IN-SITU STRESS-FIELDS AND FOCAL MECHANISM SOLUTIONS IN CENTRAL FRANCE

Focal mechanisms of 31 Central France earthquakes (m(b) = 2.9-4.4) wer e evaluated to determine if slip is compatible with stress field deter minations conducted at four nearby sites. The coherent picture that re sults indicates that the regional stress fields can be inferred by lin ear extrapolation of in-situ measurements performed between 400-1000 m depths. Furthermore, the results show that in spite of rather constan t principal stress directions from one site to the next (100 km apart) , principal stress amplitudes vary significantly. For the domain inves tigated (200 x 200 km2), the size of the homogeneous stress field regi ons appears to be fairly small (about 100 x 100 km2). In addition, wit hin these regions, important local stress perturbations as identified by extremely inconsistent focal mechanisms, are present. Thus only by integrating different data types is it possible to map properly the re gional stress field. This mapping provides means to identify the homog eneous regions as well as to highlight zones of local stress perturbat ions, most likely indicative of active faulting.