THE INSTRUMENTAL SEISMICITY OF THE WESTERN ALPS - SPATIO-TEMPARAL PATTERNS ANALYZED WITH THE WAVELET TRANSFORM

We perform a spatial and temporal analysis of the instrumental seismic ity of the Western Alps, between latitude 41 degrees-48 degrees N and longitude 5 degrees-10 degrees E, using a recently revised catalogue a vailable for the period 1962-1995 containing 7500 events in the magnit ude range 2-5.9. Taking into account the fact that the major difficult y of such an analysis in an area of moderate seismicity is the long re turn period of the events and the diffusive character of the seismic s warms, we first carry out a statistical analysis of the 3-D distributi on of the foci with the help of a 3-D wavelet transform. This smooths the location errors, which are estimated to be 1 km in epicentral coor dinates and 5 km for the depth for recent years, whereas they are more than 10 km in epicentral coordinates for the oldest events. The good agreement between the shape of the filtered volumes and geological/tec tonic features supports this new methodology for defining seismogenic zones, which are outlined better than by simple observations of the se ismicity map. The study of seismic energy release shows the major hete rogeneity in the mechanical behaviour of these seismogenic zones. An e valuation of seismic deformation has been carried out for some of thes e regions and compared with published geodetic results. The cumulative slip estimated for the Durance fault is 0.01 mm yr(-1) (1 degrees per cent of the geodetic estimation), and for the Vuache fault it is 0.19 mm yr(-1), whereas historical triangulation yields horizontal movemen ts of up to 5 mm yr(-1) in the neighbourhood of the Vuache fault. In t he Valais region, the 'seismic' shear strain rate has been evaluated t o be 5 x 10(-4) mu rad yr(-1) (0.5 per cent of the geodetic deformatio n); in the Ligurian Sea, the shortening rate deduced from seismologica l data is 1.1 mm yr(-1) (20 per cent of the geodetic evaluation). The paradoxical result of this study is that areas where instrumental seis micity is low correspond to the location of strong historical seismici ty or to regions of high geodetic deformation. Only two regions (Valai s and the Ligurian Sea) seem to correspond in historical and present s eismicity. These results show the difficulty of predicting seismic act ivity in such areas. The discrepancy between low seismic activity, hig h local deformation rate, and the moderate average velocity between Af rica and Europe in the western Alpine region needs to be explained by any tectonic theory of the western Alps.