Search

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 ). 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 × 1015 N m is found, and the source size (9.4 km2) 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. [ABSTRACT FROM AUTHOR]

A new method to investigate active tectonic structures, using soil gas composition at faults, suggests relevant information about regional stress conditions which can be obtained rapidly and at relatively low cost. In 1995, we carried out geochemical profiles around the epicentre of the M=5.1 1980 earthquake near Arudy in the French Pyrenees, where the presence of minor fractures is evident in the field, and confirmed using satellite SPOT imagery. Fractures are conduits facilitating fluid migrations in the crust, and are also pathways for the release of deep-seated gases to the atmosphere. In order to investigate the implication of these fractures in the present deformation, i.e. if they are connected to the Hercynian substratum at a depth of about 1800 m, soil gases were measured along four traverses crossing the observed structures. Gases determined were 222Rn, CO2 and 4He, each of them for their characteristic source: 222Rn has essentially a shallow origin due to its short half-life, whereas CO2 is the major soil gas component with a mainly surficial biogenic source. However, there could additionally be CO2 from crustal or mantle degassing, which would also be the principal sources of He. Data analysis clearly reveals anomalous values for each gas at specific positions along the traverses. Two sets of fractures corresponding to different observed trends are distinguished: the one characterised by He anomalies accompanied by other gases, and the second with few identified He anomalies. The agreement between the geochemical data and the field observations leads us to propose a deformation model for the area studied, analogous to a pull-apart system located in a right lateral shear zone. [Copyright &y& Elsevier]