fig. 2), which was first measured during two 220

campaigns in 1995 and 1997. In 1995, the measured sites were at longitudes east of 0°E and 221

in 1997, west of 0°E. This configuration may have induced a bias, generating this apparent 222

localized westward motion and we consider it as insignificant. 223

To interpret the velocity field, we compute five N-S profiles perpendicular to the E-W 224

Pyrenean tectonic frame, and one E-W profile, as indicated by arrows in Fig. 2. Each N-S 225


profile has a half-width of projection of 45 km. The E-W profile includes all the points. We 226

show the profiles in Fig. 3 with their corresponding topographic elevations. Profiles AA’ to 227

DD’ exhibit small strain rates from 0.0 to 0.8 nstrain/yr with uncertainties ranging from 1.1 228

nstrain/yr to 1.7 nstrain/yr. The westernmost profile (EE’) indicates N-S extension at a rate of 229

2.0 ± 1.8 nstrain/yr. Our results are compatible with the 2.5 ± 0.5 nstrain/yr estimated by 230

Asensio et al. (2012) for the western part of the range from a 3.5-year span of continuous GPS 231

data. 232

233

5 FOCAL MECHANISMS 234

The Pyrenees are shaken by a few hundred earthquakes every year. The maximum 235

magnitude recorded during the last 50 years is Ml = 5.7 for the 1967 Arette earthquake (date 236

19670813 in the table of appendix). The last major events were the Ml = 5.2 St-Paul-de-237

Fenouillet earthquake in 1996 (date 19960218 in the table of appendix; Rigo et al. 1997; 238

Pauchet et al. 1999; Rigo 2010) and the Ml = 5.0 Lourdes event in 2006 (date 20061117 in the 239

table of appendix; Sylvander et al. 2008). In order to compare qualitatively and quantitatively 240

the deformation fields inferred from the seismic and geodetic observations, we analyse the 241

available focal mechanisms. Several previous studies have used fewer than ten normal 242

faulting mechanisms to argue for north-south extension across the Pyrenees (de Vicente et al. 243

2008; Stich et al. 2010; Chevrot et al. 2011; Asensio et al. 2012). Using three different 244

methodologies in analysing the focal mechanisms, we seek to constrain the stress field when 245

others failed (Nicolas et al. 1990; Delouis et al. 1993). Does claimed N-S extension apply to 246

the whole range or is it restricted to some areas? 247

Our set of Pyrenean focal solutions is constituted by the best-constrained dataset of 248

Souriau et al. (2001). It has been updated to include the focal mechanisms from Dubos et al. 249

(2004), Ruiz et al. (2006b), Sylvander et al. (2008), and Chevrot et al. (2011) (Table 250


Appendix A). Thus, we obtain a final set of 194 focal mechanisms from 1967 to 2010 with 251

local magnitudes (Ml) ranging from 1.5 to 5.7 (Fig. 4). Like the seismicity, the focal solutions 252

are concentrated in the western part of the range and diffuse elsewhere. All types of 253

mechanisms are present: reverse faulting principally at the eastern and central parts of the 254

range with N-S and NW-SE striking nodal planes; strike-slip faulting both right-lateral and 255

left-lateral everywhere; and normal faulting mainly striking E-W principally in the central 256

part (south of the city of Tarbes). From this map, we infer that: (i) the deformation pattern is 257

not uniform across the Pyrenees; (ii) extension is not the only mode of deformation; and (iii) 258

there must be strong lateral variations in stress. 259

To characterise more precisely the Pyrenean deformation, we first analyse the focal 260

mechanisms following the methodology proposed by Delacou et al. (2004), based on the 261

plunge of the P and T axes. The P axes are vertical in an extensional domain whereas T axes 262

are vertical in a compressional regime. If the plunges of the P and T axes are equal, then the 263

stress field favours strike-slip faulting. Delacou et al. (2004) defined a factor r ranging from -264

90° for pure extension to 90° for pure compression, the null value corresponding to pure 265

strike-slip. We mapped the r factor values, one per focal solution, in Fig. 5 (top) with an 266

enlargement for the western part of the Pyrenees where the data are densest (Fig. 5, bottom). 267

Figure 5 shows that the Pyrenean deformation field is more complex than uniform 268

extension. Extension is apparent in the central part of the Pyrenees, in the area south of the 269

city of Tarbes, including three of the four normal faults mapped by Lacan & Ortuño (2012). 270

Two other areas of extension are identified: the area around the city of Pamplona in Spain, 271

and an area to the East of the Pyrenees, near the city of Olot, consistent with the presence of 272

NW-SE striking normal faults. A large E-W elongated area with a predominant strike-slip 273

regime is identified in the western part of the range, southwest to the city of Pau. We also 274

identify areas with compressional features in the eastern and western parts associated with 275


strike-slip deformation patterns and probably indicating a transpressional deformation style. 276

To the east, the compressional features correspond to the 1996 St-Paul-de-Fenouillet 277

earthquake, where the mainshock was a E-W left-lateral strike-slip mechanism and the 278

aftershocks had mostly reverse-faulting mechanisms (Rigo et al. 1997; Pauchet et al. 1999). 279

280

6 STRESS AND STRAIN-RATE TENSORS 281

6.1 Stress tensors 282

To go further in the analysis of the focal mechanisms, we compute the stress tensors. 283

We proceed by a fault-slip inversion using a Monte Carlo search method (Etchecopar et al. 284

1981) through the FSA software (Burg et al. 2005; Célérier 2011; Célérier et al. 2012). The 285

inversion searches for the stress tensor that explains the majority of focal mechanisms. The 286

magnitude of the events is not taken into account in the inversion. The focal mechanisms are 287

given by one of the nodal planes. Then, we performed two inversions, the first one for the 288

“first” nodal plane chosen randomly, the second one with the other nodal plane. Next, we 289

obtain the final stress tensor from a third inversion with the nodal planes, one per focal 290

mechanism, having the smallest misfit in the previous two inversions. The uncertainty areas 291

are defined from the five best solutions in each inversion. An inversion including all 194 focal 292

mechanisms yields an extensional stress with the most compressive principal axis σ1 oriented 293

vertically and the least axis σ3 oriented horizontally in the NE-SW direction (Table 3 and Fig. 294

6). This tensor is very similar to the one obtained by de Vicente et al. (2008), suggesting 295

active N-S extension. 296

Because of the distribution of the seismicity and diversity of the focal mechanisms, we 297

distinguish between the eastern and the western parts of the Pyrenees. In each part, we 298

determine the stress tensor using the same method (Table 3 and Fig. 6). In the eastern part, the 299

stress tensor shows a strike-slip regime where σ1 and σ3 are directed N-S and E-W, 300


respectively, as also found by Goula et al. (1999). On the western side, extension dominates 301

with σ1 vertical and σ3 horizontal but with a N-S direction instead. Figure 5 demonstrates that 302

the stress field varies across the Pyrenees, contradicting the uniformity assumed in previous 303

studies (de Vicente et al. 2008; Nocquet & Calais 2004; Nocquet 2012). Consequently, we 304

define eight zones where we estimate the stress tensors locally (Table 3, Fig. 6). This zoning 305

is similar to the one proposed by Baize et al. (2013) based on the combination of geologic, 306

seismologic, and tectonic data to assess seismic hazard. The number of data in each zone 307

varies. Zones 1, 5, and 6 include more than 30 focal mechanisms, whereas zones 2 and 7 308

include fewer than 10 data. Zone 4 is poorly constrained because it contains only 2 data. As 309

shown in Fig. 6, three zones are in compression: zones 1 and 6 with N-S compression, zone 4 310

with E-W compression. Zone 6 includes E-W striking active reverse faults (Lacan & Ortuño 311

2012). Zones 3, 5, and 7 exhibit a predominance of strike-slip deformation style with 312

extension and might be considered as transtensional areas. Zones 2 and 8 exhibit a pure 313

extension regime with NE-SW and NNW-SSE direction, respectively. Zone 5 is of particular 314

interest because the estimated σ3 direction is compatible with the normal faults proposed by 315

Lacan & Ortuño (2012). 316

Two other interesting points concern the σ3 axis. First, σ3 is the best-constrained 317

component in each case studied. In other words, the uncertainty of σ3 is less than those of 318

either σ1 or σ2. Second, the σ3 axis of least compressive stress is horizontal in all zones except 319

zone 6. The σ3 directions are consistent between zones 1, 2, and 3 in one group, zones 4 and 5 320

in another group, as well as zones 7 and 8 in a third group. This consistency and the stability 321

of a horizontal σ3 all over the range might explain why a purely extension stress tensor is 322

obtained at the Pyrenean scale. 323

324

6.2 Seismic strain-rate tensors 325


In the following, we quantify the Pyrenean deformation by determining geodetic and 326

seismic strain-rate tensors. We calculate the seismic strain-rate tensor according to the 327

formulations of Kostrov (1974) and Jackson & McKenzie (1988), as used previously in 328

different active regions (e.g., Masson et al. 2005). The average seismic strain rate !!" during a 329

time interval Δt is: 330

!!" =1

2!∆!" !!"!

