STRESS TENSOR DETERMINATION IN FRANCE AND NEIGHBORING REGIONS

The tectonic behaviour of different seismic regions of France is infer red from the properties of the local stress tensor obtained from focal mechanisms of regional earthquakes. Polarities of P-wave arrivals for several earthquakes are used in the inversion of the stress tensor an d individual focal mechanisms. The principal axes and the shape of the tensor are closely related to the forces which generate seismic ruptu res on pre-existing faults. We analyse 142 earthquakes of magnitude ab ove 3.7 which occurred between 1965 and 1989. Good results are obtaine d for some regions. The northern Alps and the southern end of the Uppe r Rhinegraben show a quasi-compressive, strike-slip tectonic regime, w ith a maximum stress-axis oriented NW-SE. The tectonic regime changes to an extensional strike-slip type in the Rhenish Massif and the Lower Rhinegraben, the direction of the minimum stress-axis being NNE-SSW. The central and western regions of France (Central and Armorican Massi fs) exhibit an extensional tectonic regime combined with strike-slip f aulting. The minimum stress-axis strikes in a NE-SW direction. A stron gly non-homogeneous stress field seems to be present in the southweste rn Alps, and no unique stress tensor could be obtained for that region . This problem is likely to be related to a rapid variation of the str ess field in space which could be expected, given the arc-like geometr y of the mountain range. The Pyrenean domain is unresolved due to lack of data and, moreover, existing focal mechanisms are globaly inconsis tent. Nevertheless, those mechanisms which are well constrained sugges t a NNW-SSE to NW-SE direction of maximum stress in the western part o f the Pyrenees. Thus, the stress field in France is fairly homogeneous at a regional scale for most of the territory except for the highly d eformed belts.