The south eastern durance fault permanent network: Preliminary results

The Durance fault area is located in South Eastern France. This fault syste m is characterized by historical earthquakes (one every century, since 1509 , with a magnitude between 5.0 and 5.3). This is the only fault in France w ith such a periodic historical seismic activity. In order to study an activ e fault in a moderate seismic context, the IPSN (Institute for Nuclear Safe ty and Protection) decided to install a permanent network in 1992, surround ing the fault area. Such a permanent seismic network has been installed in the french Pyrenees in the Arette area (Gagnepain et al., 1980). While the Arette network covers a region affected by several major faults, our networ k is devoted to the study of the specific Durance fault. Major historical e arthquakes are clearly associated with this structure. From an instrumental point of view, few earthquakes have been recorded since 1962 with the nati onal network. Our network shows a small seismic activity, with the epicente rs well aligned along the fault direction. Moreover, focal mechanisms compu ted for two events agree with the regional microstructural studies (Cushing et al., 1997). Finally, a study of the shear wave splitting underlines pre ferential S wave polarization for two stations. The H/V ratio on noise micr otremors has been computed for each station in order to check their positio ns in term of site effects. It does not exhibit any amplification effect (e xcept for two stations). The comparisons with H/V ratio on earthquake datas et show the important biases we can obtain with real earthquakes.