!

!!!

where µ is the modulus of rigidity, V is the crustal volume containing the seismic sources, and 331

!!"! gives the components of the moment tensor !! of earthquake n as calculated from the 332

double-couple focal mechanism (Appendix A). We assume the shear modulus µ = 3 x 10!"Pa. 333

We calculate the strain-rate tensors for the whole range, then for the eastern and western 334

parts, and finally in each of the 8 zones defined for the stress tensor determinations over a 335

time span of 42.6 years. To calculate the seismogenic volume V, we take its vertical 336

dimension to be the maximum focal depth in each zone (Table 4). We also compute four 337

different strain-rate tensors in each zone: one for the complete set of focal mechanisms for an 338

elementary volume and without taking into account the magnitude (black strain-rate tensors in 339

Fig. 7) to be directly compared in directions with the stress tensors shown in Fig. 6, one for 340

the total set of focal mechanisms, one for the events with magnitude M ≥ 4.5 (and M < 5.7) 341

named high moment (HM) events, and finally one for the events with magnitude M < 4.5 342

named low moment (LM) events. The focal mechanisms of the LM events are well 343

constrained because they are determined from local and/or temporary seismic networks. The 344

HM events include 18 focal solutions, which is less than 10% of our dataset of focal 345

mechanisms. Nevertheless, because of their high moment, they will dominate the magnitudes 346

and directions of the components of the strain-rate tensors. For this reason, we also determine 347


the strain-rate tensors for the set of LM events to illustrate the deformation induced by this 348

prevailing type of seismic event (90% of the activity) in the Pyrenees. 349

The results for the horizontal components of the strain-rate tensors are shown in Table 350

4 and Fig. 7. As expected, the magnitudes of the strain rates for the complete dataset (denoted 351

(a) in Table 4 and Fig. 7) and for the HM events (denoted (b) in Table 4 and Fig. 7) are 352

similar, ranging from 0.001 nstrain/yr to 2 nstrain/yr. The magnitudes of the strain rates for 353

the LM events (denoted (c) in Table 4 and Fig. 7) are smaller by a factor of 10. Where the 354

strain-rate tensors (a) and (b) are very similar, they are represented by a single tensor labelled 355

(a b) in Fig. 7. We note the consistency between the directions of horizontal components of 356

the strain-rate tensor computed without the magnitude (black strain-rate tensors in Fig. 7) and 357

the horizontal axis of the stress tensors shown in Fig. 6, except in zones 3 and 6. In zone 3, 358

the directions of the horizontal components of the strain-rate tensor fall within the 359

uncertainties of the stress tensor such that they can be considered consistent. For zone 6, the 360

large discrepancy should be due to the large uncertainty in the estimate of the σ1 axis of the 361

stress tensor. The large discrepancies in direction between the strain-rate tensors without 362

taking into account the magnitudes (black) and the strain-rate tensors labelled (a) are due to 363

the predominance in the calculations of the mechanisms with the largest magnitudes. 364

For the whole Pyrenean chain, the strain-rate tensor for the LM events characterises an 365

extension regime with a NE-SW direction consistent with the corresponding stress tensor and 366

with the results of de Vicente et al. (2008). In the cases (a) and (b) (all events and HM events, 367

respectively), the strain-rate tensors are characteristic of a transtensional regime with a NW-368

SE extensional direction. The variability of the cases (a-b) and (c) is the expression of the 369

geographical variability of the deformation style in the Pyrenees. This heterogeneity is 370

noticeable when considering the eastern and western parts of the range. For the eastern region, 371

the (a) and (b) cases have strain-rate tensors corresponding to NW-SE extension, whereas 372


they correspond to strike-slip style in zone 1, to NE-SW extension style in zone 2, to E-W 373

extension in zone 3, and to E-W compression in zone 4. Secondly, for case (c), the strain-rate 374

tensors correspond to N-S compression in zone 1, E-W extension in zone 2, strike-slip in zone 375

3, and E-W compression in zone 4. For the western part of the Pyrenees, the strain-rate tensor 376

for the (a) and (b) cases characterises an E-W transtensional regime that combines the strike-377

slip styles in zones 5 and 6, and the extensional styles in zones 7 and 8. Case (c) shows strain-378

rate tensors with NE-SW extension for the western Pyrenean region and for zones 5, 7 and 8, 379

but transpression in zone 6. 380

381

6.3 Geodetic strain-rate tensors 382

Even though the velocities estimated for individual GPS stations are negligible with 383

95% confidence, we can group them together to determine the geodetic strain-rate tensors for 384

a comparison with the seismic strain-rate tensors. The geodetic strain-rate tensor is the 385

symmetric part of the 2-D tensor of the horizontal velocity gradient (Malvern 1969). These 386

tensors are determined in each seismic zone according to the following method. To estimate a 387

unique strain-rate tensor for each zone, we compute the linear trend of the velocity field, that 388

minimizes the residual velocity on east and north components for all the GPS sites included in 389

the zone. We then compute the spatial derivatives of this linear field which constitute the 390

average velocity gradient over the zone. In order to verify if this tensor is significant, we 391

compute up to 1000 strain-rate tensors by randomly perturbing the GPS velocities within their 392

uncertainties. We then calculate the reduction function, which is the ratio of the mean wrms 393

of the perturbed solutions to the wrms of the original GPS solution. A low value of the 394

reduction function implies that even with small velocity perturbations, we find a velocity 395

gradient similar to the original solution for the study area, hence the lower the value of the 396

reduction function, the more significant the estimated strain-rate tensor. The results are given 397


in Table 4, where they are denoted (d). Zone 7 has no strain-rate tensor determination because 398

it includes only one GPS site. If the reduction function is greater than 0.7, then the 399

corresponding strain-rate tensor is not well constrained because most of the 1,000 perturbed 400

solutions are statistically compatible with the originally velocity field. Table 4 shows that the 401

reduction function values are less than 0.6 in only four areas: the western region, the eastern 402

region, the whole Pyrenean range, and zone 3. We plot only the first three strain-rate tensors 403

with the corresponding velocity field in Fig. 8 (top). Zone 3 also has a small reduction 404

function (0.29) indicating a NW-SE extension not consistent with the corresponding seismic 405

strain-rate tensor obtained with diffuse and heterogeneous focal mechanisms. 406

Discrepancies exist between the velocity field estimated by Asensio et al. (2012) and 407

our study. However this could be due to a misalignment of the reference frames. For an easier 408

comparison, we compute the strain-rate tensors from the velocity field obtained by Asensio et 409

al. (2012) and denoted (e) in Table 4. The strain-rate tensors are independent of the reference 410

frame. In order to include their GPS stations, we extend the western region to the south. The 411

velocity field from Asensio et al. (2012), modified to be in a reference frame similar to our 412

study, and the strain-rate tensors are shown in Fig. 8 (bottom). The strain-rate tensors 413

estimated from the two GPS datasets are similar and compatible within their respective 414

uncertainties. From this comparison, we conclude that both velocity fields suggest a roughly 415

NS extension. 416

The magnitudes of the geodetic estimates of strain rate, however, are slightly greater 417

than the seismic estimates considering the HM events. On the other hand, they are consistent 418

within the 2σ uncertainties, especially in zones 4, 5, 8 and for the whole Pyrenees range and 419

for its western part (Table 4). Nevertheless, compared to the profiles (Fig. 3), the geodetic 420

strain rates are consistent with the seismic ones for zones 1 and 2 with profiles AA’, and 421

zones 5 and 6 with profile EE’. A deformation style of N-S extension seems to characterise 422


the whole Pyrenees range in both cases. Transtension is apparent in the western region of the 423

range with a NW-SE extensional direction for our data and a NE-SW direction of greater 424

extension for Asensio’s observations, consistent with the overall seismic strain-rate tensor for 425

all events and the HM subset within this zone. The eastern part of the range shows poorly 426

constrained strain-rate tensors with a strike-slip deformation regime for our data and with NE-427

