The 2006 November, ML=5.0 earthquake near Lourdes (France): new evidence for NS extension across the Pyrenees

A widely felt, M-L = 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 km(2) 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 degrees 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). Moment tensors and other source parameters (corner frequency, stress drop and source radius) are determined from body wave modelling for the strongest events of the sequence (Table 3). For the main shock, a seismic moment of 5.32 x 10(15) N m is found, and the source size (9.4 km(2)) is consistent with the aftershock distribution. Stress drops are about four times smaller for the aftershocks than for the main shock, except for a late event (2006 December 16) located 10 km to the west. We show that it could have been triggered by the main shock through viscoelastic stress transfer in the lower crust. Hydrological manifestations such as water level changes in hydrothermal wells are also discussed. The rupture occurred on a segment of an E-W trending normal fault. It is the clearest active structure identified in the whole Pyrenean range, and might be the cause of large historical earthquakes. Although paradoxical in an active mountain belt generally depicted as enduring a compressional regime, its extensional mechanism is consistent with recent geomorphological evidences of quaternary normal faults, and with the present-day strain and stress fields inferred from permanent GPS measurements and CMT moment tensors. This new seismic sequence provides an additional support to the rising idea of extensional stress and strain regimes across the Pyrenees.