Seismicity in central-western Pyrenees (France): A consequence of the subsidence of dense exhumed bodies

- We observe periodic features in the geographic distribution of W-Pyrenean seismicity.
- Earthquakes are mostly located south of large positive Bouguer anomalies.
- Subsidence of dense intra-crustal blocks explains the main features of the seismicity.
- Normal focal solutions are a consequence of the intra-crustal block subsidence.
- The stress field may not be uniform along the Pyrenean range.

The Pyrenees results from the convergence of the Iberian and Eurasian plates since Cenozoic, but the present stress regime of the range is debated, as most of the recent earthquakes exhibit normal solutions. We analyse the seismicity in central-western Pyrenees, which is the most active part of the range with the largest events. Seismicity maps obtained at different depths reveal quasi-periodic features in focus distribution, and a preferential occurrence of large earthquakes at the base of the upper crust. The superimposition of the seismicity to the gravity anomalies shows that earthquakes are mostly located on the southern border of positive Bouguer anomalies, which correspond to dense lower crust blocks trapped in the upper crust during the Pyrenean convergence. We propose that the seismicity results from the subsidence of these blocks previously exhumed inside pull-apart basins. This scenario explains all together the geographic distribution of the seismicity, its magnitude distribution and the predominance of normal focal solutions. It shows that the normal mechanisms do not necessarily imply a general north-south extension of the range, but may be compatible with a weak compressive regime, and that the stress field may not be uniform along the range.