SW extension for Asensio’s data. 428

The seismic catalog we used spans only ~40 years, a small fraction (2 percent) of the 429

recurrence interval if of the order of ~2500 years for magnitude 6.5 earthquakes estimated 430

from a previous study of geodetic strain rates spatially averaged over the entire Pyrenean 431

range (Asensio et al. 2012). Since large earthquakes occur irregularly, any estimates of strain-432

rate and its attendant uncertainty that are based on seismicity are very likely to be biased by 433

temporal aliasing. Furthermore, the assumption of a periodic or “characteristic” earthquake is 434

doubtful at best. In California, where the rate of deformation is arguably at least an order of 435

magnitude faster than in the Pyrenees, large earthquakes occur on the San Andreas fault at 436

irregular intervals with large variance in timing (Sieh et al., 1989). Similarly, extrapolating 437

the deformation rate in time from the 1966 Parkfield earthquake failed to predict the next 438

large earthquake there with 95% confidence (Murray & Segall 2002). 439

Consequently, we compare the geodetic and seismic strain tensors in terms of spatial 440

orientation rather than temporal rate. Such an approach has been used successfully in 441

California, where the catalog of earthquakes for which a centroid moment tensor can be 442

calculated spans only a small fraction of the time since a large earthquake with magnitude 443

greater than 7 on the San Andreas fault (e.g., Ekström & England 1989). Nonetheless, the 444

“orientations of relative velocity … determined from the seismicity between 1977 and 1987 445

agree within a few degrees with those determined from plate motions” (Ekström & England 446

1989). The approach is further justified mathematically by the fact that the orientation of the 447


principal axis is the only directional quantity of the three required to define a horizontal strain 448

rate tensor. Since this orientation does not involve time, it cannot be biased by temporal 449

aliasing. Furthermore, a calculation including the focal mechanisms for small-magnitude 450

earthquakes is likely to lead to an estimate of the orientation of the strain field that better 451

represents the tectonic style than one neglecting them. 452

453

7 DISCUSSIONS AND CONCLUSION 454

We have analysed GPS surveys over the whole Pyrenean range at 74 sites spanning a 455

time interval of 18 years. The resulting velocity field (Fig. 2) shows that only a few of the 456

velocities are significantly different from zero with 95% confidence. Projected along profiles, 457

only the westernmost N-S profile (profile EE’ in Fig. 3), with a deformation rate of 2.0 ± 1.8 458

nstrain/yr, approaches the value of 2.5 nstrain/yr given by Asensio et al. (2012). In the east, 459

Asensio’s study and ours show insignificant horizontal deformation rates. Moreover, since the 460

measured networks include sites on both sides of the Pyrenees, we infer that the continental 461

collision that generated this range appears now to have ceased to deform measurably. Thus 462

we conclude that the Iberian plate is attached to the Eurasian one to within a few tenths of a 463

millimeter per year. 464

Despite this very low deformation rate, we observe a diversity of deformational styles. 465

Extension is principally present in the central part of the range, where most of the earthquakes 466

analysed by de Vicente et al. (2008) and Chevrot et al. (2011) occur. The westernmost zone of 467

the Pyrenees also exhibits extension, principally in the Spanish side around the city of 468

Pamplona, with a predominant strike-slip regime. At the ends of the range, horizontal 469

compression is apparent, especially in the eastern part, where the deformation is probably 470

amplified by the 1996 seismic crisis of St-Paul-de-Fenouillet (Rigo et al. 1997; Pauchet et al. 471

1999; Rigo 2010). In the western part, an area of right-lateral strike-slip deformation regime 472


(zone 6) connects two areas (zones 5 and 7-8) with perpendicular extension directions. This 473

result suggests a rotation of the principal axis σ2 from vertical to horizontal around a NE-SW 474

horizontal σ3 axis. This interpretation may explain why the fault-slip inversion of all focal 475

mechanisms gives a stress tensor similar to the one obtained by de Vicente et al. (2008). We 476

divide the Pyrenean range in eight zones in addition to the whole Pyrenees range and its 477

western and eastern parts, for which we determined the seismic and geodetic stress and strain-478

rate tensors (Fig. 6, 7 and 8). Overall, the orientations of seismic stress and strain-rate tensors 479

are consistent with the mapped active faults. Schematically, the Pyrenean deformation regime 480

can be described as varying from transpression to the east to transtension to the west. Thus, 481

deformation in the Pyrenees cannot be reduced to a single style but varies from east to west 482

and from north to south over length scale of 20-40 km. 483

We conclude with three remarks. First, regarding the distribution of the seismicity, we 484

note diffuse seismicity in the eastern part with a concentration at the Maladeta area, which is 485

constituted by a granitic massif (Ortuño et al. 2008). In the central part of the range, the 486

seismicity is principally located in the boundary zone between the North Pyrenean Zone and 487

the Axial Zone. Since there is almost no seismic activity in the internal part of the range, this 488

seismicity might be the expression of the deformation of the contact of two zones and not of 489

the entire range. Because of the sparsity of seismicity, we cannot characterise or quantify the 490

deformation at the northern and southern fronts of the Pyrenees. Measuring the rate of 491

deformation there with geodetic techniques would require to resurvey the survey mode GPS 492

networks and develop a denser network of continuously operating GNSS stations. 493

The second remark concerns the heterogeneity of the internal deformation of the 494

Pyrenees, which is clear in Figs 5, 6, and 7, where the style of deformation changes from one 495

area to another. Since we divided the Pyrenees in zones according to seismicity and tectonics, 496

the estimated strain-rate tensors capture the spatial complexity of the deformation field with a 497


resolution of around 50 km. Camelbeeck et al (2013) argued from the second derivative of the 498

gravitational potential energy that the deformation styles in the Pyrenees may vary laterally 499

over a few tens of kilometres. Although seismic and geodetic data cannot detect such details, 500

but our and their observations are consistent, for example, in the central and eastern parts of 501

the range with extension and strike-slip deformation styles, respectively. 502

Thirdly, we have analysed only the horizontal deformation. Further research is 503

required to constrain the vertical motion in the Pyrenees. Although Gimenez et al. (1996) 504

analysed levelling data to find 1-4 mm/yr of uplift in the southeastern part of the range 505

(Catalonia), Rigo & Cushing (1999) showed that there is no significant vertical movement 506

along a short line in the central part of the range. However, with these fragmentary results, we 507

cannot exclude the possibility of mostly vertical motion, as modelled by Vernant et al. (2013). 508

To evaluate this possibility, it would be necessary to measure levelling lines across the 509

Pyrenees to capture the differential vertical motion between the internal and higher part of the 510

range and the forelands. 511

512

Acknowledgements 513

We are particularly grateful to the numerous participants to the GPS campaigns in the field, 514

coming from ESGT, IRSN, IGN, ICGC and the universities of Toulouse, Montpellier and 515

Barcelona. We thank Annie Souriau and Sébastien Chevrot for constructive discussions. We 516

thank R.W. King and an anonymous reviewer for their pertinent and very helpful comments 517

on the manuscript. This work benefited from the instruments of Parc GPS INSU and from the 518

financial support of the Tectoscope Positionnement, PNRN, 3F and CT3 programs from 519

INSU/CNRS and of the Regional Council of Midi-Pyrénées and of Cataluña Province. The 520

figures were prepared using the Generic Mapping Tool GMT (Wessel & Smith 1991). 521

522


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Perea, H., 2009. The Catalan seismic crisis (1427 and 1428; NE Iberian Peninsula): 687

Geological sources and earthquake triggering, J. Geodynamics, 47, 259-270. 688

Philip, H., Bousquet, J. C., Escuer, J., Fleta, J., Goula, X. & Grellet, B., 1992. Présence de 689

failles inverses d'âge quaternaire dans l'Est des Pyrénées: implications 690

sismotectoniques, C. R. Acad. Sci. Paris, 314, 1239-1245. 691

Pous, J., Ledo, J., Marcuello, A. & Daignères, M., 1995. Electrical resistivity model of the 692

crust and upper mantle from a magnetotelluric survey through the central Pyrenees, 693

Geophys. J. Int., 121, 750-762. 694

Reilinger, R., McClusky, S., Vernant, P., Lawrence, S., Ergintav, S., Cakmak, R., Ozener, H., 695

Kadirov, F., Guliev, I., Stepanyan, R., Nadariya, M., Hahubia, G., Mahmoud, S., Sakr, 696


K., ArRajehi, A., Paradissis, D., Al-Aydrus, A., Prilepin, M., Guseva, T., Evren, E., 697

