Advances in the knowledge of seismogenetic structures in the eastern edge of the Pyerenees

Although the seismic activity at the eastern Pyrenees is nowadays moderate and sparse, with events usually not exceeding magnitudes 4.5, this area has been affected in the past by the most destructive event occurred in the Pyrenees, reaching an intensity of IX, whose seismogenic structure is not well  understood and still under debate. In order to progress in the knowledge of these structures, we present here the results derived from a 14 months-long broad-band seismic deployment focused on the Cerdanya Basin, but encompassing the eastern termination of the Pyrenees. The dense station coverage has allowed us to obtain accurate hypocentral locations, as well as up to 23 focal mechanisms from local low-magnitude earthquakes. In addition to a relatively sparce seismicity, several clusters of seismic events located in well-defined, small areas and depth ranges have been identified. The results show a few low-magnitude seismic events located in the southern limit of the Cerdanya Basin that could be related to oblique secondary faults within the footwall of the Têt Fault, the major tectonic structure in the area. Our data shows also that the Capcir Fault has associated seismicity, with some of the events located out of the fault plane, perhaps on secondary fault branches. To the east, a cluster of low-magnitude events is detected in the epicentral area of two relatively large earthquakes occurred recently, probably indicating the development of the preparatory phase. Further west, the Maladeta Massif has a sustained seismic activity, although its origin does not seem to be related to the most relevant structure in the area, the North Maladeta Fault. Regarding focal mechanisms, most of them show normal fault solutions with nodal planes NW-SE oriented, which are in agreement with the extensional regime perpendicular to the axis of the chain derived from the seismic and geodesic data.