Analysis of the Mw = 4.3 Lorient earthquake sequence : a multidisciplinary approach to the geodynamics of the Armorican Massif, Westernmost France

International audienceSince it was one of the largest events ever recorded in this region, this was the opportunity to improve our seismotectonic knowledge of the Armorican Massif. We performed a post-seismic survey (SISBREIZH), which allowed us to locate accurately 62 aftershocks within 14 days. An analysis of the main shock using broadband records provided a normal fault mechanism with a dextral strike-slip component located at 12-km depth. The aftershock sequence exhibits: (1) a combination of almost pure right-lateral strike-slip and dominant normal faulting similar to the main shock; (2) magnitudes ranging from 0.4 to 1.9 and (3) depths ranging from 11.5 to 13.5 km, that is, close to the main shock hypocenter. The distribution of the aftershocks defines a rupture plane dipping 60 • to the south with a fault length of ≈2 km consistent with the source parameters of the main shock. Beside the SISBREIZH survey, a morpho-structural analysis has been conducted: we found fault plane solutions with southward-dipping N120-150 normal fault planes. The stress tensor computed after the aftershock focal mechanisms is a strike-slip regime with a NE-SW extensional direction. The Lorient earthquake appears to reactivate Late Hercynian structures and the whole sequence is reflecting the regional-scale tectonic stress field expressed by a combination of strike-slip and normal faulting