New seismotectonic data from an intraplate region: focal mechanisms in the Armorican Massif (northwestern France)

Abstract

Focal mechanism solutions are determined for 11 small intraplate earthquakes that occurred between 1990 and 1998 in Normandy and the Channel Islands. These mechanisms are obtained from the P-wave first-motion polarities recorded by stations from local and regional seismic networks. The accuracy of each hypocentral location is closely examined and the quality of each fault plane solution is discussed by considering the influence of the velocity structure. The predominant feature of the computed focal mechanisms is the relatively widespread near-horizontal NE–SW T-axis orientation. Horizontal P-axes strike roughly NW–SE. Mechanism solutions for the earthquakes in the Avranches region show left-lateral strike slip on a NNW–SSE fault zone. For the overall region, it seems that nodal planes of normal faulting solutions trend NW–SE or WNW–ESE, whereas those of thrust faulting solutions trend NE–SW. This is in agreement with the general regional stress pattern. The NE–SW normal fault plane solution of the 1990 Jersey event is unique because it is not consistent with the regional style of faulting.