Identifying an asperity through 3-D mapping of the frequency-magnitude distribution

Following the idea introduced by Wiemer and Wyss [1997], here is an attempt to map the seismic hazard, in terms of the b value in the frequency-magnit ude distribution, in a region with moderate seismic activity (Bearn region, French Pyrenees). In this area, the complexity Of the geology makes it nec essary to perform a 3-D mapping, b values are computed within 5 km radius s pherical volumes, that contain an average of 140 events. Strong lateral var iations, between 0.45 and 1.8, are observed at short scales in the whole in vestigated volume. The minimum of the b value is clearly correlated with a historically active zone, which last ruptured in 1980 (M-b = 5.1). Interpre tating low b values in terms of high stresses may lead us to consider this minimum-b patch as a 5 km radius asperity. This study allows to define its location and dimensions.