Dmitrotsa, A., Filikov, S. V., Gomez, F., Al-Ghazzi, R. & Karam, G., 2006. GPS 698

constraints on continental deformation in the Africa‐Arabia‐Eurasia continental 699

collision zone and implications for the dynamics of plate interactions, J. Geophys. 700

Res., 111, 2156-2202. 701

Rigo, A., 2010. Precursors and fluid flows in the case of the 1996, ML=5.2 Saint-Paul-de-702

Fenouillet earthquake (Pyrenees, France): A complete pre-, co- and post-seismic 703

scenario, Tectonophysics, 480, 109-118. 704

Rigo, A., Souriau, A., Dubos, N., Sylvander, M. & Ponsolles, C., 2005. Analysis of the 705

seismicity in the central part of the Pyrenees (France), and tectonic implications, J. 706

Seismology, 9, 211-222. 707

Rigo, A. & Massonnet, D., 1999. Investigating the 1996 Pyrenean earthquake (France) with 708

SAR interferograms heavily distorted by atmosphere, Geophys. Res. Lett., 26, 3217-709

3220. 710

Rigo, A. & Cushing, M., 1999. Effets topographiques sur les comparaisons de profils de 711

nivellement: cas français de Saint-Paul-de-Fenouillet (Pyrénées-Orientales) et d'Arudy 712

(Pyrénées-Atlantiques), C.R. Acad. Sci. Paris, 329, 697-704. 713

Rigo, A., Pauchet, H., Souriau, A., Grésillaud, A., Nicolas, M., Olivera, C. & Figueras, S., 714

1997. The February 1996 earthquake sequence in the eastern Pyrenees: first results, J. 715

Seismology, 1, 3-14. 716

Ruiz, M., Gaspà, O., Gallart, J., Diaz, J., Pulgar, J. A., Garcia-Sansegundo, J., Lopez-717

Fernandez, C. & Gonzalez-Cortina, J. M., 2006a. Aftershocks series monitoring of the 718

September 18, 2004 M=4.6 earthquake at the western Pyrenees: A case of reservoir-719

triggered seismicity?, Tectonophysics, 424, 223-243. 720

Ruiz, M., Gallart, J., Diaz, J., Olivera, C., Pedreira, D., Lopez, C., Gonzalaz-Cortina, J. M. & 721


Pulgar, J. A., 2006b. Seismic activity at the western Pyrenean edge, Tectonophysics, 722

412, 217-235. 723

Sieh, K., Stuiver, M. & Brillinger, D., 1989. A more precise chronology of earthquakes 724

produced by the San Andreas fault in southern California, J. Geophys. Res., 94, 603-725

623. 726

Smith, W.H.F. & Wessel, P., 1990. Gridding with continuous curvature splines in tension, 727

Geophysics, 55, 293–305. 728

Souriau, A., Chevrot, S. & Olivera, C., 2008. A new tomographic image of the Pyrenean 729

lithosphere from teleseismic data, Tectonophysics, 460, 206-214. 730

Souriau, A., Sylvander, M., Rigo, A., Fels, J.-F., Douchain, J.-M. & Ponsolles, C., 2001. 731

Sismotectonique des Pyrénées: principales contraintes sismologiques, Bull. Soc. géol. 732

Fr., 172, 25-39. 733

Souriau, A. & Pauchet, H., 1998 A new synthesis of Pyrenean seismicity and its tectonic 734

implications, Tectonophysics, 290, 221-244. 735

Souriau, A. & Granet, M., 1995. A tomographic study of the lithosphere beneath the Pyrenees 736

from local and teleseismic data, J. Geophys. Res., 100, 18,117-18,134. 737

Stich, D., Martin, R. & Morales, J., 2010. Moment tensor inversion for Iberia-Maghreb 738

earthquakes 2005-2008, Tectonophysics, 483, 390-398. 739

Stich, D., Serpelloni, E., Mancilla, F. L. & Morales, J., 2006. Kinematics of the Iberia-740

Maghreb plate contact from seismic moment tensors and GPS observations, 741


Sylvander, M., Souriau, A., Rigo, A., Tocheport, A., Toutain, J. P., Ponsolles, C. & 743

Benahmed, S., 2008. The November 2006, ML = 5.0 earthquake near Lourdes (French 744

Pyrenees): normal faulting in an active mountain belt, Geophys. J. Int., 175, 649-664. 745


Talaya, J., Feigl, K., Térmens, A. & Colomina, I., 1999. Practical lessons from analysis of a 746

GPS network designed to detect movements of = 1 mm/year in the eastern Pyrenees, 747

Phys. Chem. Earth, 24, 355–359. 748

Torné, M., Cabissole, B. D., Bayer, R., Casas, A., Daignières, M. & Rivero, A., 1989. Gravity 749

constraints on the deep structure of the Pyrenean belt along the ECORS profile, 750


Vacher, P. & Souriau, A., 2001. A three-dimensional model of the Pyrenean deep structure 752

based on gravity modelling, seismic images and petrological constraints, Geophys. J. 753

Int., 145, 460-470. 754

Vernant P., Hivert F., Chéry J., Steer P., Cattin R. & Rigo A., 2013. Erosion-induced isostatic 755

rebound triggers extension in low convergent mountain ranges, Geology, 41, 467-470. 756

Vissers, R. L. M. & Meijer, P. T., 2012. Iberian plate kinematics and Alpine collision in the 757

Pyrenees, Earth-Science Reviews, 114, 61-83. 758

Wessel, P. & Smith, W.H.F., 1991. Free software helps map and display data, EOS, Trans. 759

Am. Geophys. Un., 72(41), 441. 760

761


Tables 762

763

764

Table 1: Schedule of occupation of the PotSis and ResPyr GPS Pyrenean sites. 765

766


767

Site Longitude Latitude VE ± 1 σ VN ± 1 σ ρ ARD0 -2.10 42.25 -0.74 0.32 0.56 0.32 -0.005 ARG0 -0.53 43.54 -0.21 0.32 -0.07 0.32 -0.006 ASCO 1.86 42.72 -0.91 0.32 -0.30 0.32 -0.005 ASPI 0.33 42.94 -0.34 0.30 -0.19 0.30 -0.003 ATC0 -1.19 43.33 -0.35 0.31 0.43 0.32 -0.002 AYER -0.72 42.29 0.06 0.35 0.10 0.35 -0.004 AYG0 -2.05 42.66 0.30 0.32 -0.25 0.32 -0.003 BEL0 -1.62 43.05 -0.30 0.31 0.57 0.32 0.000 BEN0 0.47 42.11 -0.46 0.30 -0.01 0.30 -0.007 BGRN 0.13 43.06 -0.82 0.33 -0.18 0.34 -0.003 BGS0 0.10 42.91 -0.50 0.33 -0.10 0.32 -0.012 BIZ0 -0.32 43.31 0.06 0.34 0.41 0.33 -0.009 BOHO -1.01 43.10 -0.15 0.34 -0.09 0.34 -0.001 BOR0 -0.40 42.56 -0.22 0.31 -0.31 0.32 -0.001 BOUI 2.44 43.00 -0.30 0.31 -0.14 0.31 -0.010 CAAR 0.01 42.42 0.19 0.34 -0.06 0.34 -0.003 CAR0 -1.62 42.33 -0.38 0.31 -0.32 0.32 0.000 COUP 1.07 43.14 -0.81 0.32 -0.11 0.32 0.009 DUH0 -0.32 43.71 0.14 0.32 -0.13 0.32 -0.002 EMBU -0.72 42.63 -0.85 0.39 -0.34 0.40 0.024 ETXA -1.78 42.80 -0.58 0.34 0.08 0.35 -0.002 FUE0 -0.88 42.36 -0.39 0.32 0.16 0.32 -0.006 ISSA -0.79 43.02 0.20 0.34 0.18 0.35 -0.004 JAUT -0.34 43.03 -0.88 0.35 0.48 0.36 -0.007 LHRZ 1.39 42.81 -0.06 0.30 -0.42 0.30 -0.008 LIE0 -1.32 42.63 0.02 0.31 0.11 0.32 -0.001 MARC 0.16 43.53 -0.28 0.35 -0.13 0.37 -0.026 MCA0 3.02 43.60 -0.64 0.29 0.25 0.30 -0.006 MGA0 1.64 42.94 -0.85 0.30 0.16 0.30 0.008 MRTE 0.75 42.92 -0.67 0.32 0.26 0.33 -0.007 NEN0 0.70 43.36 -0.41 0.32 0.19 0.33 -0.002 PANT -0.24 42.76 -0.01 0.34 -0.32 0.35 -0.004 PER0 -0.02 42.05 0.11 0.32 0.15 0.32 -0.003 PLAB 0.95 42.71 0.17 0.33 -0.13 0.32 -0.016 POM0 -0.83 43.63 0.21 0.32 -0.01 0.32 -0.001 PPY0 1.71 42.64 1.05 0.33 0.42 0.30 -0.016 PYG0 0.46 42.77 -2.92 1.09 1.55 1.11 -0.055 QUI0 2.15 42.87 -0.22 0.31 0.04 0.30 -0.019 RGS0 2.63 43.16 0.07 0.29 -0.29 0.29 -0.006 RIEB 1.34 43.06 -0.16 0.30 -0.15 0.30 -0.011 RON0 -0.98 42.74 -0.09 0.32 0.21 0.32 -0.002 RPE0 2.53 43.42 -0.70 0.31 -0.34 0.31 -0.017 SEIX 1.13 42.87 -0.13 0.33 -0.29 0.32 -0.014 SEPE -0.41 42.25 0.58 0.37 -0.29 0.35 -0.018 SERR 0.71 42.56 0.05 0.32 -0.53 0.32 -0.011 SOMP -0.55 42.80 -0.69 0.34 -0.20 0.34 -0.001 TRMO 0.10 42.73 -0.69 0.31 -0.01 0.31 -0.001


