International audienc

The lithospheric structures beneath the Pyrenees, which holds the key to settle long-standing controversies regarding the opening of the Bay of Biscay and the formation of the Pyrenees, are still poorly known. The temporary PYROPE and IBERARRAY experiments have recently filled a strong deficit of seismological stations in this part of western Europe, offering a new and unique opportunity to image crustal and mantle structures with unprecedented resolution. Here we report the results of the first tomographic study of the Pyrenees relying on this rich data set. The important aspects of our tomographic study are the precision of both absolute and relative traveltime measurements obtained by a nonlinear simulated annealing waveform fit and the detailed crustal model that has been constructed to compute accurate crustal corrections. Beneath the Massif Central, the most prominent feature is a widespread slow anomaly that reflects a strong thermal anomaly resulting from the thinning of the lithosphere and upwelling of the asthenosphere. Our tomographic images clearly exclude scenarios involving subduction of oceanic lithosphere beneath the Pyrenees. In contrast, they reveal the segmentation of lithospheric structures, mainly by two major lithospheric faults, the Toulouse fault in the central Pyrenees and the Pamplona fault in the western Pyrenees. These inherited Hercynian faults were reactivated during the Cretaceous rifting of the Aquitaine and Iberian margins and during the Cenozoic Alpine convergence. Therefore, the Pyrenees can be seen as resulting from the tectonic inversion of a segmented continental rift that was buried by subduction beneath the European plate.

[1] We locate the sources of double-frequency (or secondary) microseisms in western Europe by frequency slowness analysis of array data as well as polarization and amplitude analysis at individual stations. Array analysis uses data recorded by a temporary array of broadband stations that we deployed in the Quercy region (southwest of France) and those from the Gräfenberg array, from 2 December 2005 to 30 January 2006. We determine attenuation laws for microseisms generated in the Mediterranean Sea and in the Atlantic Ocean, which allow us to use noise amplitudes to estimate distances from the source. We then combine azimuth and amplitude measurements to obtain precise locations of microseisms and estimate their source dimensions. Most of the time, microseismic noise originates in coastal regions where the swell reaches steep rocky coasts with normal incidence, in good agreement with the Longuet-Higgins model for the generation of secondary microseisms. In addition, we find evidence of occasional pelagic sources, which are closely related to moving storms, suggesting that nonlinear interaction between wave components can also generate secondary microseisms.

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S U M M A R YLocal seismic networks are usually designed so that earthquakes are located inside them (primary azimuthal gap <<180• ) and close to the seismic stations (0-100 km). With these local or near-regional networks (0• -5 • ), many seismological observatories still routinely locate earthquakes using 1-D velocity models. Moving towards 3-D location algorithms requires robust 3-D velocity models. This work takes advantage of seismic monitoring spanning more than 30 yr in the Pyrenean region. We investigate the influence of a well-designed 3-D model with station corrections including basins structure and the geometry of the Mohorovicic discontinuity on earthquake locations. In the most favourable cases (GAP < 180• and distance to the first station lower than 15 km), results using 1-D velocity models are very similar to 3-D results. The horizontal accuracy in the 1-D case can be higher than in the 3-D case if lateral variations in the structure are not properly resolved. Depth is systematically better resolved in the 3-D model even on the boundaries of the seismic network (GAP > 180• and distance to the first station higher than 15 km). Errors on velocity models and accuracy of absolute earthquake locations are assessed based on a reference data set made of active seismic, quarry blasts and passive temporary experiments. Solutions and uncertainties are estimated using the probabilistic approach of the NonLinLoc (NLLoc) software based on Equal Differential Time. Some updates have been added to NLLoc to better focus on the final solution (outlier exclusion, multiscale grid search, S-phases weighting). Errors in the probabilistic approach are defined to take into account errors on velocity models and on arrival times. The seismicity in the final 3-D catalogue is located with a horizontal uncertainty of about 2.0 ± 1.9 km and a vertical uncertainty of about 3.0 ± 2.0 km.

