The Western Pyrenees presents a diffuse and moderate (M<=5.7) instrumental seismicity. It nevertheless historically suffered from strong earthquakes (I=IX MSK). The seismic sources of these events are not yet clearly identified. We evidence in the Arudy (1980) epicentral area (M=5.1) a tectonic activity revealed by folding of terrace remnants and alluvial fans, middle Pleistocene in age. These superficial folds shows wavelengthes of 1500 m and amplitudes of 5-10 m. A potential source of this folding could be a blind early Cretaceous crustal normal fault of the Iberian margin.
The current geodynamic arrangement, as revealed by focal solutions, microtectonics and plate kinematics, suggests that this accident could be reactivated in a right lateral mode. This reactivation leads to a strain partitioning between the deep discontinuity that accommodates the lateral component of the motion and shallow thrusts, rooted on this discontinuity. These thrusts accommodate the normal component of the strain.
The crustal fault is likely the major seismic source for the area. Its length reaches ca 25-30 km and its potential reactivation in a strike-slip mode suggest that a maximum earthquake magnitude of ca 6.5 could be expected. But the seismic potential of this buried fault remains difficult to characterize accuratly.
Magnitude of displacement and strain rates of thrusts leading to the shallow folding of terraces is measured here by a trishear model of deformation. This model takes into account the kinematics of the ramp and, in particular, the fold wavelength is driven by the slip rate above the ramp. Thus these thrusts allow to quantify the shallow normal component of strain.
The lateral components of displacement, accommodated above the crustal blind fault, can be estimated if the strain ellipsoid is known. The normal component accommodated by the shallow thrusts increases with the obliquity of the buried fault with respect to shortening direction. We implement a Kostroff analysis, by using the Arudy (1980) seismic crisis and later events, to characterize the strain ellipsoid in the area. Thus, as the strike of the crustal fault is known and the normal component yielded by shallow thrusts, the parallel component of displacement, i.e. the activity of the buried crustal fault, can be evaluated what gives informations about return period of major earthquakes.