TRON 0.31 42.31 -0.45 0.32 -0.59 0.32 -0.014 UNC0 -1.19 42.33 -0.43 0.33 0.00 0.33 -0.005 UNZU -1.63 42.66 0.21 0.36 -0.40 0.36 -0.008 VCGE 2.82 43.04 -0.11 0.31 -0.42 0.31 0.000 VGR0 -2.53 42.31 -0.61 0.31 0.12 0.31 0.000 0002 2.10 42.23 -0.18 0.32 -0.19 0.32 0.012 0003 2.17 42.34 -0.43 0.29 0.05 0.29 0.004 0004 2.32 42.32 -0.56 0.32 -0.04 0.32 0.011 0005 2.26 42.23 -0.35 0.31 0.10 0.31 0.008 0007 2.52 42.17 0.02 0.31 0.03 0.31 0.006 0008 2.53 42.26 -0.01 0.31 -0.16 0.31 0.007 0009 2.33 41.99 -0.13 0.31 -0.12 0.31 0.009 0010 2.59 42.08 0.08 0.31 0.18 0.31 0.005 0011 2.80 42.19 0.12 0.31 -0.15 0.31 0.005 0013 2.82 42.40 -0.15 0.33 0.27 0.32 0.024 0014 2.27 42.12 0.06 0.31 -0.05 0.31 0.007 0102 2.29 42.54 -0.06 0.30 0.23 0.30 0.003 0103 2.50 42.38 -0.63 0.29 0.40 0.29 0.008 0104 2.45 42.73 -0.37 0.32 -0.09 0.32 0.012 0105 2.52 42.80 -0.20 0.32 -0.26 0.32 0.008 0106 2.41 42.58 -0.04 0.33 0.15 0.33 0.007 0107 2.58 42.64 -0.19 0.31 -0.19 0.31 0.009 0108 2.64 42.48 -0.16 0.33 0.60 0.34 0.009 0109 2.68 42.73 -0.77 0.30 0.25 0.31 0.002 0110 2.76 42.88 -0.19 0.32 -0.07 0.32 0.012 0111 2.72 42.62 -0.66 0.32 -0.08 0.32 0.013 0112 2.87 42.51 -0.21 0.30 0.55 0.31 0.001 BELL 1.40 41.60 -0.33 0.33 -0.03 0.33 0.000 CAGL 8.97 39.14 -0.37 0.32 0.03 0.32 0.011 CANT -3.80 43.47 -0.93 0.37 0.22 0.37 0.001 CREU 3.32 42.32 -0.45 0.32 -0.07 0.32 0.002 EBRE 0.49 40.82 -0.10 0.28 -0.34 0.28 0.001 ESCO 0.98 42.69 -0.98 0.37 0.26 0.37 0.001 FJCP 2.80 43.05 -0.46 0.44 0.44 0.44 0.001 GRAS 6.92 43.76 -0.15 0.15 0.22 0.15 0.067 LACA 2.73 43.68 -0.26 0.46 0.57 0.46 0.002 LLIV 1.97 42.48 -0.40 0.36 -0.05 0.36 0.001 MADR -4.25 40.43 -0.01 0.17 -0.41 0.17 0.009 MARS 5.35 43.28 -0.65 0.32 0.11 0.32 0.003 MTPL 3.87 43.64 0.05 0.32 -0.02 0.32 0.002 POTS 13.07 52.38 -0.38 0.04 -0.15 0.04 0.354 TLSE 1.48 43.56 -0.16 0.36 0.11 0.36 0.001 VILL -3.95 40.44 -0.13 0.18 0.31 0.18 0.011 WETT 12.88 49.14 1.35 0.77 1.35 0.74 0.075 WSRT 6.60 52.92 -0.07 0.32 0.33 0.32 0.000 ZIMM 7.47 46.88 0.01 0.20 0.19 0.20 0.009 Table 2: Coordinates and velocities (mm/yr) with 1σ uncertainties of the 74 GPS pyrenean 768

sites of the ResPyr and PotSis networks plus the 19 permanent GPS sites used for the 769


transformation of the velocities into the Eurasian reference frame. VE: east component; VN: 770

north component; ρ: correlation coefficient between east and north components. 771

772


773

S1 S2 S3 Zone Nb f.m. Azimuth Plunge Azimuth Plunge Azimuth Plunge Total 195 86 82 314 5 223 45 East 71 351 17 128 67 256 14 West 124 217 60 110 10 15 28

1 47 15 27 125 33 255 44 2 7 306 37 166 45 53 21 3 15 307 66 178 15 83 17 4 2 267 21 113 65 1 10 5 69 294 16 119 73 24 1 6 35 213 8 122 8 346 78 7 13 229 34 110 35 349 36 8 7 225 63 77 23 342 12

Table 3: Principal stress axis S1 (σ1, maximum), S2 (σ2) and S3 (σ3, minimum) resulting 774

from FSA inversion. Nb f.m.: number of focal mechanisms used in the inversion. 775

776


Zone 1 2 3 (°) Reduct. F.

1 (11)

a (47) -0.78 0.89 143 b (1) -0.74 0.91 144 c (46) d

-0.06 -17.59 ± 9.75

0.004 -2.13 ± 6.28

107 89 ± 29

0.75

2 (9)

a (7) 0.01 0.28 49 b (1) -0.002 0.23 36 c (6) d

-0.024 -1.63 ± 6.50

0.09 25.25 ± 18.31

80 165 ± 23

0.85

3 (17)

a (15) 0.02 0.04 73 b (5) 0.02 0.04 87 c (10) d

-0.004 -3.90 ± 4.95

0.005 7.39 ± 5.93

48 136 ± 54

0.29

4 (5)

a (2) -0.01 0.005 177 b (0) - - - c (2) d

-0.01 -23.28 ± 24.05

0.005 25.26 ± 25.28

177 73 ± 23

0.89

5 (16)

a (69) -1.88 2.31 127 b (6) -2.00 2.34 127 c (63) d

-0.04 -8.58± 9.83

0.14 29.43 ± 13.76

21 143 ± 13

1.06

6 (20)

a (35) -4.89 6.77 90 b (4) -6.50 9.01 90 c (31) d

-0.01 -9.22 ± 34.07

0.01 48. 40 ± 58.68

62 19 ± 47

0.81

7 (15)

a (13) -0.0002 0.0004 15.8 b (0) - - - c (13) d

-0.0002 -

0.0004 -

15.8 -

8 (7)

a (7) -0.12 0.61 7 b (2) -0.09 0.51 6 c (5) d

-0.03 -12.91 ± 15.91

0.10 21.27 ± 20.53

8 142 ± 58

1.18

all (20)

a (195) -0.22 0.36 109 b (19) -0.25 0.39 109 c (176) -0.003 0.01 30

d e

-0.86 ± 1.00 0.77 ± 0.56

2.21 ± 1.69 3.85 ± 1.79

169 ± 20 21 ± 22

0.52 0.06

east (17)

a (71) -0.02 0.07 142 b (7) -0.02 0.08 145 c (64) -0.006 0.006 78

d e

-0.88 ± 1.38 -0.06 ± 3.92

0.62 ± 1.96 3.27 ± 2.84

43 ± 72 164 ±76

0.39 0.42

west (20)

a (124) -0.46 0.69 106 b (12) -0.55 0.79 106 c (112) -0.008 0.03 20

d e

-1.37 ± 1.47 2.44 ± 1.61

3.76 ± 1.94 4.46 ± 1.70

155 ±14 44 ± 81

0.54 0.25


Table 4: Horizontal components of the strain-rate tensors. The vertical dimension of the 777

seismogenic volume V is indicated between brackets and below at each zone. 1: amplitude of 778

the first axis in nstrain/yr (negative is compression); 2: amplitude of the second axis in 779

nstrain/yr (positive is extension); 3: azimuth of the second axis; Reduct. f.: reduction function 780

value; a: all focal mechanisms; b: focal mechanisms with M ≥ 4.5 (HM events); c: focal 781

mechanisms with M < 4.5 (LM events); d: GPS with ± 2σ, this study; e: GPS with ± 782

