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

We present the spatio-temporal distribution ofmore than 2000 earthquakes that occurred during theUmbria-Marche
seismic crisis, between September 26 and November 3, 1997. This distribution was obtained from recordings of a
temporary network that was installed after the occurrence of the ?rst two largest shocks (Mw= 5.7, Mw= 6.0) of
September 26. This network was composed of 27 digital 3-components stations densely distributed in the epicentral
area. The aftershock distribution covers a region of about 40 km long and about 12 km wide along the NW-SE
central Apennines chain. The activity is shallow, mostly located at less than 9 km depth. We distinguished three
main zones of different seismic activity from NW to SE. The central zone, that contains the hypocenter of four
earthquakes of magnitude larger than 5, was the more active and the more complex one. Sections at depth identify
40–50 dipping structures that agree well with the moment tensor focal mechanisms results. The clustering and
the migration of seismicity from NW to SE and the general features are imaged by aftershock distribution both
horizontally and at depth.