The instrumental seismicity of the western Alps: spatio-temparal patterns analysed with the wavelet transform

We perform a spatial and temporal analysis of the instrumental seismicity of the Western Alps, between latitude 41 degrees-48 degrees N and longitude 5 degrees-10 degrees E, using a recently revised catalogue available for the period 1962-1995 containing 7500 events in the magnitude range 2-5.9. Taking into account the fact that the major difficulty of such an analysis in an area of moderate seismicity is the long return period of the events and the diffusive character of the seismic swarms, we first carry out a statistical analysis of the 3-D distribution 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 coordinates 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/tectonic features supports this new methodology for defining seismogenic zones, which are outlined better than by simple observations of the seismicity map. The study of seismic energy release shows the major heterogeneity in the mechanical behaviour of these seismogenic zones. An evaluation of seismic deformation has been carried out for some of these 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 movements of up to 5 mm yr(-1) in the neighbourhood of the Vuache fault. In the Valais region, the 'seismic' shear strain rate has been evaluated to be 5 x 10(-4) mu rad yr(-1) (0.5 per cent of the geodetic deformation); in the Ligurian Sea, the shortening rate deduced from seismological 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 seismicity is low correspond to the location of strong historical seismicity or to regions of high geodetic deformation. Only two regions (Valais and the Ligurian Sea) seem to correspond in historical and present seismicity. These results show the difficulty of predicting seismic activity in such areas. The discrepancy between low seismic activity, high local deformation rate, and the moderate average velocity between Africa and Europe in the western Alpine region needs to be explained by any tectonic theory of the western Alps.