2σ, Asensio et al. (2012). In brackets after a, b, c the number of focal mechanisms used. 783

784


APPENDIX 785

786

Parameters of the 194 focal mechanisms. bul**: bulletins from OMP/SGC, year 19**; 787

SGC**: bulletins from SGC, year 19** (from 1984 to1995, published by Servei Geològic de 788

Catalunya (SGC); 1995-2005, published by Institut Cartogràfic de Catalunya (ICC); 2006-789

2011, published by Institut Geològic de Catalunya (IGC); HR71: Hoang Trong & Rouland 790

1971; GMa80: Gagnepain et al. 1980; Ga82: Gallart et al. 1982; GDa84: Gallart et al. 1984; 791

GDa85: Gallart et al. 1985; G87: Gagnepain-Beyneix 1987; OG87: Olivera & Gallart 1987; 792

LDG88: Cansi et al. 1988; NSD90: Nicolas et al. 1990; Ga92: Grandjean 1992 and Grandjean 793

et al. 1994; Gui92: Guilbert 1992; Oa92: Olivera et al. 1992; Da93: Delouis et al. 1993; Ra97: 794

Rigo et al. 1997; Pa99: Pauchet et al. 1999; Du04: Dubos et al. 2004; Ru06: Ruiz et al. 2006b; 795

Syl08: Sylvander et al. 2008; Che11: Chevrot et al. 2011. Depth is in km (U: unknown). 796

797

Date Longitude Latitude Depth Magnitude Ml Strike Dip Rake Reference 19670813 -0.76 43.09 U 5.7 41.0 63.0 -19.2 HR71 19690311 0.70 42.70 5 4.0 2.0 72.0 -99.3 NSD90 19700314 1.70 42.52 5 4.0 270.0 79.0 108.3 NSD90 19700314 1.68 42.52 5 4.7 28.0 59.0 85.9 NSD90 19700316 1.71 42.54 5 4.6 290.0 82.0 134.7 NSD90 19700405 1.66 42.47 5 4.9 216.0 55.0 -85.4 NSD90 19770912 -0.99 43.06 7 4.5 267.0 75.0 126.9 NSD90 197811 -0.36 43.06 11 2.5 317.0 16.0 92.9 GMa80 197811 -0.48 43.06 3 1.8 12.0 20.0 -163.0 GMa80 197811 -0.50 43.07 2 1.7 12.0 20.0 -163.0 GMa80 197811 -0.52 43.09 1 U 38.0 78.0 -26.8 GMa80 197811 -0.57 43.06 5 U 74.0 80.0 90.2 GMa80 197811 -0.62 43.06 9 U 141.0 63.0 162.8 GMa80 197811 -0.65 43.10 10 1.7 91.0 70.0 -40.0 GMa80 197811 -0.66 43.09 9 U 350.0 85.0 149.0 GMa80 197811 -0.74 43.09 3 2.8 80.0 54.0 168.2 GMa80 197811 -0.75 43.06 3 2.0 78.0 45.0 167.4 GMa80 197811 -0.75 43.06 1 U 25.0 49.0 87.4 GMa80 197811 -0.75 43.06 1 U 62.0 60.0 89.5 GMa80 197811 -0.75 43.06 1 U 7.0 40.0 93.1 GMa80 197811 -0.87 43.07 5 U 84.0 76.0 90.0 GMa80 19781103 2.70 42.84 8 4.3 286.0 52.0 48.7 Ga92 19790928 0.54 43.15 16 4.1 276.0 72.0 173.6 NSD90 19800229 -0.39 43.17 6 5.7 80.0 50.0 0.0 NSD90 19800302 -0.37 43.07 6 3.3 254.0 75.0 -145.7 G87 19800303 -0.36 43.07 6 3.9 160.0 48.0 -20.4 G87 19800303 -0.38 43.08 5 3.2 50.0 22.0 -118.2 G87 19800303 -0.40 43.06 5 3.8 258.0 82.0 -153.3 G87 19800303 -0.42 43.07 6 3.3 188.0 52.0 -12.9 G87 19800304 -0.38 43.06 5 3.7 154.0 76.0 180.0 G87 19800304 -0.37 43.07 5 3.1 22.0 10.0 0.0 G87


19800305 -0.40 43.07 4 3.6 266.0 73.0 -94.5 G87 19800307 -0.37 43.06 5 3.0 129.0 40.0 -106.4 G87 19800307 -0.37 43.07 4 3.3 139.0 52.0 -56.6 G87 19810423 1.95 42.90 4 4.5 176.0 70.0 174.2 Ga82 19810719 0.07 43.09 6 4.6 132.0 40.0 39.2 NSD90 19810928 -0.03 43.17 13 4.3 27.0 75.0 -164.9 NSD90 19820106 -0.98 43.21 15 5.0 210.0 81.0 -59.7 NSD90 19820522 -1.72 42.80 3 4.7 80.0 48.0 -101.6 OG87 19820522 -1.69 42.81 1 4.4 70.0 60.0 -124.5 OG87 19820522 -1.68 42.77 2 3.8 70.0 76.0 -134.1 OG87 19820622 -1.69 42.80 7 4.7 71.0 66.0 -128.6 OG87 19820825 -0.28 43.07 8 4.3 290.0 80.0 -39.0 NSD90 19821023 -0.42 43.09 8 1.7 277.0 73.0 -84.6 GDa85 19821024 -0.61 43.08 8 2.1 63.0 82.0 54.4 GDa85 19821030 -0.37 43.07 5 2.0 340.0 80.0 -53.1 GDa85 19821031 -0.51 42.92 4 2.1 290.0 80.0 -26.7 GDa85 19821101 -0.52 42.92 2 1.9 34.0 75.0 -143.1 GDa85 19821101 -0.82 43.08 13 1.9 306.0 80.0 -122.9 GDa85 19821103 -0.75 43.15 15 1.5 348.0 65.0 90.4 GDa85 19821105 -0.97 43.14 16 2.1 171.0 83.0 52.4 GDa85 19821105 -1.04 43.09 3 1.0 98.0 50.0 -99.1 GDa85 19821105 -0.82 43.15 19 1.6 21.0 65.0 -132.2 GDa85 19821111 -0.34 43.05 4 2.1 343.0 75.0 -28.5 GDa85 19821115 -0.12 43.07 16 2.2 289.0 60.0 -79.1 GDa85 19821115 -0.60 43.10 5 1.6 205.0 80.0 98.9 GDa85 19821122 -1.22 43.17 4 1.8 166.0 50.0 113.4 GDa85 19821203 -0.45 43.11 14 1.8 124.0 53.0 -71.6 GDa85 19821215 -0.24 43.10 15 1.7 121.0 70.0 -94.2 GDa85 19821224 0.50 42.55 5 4.0 126.0 73.0 51.2 NSD90 19830606 -0.42 43.11 7 4.0 221.0 29.0 61.3 Da93 19830720 2.23 42.43 4 3.9 216.0 43.0 110.4 GDa84 19840223 0.17 43.02 12 4.5 105.0 68.0 144.4 NSD90 19840225 -1.17 43.21 11 4.7 309.0 86.0 0.0 NSD90 19840926 2.17 42.15 5 4.4 344.0 75.0 180.0 SGC84 19850208 0.22 43.04 10 4.1 70.0 20.0 90.0 LDG88 19850930 -0.44 43.03 6 3.8 40.0 40.0 118.2 LDG88 19851002 1.38 42.93 13 3.8 310.0 20.0 45.8 LDG88 19860419 -0.51 43.09 10 3.9 30.0 77.0 90.0 LDG88 19861203 -0.67 43.16 8 3.6 240.0 50.0 -150.5 LDG88 19870303 -0.65 43.14 2 3.9 222.0 74.0 37.6 LDG88 19870413 0.16 43.05 13 3.6 270.0 85.0 93.0 LDG88 19870526 -0.38 43.13 4 3.8 35.0 45.0 7.1 LDG88 19870626 -0.41 43.08 3 3.9 280.0 5.0 -132.9 LDG88 19871105 -0.76 42.91 3 3.9 14.0 65.0 -78.9 LDG88 19871111 -0.18 43.07 8 4.1 250.0 10.0 -30.4 LDG88 19871112 -0.19 43.06 10 3.8 250.0 10.0 -30.4 LDG88 19880220 1.43 42.36 16 3.8 312.0 51.0 -12.6 LDG88 19880316 2.20 42.34 8 3.8 192.0 85.0 21.8 LDG88 19890106 0.17 43.04 10 4.4 276.0 75.0 -143.1 SGC89 19900426 -1.18 43.11 12 2.4 354.0 73.0 88.2 Ga92 19900530 -0.55 43.10 7 2.4 13.0 86.0 153.4 Ga92 19900602 -1.83 43.21 9 3.0 295.0 52.0 -90.0 Ga92 19900612 -0.31 43.10 6 4.2 92.0 46.0 104.2 Ga92 19900616 -1.03 43.10 20 1.5 72.0 85.0 89.5 Ga92 19900628 -1.31 43.29 15 1.5 74.0 28.0 -90.0 Ga92 19900705 -1.39 43.06 11 1.5 105.0 80.0 90.0 Ga92 19900720 -1.56 43.12 12 2.1 60.0 68.0 90.0 Ga92 19900722 -0.38 43.08 7 3.6 90.0 90.0 90.0 Ga92 19900722 -0.39 43.09 7 3.1 274.0 70.0 -100.7 Ga92 19900724 -0.56 43.06 3 3.6 302.0 10.0 -109.7 Ga92 19900805 1.09 42.27 U 3.7 357.0 74.0 -10.8 bul90 19910819 0.68 42.66 1 3.0 64.0 49.0 -122.4 Gui91 19911202 0.90 42.73 7 2.9 292.0 80.0 -134.6 Gui91 19920319 2.06 42.23 2 4.2 262.0 51.0 155.5 bul92 19920725 1.22 42.79 12 3.1 343.0 39.0 -125.0 bul92 19920824 2.32 42.27 u 2.8 150.0 46.0 -43.1 bul92 19921002 1.86 42.43 4 3.2 193.0 35.0 152.6 bul92


