STRESS TENSOR DETERMINATION IN FRANCE AND NEIGHBORING REGIONS

The tectonic behaviour of different seismic regions of France is inferred 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 and individual focal mechanisms. The principal axes and the shape of the tensor are closely related to the forces which generate seismic ruptures on pre-existing faults. We analyse 142 earthquakes of magnitude above 3.7 which occurred between 1965 and 1989. Good results are obtained for some regions. The northern Alps and the southern end of the Upper Rhinegraben show a quasi-compressive, strike-slip tectonic regime, with 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 Massifs) exhibit an extensional tectonic regime combined with strike-slip faulting. The minimum stress-axis strikes in a NE-SW direction. A strongly non-homogeneous stress field seems to be present in the southwestern 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 stress field in space which could be expected, given the arc-like geometry of the mountain range. The Pyrenean domain is unresolved due to lack of data and, moreover, existing focal mechanisms are globaly inconsistent. Nevertheless, those mechanisms which are well constrained suggest a NNW-SSE to NW-SE direction of maximum stress in the western part of the Pyrenees. Thus, the stress field in France is fairly homogeneous at a regional scale for most of the territory except for the highly deformed belts.