SUMMARY A widely felt, ML= 5.0 earthquake occurred in the central French Pyrenees on 2006 November 17, close to the pilgrimage city of Lourdes, in a region where strong historical earthquakes produced severe damage and casualties in the 17th and 18th centuries. Seismic recordings performed by dense permanent networks and temporary stations allowed an exhaustive study of this event and its aftershock sequence, revealing a great coherency of all the parameters which characterize the rupture. More than 250 aftershock hypocentres, located in a 3‐D tomographic model, are remarkably distributed on a 10 km2 quasi‐planar surface which extends at depth between 6 and 10 km. This surface coincides with one of the main shock nodal planes, as inferred from P‐wave polarities and body waveform modelling. The tectonic structure responsible for the earthquake is identified as an E–W oriented normal fault, dipping 56° north, a few kilometres south of the North Pyrenean Fault, recognized as the former boundary between the Iberian and Eurasian Plates. The mechanisms of the strongest aftershocks are also clearly extensional (Tables 1 and 2). Focal solution parameters for the main shock, the seven strongest aftershocks, the 2006 December 16 and the 2007 November 15 events. Date Origin time Lat (°N) Lon (°E) Depth M L Plane 1 Plane 2 P‐axis T‐axis (km) St Dip Rk St Dip Rk St Pln St Pln 2006 November 1718:1943.02820.00329.75.028456−849334−992157810112006 November 1820:3443.01230.00276.93.312230−7428461−991727220162006 November 1822:1743.0120−0.00026.63.013729−6028465−1051666625192006 November 1905:1043.0227−0.00638.02.829940−9011950−902985209052006 November 1909:1443.0285−0.00358.92.532264−6910133−1262686537162006 November 1913:1643.0223−0.00958.43.326065−867125−9917870347202006 November 2004:0143.0190−0.00827.72.910470−7224027−1304161180232006 November 2213:5543.0255−0.00578.72.824955−906935−9015980339102006 December 0206:2343.0213−0.00058.02.928528−9010562−901573195172006 December 1608:1743.0208−0.11009.74.031258−8913032−922257741132007 November 1513:4743.0207−0.00227.84.129630−507267−1103106217720 Notes: Projection on the lower hemisphere. The strike (St), dip and rake (Rk) angles for each event are reported (in degrees) for each plane, as well as the strike (St) and plunge (Pln) angles for the P‐ and T‐axes. Source parameters obtained from body waveform modelling, with their uncertainties, for the main shock, the three strongest aftershocks, and the December 16 event. Date‐time N M 0 Plane 1 Plane 2 (Nm) Strike Dip Rake Strike Dip Rake 2006 November 17–18:19255.32 × 1015± 5.00 × 1013267 ± 152 ± 1−107 ± 1114 ± 140 ± 1−69 ± 12006 November 18–20:3462.29 × 1013± 5.60 × 1012231 ± 972 ± 6−132 ± 2122 ± 445 ± 3−25 ± 72006 November 18–22:1771.14 × 1013± 1.95 × 1012148 ± 551 ± 1−25 ± 9254 ± 471 ± 6−139 ± 32006 November 19–13:16103.64 × 1013± 3.82 × 1012332 ± 9531 ± 1−128 ± 23195 ± 766 ± 2−69 ± 132006 December 16–08:17167.80 × 1013± 3.57 × 1012350 ± 467 ± 2−59 ± 4112 ± 338 ± 2−141 ± 6 ...

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S U M M A R YThree earthquakes of magnitudes 4.6, 4.3 and 3.7 occurred in 2002 May at two locations 20 km from the pilgrimage city of Lourdes in the French Pyrenees. They were well recorded by the permanent Pyrenean seismic networks, by a temporary local network, as well as by accelerometric stations. In order to understand their tectonic contexts, and to come to a better evaluation of the seismic risk at Lourdes, a detailed analysis of these events is performed.The first two events are located south of Lourdes in an area where only a few earthquakes have occurred up to now. Their focal solutions derived from first-motion polarities indicate reverse faulting, with a N110 • E strike consistent with the geological structures. 10 aftershocks were recorded and relocated with respect to the main events, benefiting from the waveform similarity of the various events. This analysis reveals that the two main events concern probably the same fault, the second rupture being in the prolongation of the first one, whereas the other small aftershocks are located on fault segments in the vicinity of the hypocentre of the second event.The third large event, located to the SE of Lourdes, involves a normal mechanism with a N120 • E plane parallel to the main geological structures. It occurred in a region of intense activity, including in particular an event of maximum macroseismic intensity IX in 1660.The first two events are at the boundary of a large quiet zone. In order to understand the related structural context, a new crustal tomographic model has been computed. It reveals that this quiet zone coincides with a block of high P-velocity. In contrast, the seismicity appears to be stronger at the northern and eastern boundaries of this block.The accelerometric data of the three main events recorded at Lourdes have been used to estimate the maximum peak ground accelerations in this city if a large event occurred, similar to those which damaged the city in the seventeenth and eighteenth centuries. Horizontal accelerations of 0.25 ± 0.07 g are predicted in the frequency domain 1-5 Hz at the location of the Sanctuary for a magnitude 6 event occurring 10 km away from the city. Taking into account the error bars, these values could in some cases exceed those specified by the building codes in this region.