19921215 1.06 42.65 9 3.1 49.0 11.0 123.5 bul92 19960218 2.54 42.80 8 5.2 96.0 84.0 -45.2 Ra97 19960218 2.54 42.79 11 3.2 53.0 43.0 24.0 Ra97 19960218 2.52 42.77 10 3.4 116.0 69.0 101.7 Ra97 19960219 2.53 42.77 7 3.3 287.0 80.0 174.3 Ra97 19960219 2.53 42.77 8 3.1 259.0 66.0 38.6 Pa99 19960219 2.54 42.79 10 3.8 103.0 53.0 153.1 Pa99 19960219 2.54 42.79 9 3.4 257.0 62.0 -105.8 Pa99 19960219 2.55 42.79 10 3.8 66.0 66.0 7.3 Pa99 19960219 2.55 42.79 9 3.7 80.0 26.0 90.0 Pa99 19960219 2.55 42.79 10 3.7 320.0 77.0 88.0 Pa99 19960219 2.54 42.78 7 3.4 198.0 89.0 -63.4 Pa99 19960220 2.55 42.79 9 3.6 323.0 23.0 -112.3 Pa99 19960220 2.53 42.79 7 3.3 48.0 61.0 -42.6 Pa99 19960220 2.54 42.78 9 3.4 279.0 34.0 147.3 Pa99 19960220 2.55 42.79 9 3.3 359.0 29.0 -82.1 Pa99 19960220 2.53 42.78 10 3.6 279.0 51.0 84.3 Pa99 19960220 2.53 42.80 9 3.7 276.0 30.0 -162.8 Pa99 19960220 2.51 42.78 9 3.1 306.0 44.0 71.5 Pa99 19960220 2.53 42.78 7 3.2 354.0 55.0 -23.5 Pa99 19960220 2.53 42.79 8 3.4 252.0 56.0 64.9 Pa99 19960220 2.52 42.77 11 3.3 185.0 86.0 0.0 Pa99 19960220 2.53 42.79 8 3.3 344.0 46.0 154.9 Pa99 19960220 2.52 42.78 8 2.8 30.0 69.0 -36.8 Pa99 19960220 2.53 42.79 9 3.1 40.0 74.0 -37.6 Pa99 19960220 2.54 42.79 9 2.8 83.0 69.0 -5.6 Pa99 19960220 2.55 42.78 8 3.3 316.0 79.0 -74.8 Pa99 19960220 2.54 42.79 8 3.2 273.0 48.0 -75.6 Pa99 19960220 2.52 42.78 9 3.2 140.0 50.0 -164.7 Pa99 19960220 2.52 42.78 9 2.8 140.0 50.0 -164.7 Pa99 19960220 2.54 42.79 8 2.9 15.0 47.0 -56.6 Pa99 19960221 2.55 42.79 8 3.2 28.0 51.0 -91.3 Pa99 19960221 2.54 42.79 8 3.1 338.0 52.0 -88.2 Pa99 19960221 2.53 42.78 8 3.6 317.0 43.0 -99.6 Pa99 19960221 2.54 42.79 8 3.4 352.0 35.0 -86.7 Pa99 19960221 2.53 42.79 8 3.1 306.0 44.0 87.8 Pa99 19960221 2.54 42.79 8 3.0 50.0 61.0 -20.0 Pa99 19960221 2.53 42.79 8 3.3 68.0 70.0 8.7 Pa99 19960221 2.54 42.79 8 3.4 304.0 47.0 139.8 Pa99 19960221 2.54 42.80 8 3.4 249.0 52.0 161.0 Pa99 19960221 2.52 42.78 10 3.6 3.0 21.0 -35.8 Pa99 19960222 2.54 42.79 9 3.5 14.0 61.0 10.2 Pa99 19960222 2.53 42.79 8 3.2 360.0 10.0 -71.3 Pa99 19960223 2.52 42.79 9 3.6 300.0 46.0 65.7 Pa99 19970807 2.78 42.57 6 3.2 88.0 41.0 76.2 SGC97 19990717 -1.83 42.80 6 2.5 330.0 65.0 -139.3 Ru06 19991004 0.64 42.87 17 4.8 108.0 39.0 -108.3 SGC99 20001023 -1.15 43.13 13 1.9 75.0 85.0 31.0 Ru06 20001202 -1.17 43.09 9 1.9 165.0 40.0 -10.4 Ru06 20001210 -1.56 43.10 8 1.8 70.0 65.0 -90.0 Ru06 20010118 -1.06 43.13 16 2.6 95.0 85.0 141.3 Ru06 20010201 -1.45 43.13 18 2.9 40.0 60.0 160.6 Ru06 20010203 -1.38 43.02 9 2.0 65.0 90.0 0.0 Ru06 20010210 -1.65 43.10 11 1.5 105.0 45.0 -29.7 Ru06 20010602 -1.74 42.87 1 2.3 110.0 70.0 -8.7 Ru06 20010602 -1.74 42.87 1 2.1 110.0 70.0 -8.7 Ru06 20011112 0.07 43.00 12 1.9 96.0 87.0 -116.5 Du04 20020105 -0.38 43.07 11 1.5 201.0 34.0 162.1 Du04 20020105 -0.38 43.08 8 1.9 26.0 48.0 3.0 Du04 20020113 -0.08 43.00 8 U 160.0 73.0 61.2 Du04 20020319 -0.23 43.08 15 U 197.0 43.0 142.8 Du04 20020322 -0.37 43.06 4 2.7 298.0 64.0 -118.4 Du04 20020410 -0.10 43.05 13 1.5 208.0 53.0 163.7 Du04 20020418 -0.19 43.01 8 1.8 101.0 74.0 105.4 Du04 20020422 -0.10 43.05 14 2.3 68.0 71.0 -9.1 Du04 20020505 -0.23 43.07 6 U 308.0 90.0 90.0 Du04 20020512 -0.12 43.04 12 U 351.0 81.0 -134.6 Du04


