Spatio-temporal distribution of seismic activity during the Umbria-Marche crisis, 1997

We present the spatio-temporal distribution of more than 2000 earthquakes t hat occurred during the Umbria-Marche seismic crisis, between September 26 and November 3, 1997. This distribution was obtained from recordings of a t emporary network that was installed after the occurrence of the first two l argest shocks (Mw =, 5.7, Mw = 6.0) of September 26. This network was compo sed of 27 digital 3-components stations densely distributed in the epicentr al area. The aftershock distribution covers a region of about 40 km long an d about 2 km wide along the NW-SE central Apennines chain. The activity is shallow, mostly located at less than 9 km depth. We distinguished three mai n zones of different seismic activity from NW to SE. The central zone, that contains the hypocenter of four earthquakes of magnitude larger than 5, wa s the more active and the more complex one. Sections at depth identify 40-5 0 degrees dipping structures that agree well with the moment tensor focal m echanisms results. The clustering and the migration of seismicity from NW t o SE and the general features are imaged by aftershock distribution both ho rizontally and at depth.