Sismotectonique des Pyrenees; principales contraintes sismologiques

Abstract

Abstract This paper presents a critical review of the main seismological data which constrain the Pyrenean tectonics, mostly the spatial distribution of the hypocenters and the fault plane solutions. A new seismicity map is computed for the period 1989-99, and a compilation of the published focal solutions is presented and discussed. The Pyrenean range results from the convergence of the Iberian and Eurasian plates for about 65 Ma. This convergence followed a period of extension (-115 to -80 Ma), which is related to the opening of the Bay of Biscay. The boundary between the two plates, the North Pyrenean Fault (NPF), is recognized as a major tectonic feature running E-W to the north of the range. The eastern part of the range has been later affected by the Mediterranean tectonics (about 30 Ma ago), which generated some faults with NE-SW and NW-SE orientations in Catalonia. The Pyrenean structure is rather well known thanks to systematic studies carried out during the last two decades: crustal exploration from refraction and reflection seismic experiments, gravimetric studies, and tomographic imaging of the lithosphere using either seismological methods or magnetotelluric methods. The most important results are the detection of a 10-15 km Moho jump at the NPF with a thicker crust at the Iberian side, the evidence for a subduction of the Iberian lower crust beneath the North Pyrenean Zone along the eastern and central parts of the range, and the imaging, in the central and western Pyrenees, of two large blocks of lower crust material uplifted through the upper crust. Since 1989, several seismic networks set up by various institutions in the different parts of the range have allowed to produce a rather accurate seismicity map. Since 1997, the seismic network on the French side has been significantly upgraded, and the geographic distribution of the stations has been made more even. This network consists now in fifteen three-component, short period stations with telephonic transmissions, and five one-component stations with Meteosat satellite transmissions, which ensure an access to the data in case of damaging earthquake. Four stations of the French Atomic Energy Commission (CEA) are running in addition on the French side. 22 stations are set up on the Spanish side, with a higher density in Catalonia. With all these stations together, about 450 earthquakes with magnitudes > or =1.0 can be located each year in the Pyrenees, with a mean accuracy of + or -1.5 km for the horizontal coordinates, + or -3 km for depth, i.e. a factor of two better than with the previous networks. The new seismicity map reveals some new features which were not apparent in the previous map. In particular, the NPF is not a single linear feature, but it includes a multiplicity of segments which are not necessarily E-W oriented. The Adour Fault (also called "Bigorre Fault" in some previous articles), a NW-SE oriented fault in central Pyrenees, appears as a complex zone which possibly includes several segments parallel to each others. The seismicity beneath the Maladeta, which is the largest unit of the Paleozoic Axial Zone, appears more linearly distributed than in the previous map. Most of the events are located inside the upper crust (depth less than 12-15 km). Deeper events are mostly located in the western part of the range.