20020515 -0.08 43.07 15 1.1 249.0 78.0 -165.9 Du04 20020516 0.17 43.02 11 1.4 63.0 47.0 -100.4 Du04 20020516 -0.15 42.92 9 4.5 120.0 80.0 -105.0 Che11 20020516 -0.15 42.92 8 4.2 130.0 70.0 -89.7 Che11 20020519 0.14 42.98 11 3.7 121.0 75.0 -121.4 Du04 20020905 -0.40 43.08 9 4.0 130.0 5.0 -69.1 Che11 20021211 -0.36 43.09 8 4.2 130.0 45.0 -64.5 Che11 20021212 -0.27 43.08 9 4.6 175.0 30.0 -25.0 Che11 20030121 -0.33 43.06 9 4.3 120.0 45.0 -74.8 Che11 20030226 2.21 42.29 9 4.4 160.0 35.0 -70.0 Che11 20040601 2.22 42.29 9 4.1 155.0 35.0 -44.7 Che11 20040718 1.02 42.89 9 3.9 90.0 55.0 -159.7 Che11 20040921 2.17 42.34 8 4.8 120.0 65.0 -95.1 Che11 20061117 0.01 43.03 10 5.1 105.0 35.0 -80.1 Che11 20061118 0.00 43.00 6 3.3 133.0 24.0 -61.2 Syl08 20061118 0.00 43.00 5 3.0 130.0 33.0 -66.9 Syl08 20061119 -0.01 43.01 6 2.8 242.0 66.0 -75.2 Syl08 20061119 -0.02 43.01 8 2.5 329.0 56.0 -63.3 Syl08 20061119 -0.01 43.01 7 3.3 239.0 59.0 -74.1 Syl08 20061120 -0.01 43.00 6 2.9 104.0 62.0 -67.1 Syl08 20061122 -0.01 43.02 8 2.8 233.0 55.0 -90.0 Syl08 20061202 0.00 43.01 6 2.9 287.0 30.0 -90.0 Syl08 20061216 -0.11 43.02 9 4.0 105.0 35.0 -119.8 Che11 20071115 0.00 43.02 8 4.0 130.0 60.0 -60.1 Che11 20080503 0.22 42.96 12 3.8 145.0 70.0 -49.8 Che11 20080518 -0.18 43.03 10 4.2 100.0 35.0 -75.1 Che11 20080716 -0.41 43.08 16 4.1 155.0 80.0 -5.7 Che11 20080918 -0.33 43.04 9 3.8 115.0 55.0 -95.2 Che11 20100401 0.32 42.97 13 4.3 110.0 25.0 -90.0 Che11 798

799

−2˚00' −1˚30' −1˚00' −0˚30' 0˚00' 0˚30' 1˚00' 1˚30' 2˚00' 2˚30' 3˚00'

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Auch

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Pau

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NPFT

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SPFT

44˚00'

0 50km

DUH0

MARC

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ARG0

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FJCP VCGEBOUI

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CREU00130103

00040003

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PLABESCO

COUP

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−2˚00' −1˚30' −1˚00' −0˚30' 0˚00' 0˚30' 1˚00' 1˚30' 2˚00' 2˚30' 3˚00'

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Pamplona

2.0 ≤ Ml < 3.03.0 ≤ Ml < 4.04.0 ≤ Ml < 5.0

Ml ≥ 5.0

Rigo et al, 2014 - Revised - GJInt

A

B

Figure 1: Seismotectonic maps of Pyrenees on the shaded relief. In black the main faults of the range: NPFT: North Pyrenean Frontal thrust, NPF: North Pyrenean Fault, SPFT: South Pyrenean Frontal Thrust. Grey area: Palaeozoic domain. In red the faults considered as active at present time from Lacan & Ortuño (2012). A) Seismicity for the period 1989-2011. B) GPS points of the PotSis and ResPyr networks for which velocities have been determined after the measurements done in 2008 and 2010.

−2˚00' −1˚30' −1˚00' −0˚30' 0˚00' 0˚30' 1˚00' 1˚30' 2˚00' 2˚30' 3˚00'

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t

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Pau

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Pa −de−F.

Tarbes

Toulouse

AndorraMaladetta

NPFT

NPF

SPFT

44˚00'

Figure 2: GPS horizontal velocity field into the Eurasian reference frame (Table 2) with their 95% confi-dence ellipses superimposed on the shaded topographic map of Pyrenees. Arrows with letters from AA’ to FF’ locate the profiles of Fig.3.

2 mm/yr

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0.5 ± 1.1 nstrain/yr 0.0 ± 1.7 nstrain/yr

0.7 ± 1.3 nstrain/yr 0.8 ± 1.2 nstrain/yr

2.0 ± 1.8 nstrain/yr -0.2 ± 1.2 nstrain/yr


Figure 3: N-S (AA’ to EE’) and E-W (FF’) profiles located in fig. 2 with the projected GPS velocities and their 1σ uncertainties. For the N-S profiles, the half-width of projection is 45 km and all the GPS velocities are projected on the E-W profile. At each profile, the corresponding topographic envelope and deformation rate are done.

−1 5

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44˚00'

0 50km

Figure 4: Focal mechanisms over the period 1967-2011 (Table Appendix A) on the shaded topographic map of Pyrenees. The size of the focal mechanisms is proportional to the magnitude. The 1989-2011 seismicity and the faults are reported in grey.


−2˚00' −1˚30' −1˚00' −0˚30' 0˚00' 0˚30' 1˚00' 1˚30' 2˚00' 2˚30' 3˚00'

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NPF

SPFT

44˚00'

−2˚00' −1˚30' −1˚00' −0˚30' 0˚00' 0˚30'

43˚00'

u

gnères−de−B.

Lou des

Pamplona

Tarbes43˚15'

Extension Strike-slip Compression

Figure 5: Map of ‘r’ factor on the shaded topographic map of Pyrenees (top), inset locates the close-up on the western part (bottom).


{{ {

σ2

σ1

σ3

σ3

σ2

σ1

σ2σ1

σ3

−2˚00' −1˚30' −1˚00' −0˚30' 0˚00' 0˚30' 1˚00' 1˚30' 2˚00' 2˚30' 3˚00'

42˚00'

42˚15'

42˚30'

42˚45'

43˚00'

43˚15'

43˚30'

0 50km


Figure 6: Stress tensors from FSA inversion. Colours correspond to the stress axis and their uncertain-ties: red σ1; green σ2 and blue σ3.The active faults from Lacan & Ortuño (2012) are indicated in red.

7

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13

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σ3

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σ2σ3

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σ3

σ1σ2

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42˚00'

42˚30'

43˚00'

43˚30'

Auch

Bagnères−de−B.

Bayonn

Carcassonne

Foix

GironaHuesca

Irun

Jaca

Lourdes

Olot

Pam lona

Pau

Perpignan

rad s

Quillan

St−Paul−de−F.

Tarbes

Toulouse

AndorraMaladetta

NPFT

NPF

SPFT

44˚00'

0 50km


Figure 7: Seismic strain-rate tensors (Table 5). a: all events; b: HM events (M ≥ 4.5); c: LM events M < 4.5). In black, tensors are caluclated without magnitude in elementary volumes. Amplitudes of strain-rates are given by the coloured scale.

1

2

3

4

5

67

8

abc

a bc

a bc

a c

cb

a

a b

c

a

c

a b c

nstr yr-1

0.0001 0.01 0.1 1 10 1000.001

ba

c

Westa

bc

East

ba

c

All

−3˚ −2˚ −1˚ 0˚ 1˚ 2˚ 3˚ 4˚41˚

42˚

43˚

44˚

BIAZ

UNMEORON

ALSA

LOSA

PAMP

SANG

FUEN

EPSH

ASIN

CREULLIV

MSGTESCO

BARY

SORI

CASS

SBAR

2 nanostrain/yr

1 mm/yr / 95% confidence interval


TAFA

MATA

GARR

FJCP

TUDE

LNDA

TLSE

BELL

AVEL

ZARA

SCOA

−3˚ −2˚ −1˚ 0˚ 1˚ 2˚ 3˚ 4˚

41˚

42˚

43˚

44˚

2 nanostrain/yr

1 mm/yr / 95% confidence interval

MTDMAUCH

PERP

Figure 8: Geodetic strain-rate tensors (black, Table 4) for the eastern and western parts delimited by the red contour, and for the all Pyrenees in the inset. In red (extension axis) and blue (compression axis) are the 1000 tensors obtained by varying randomly the GPS velocities in their uncertainties (see text for details). Top: this study; bottom: from Asensio et al. (2012), the GPS velocities shown here being com-puted in the same European reference frame as our study.

present-day deformation of the pyrenees revealed by gps surveying and earthquake focal mechanisms...

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