EVIDENCE FOR SURFACE FAULTING DURING THE SEPTEMBER 26, 1997, COLFIORITO (CENTRAL ITALY) EARTHQUAKES

On September 26, 1997, two moderate earthquakes (Mw = 5.7 at 00:33 GMT and Mw = 6.0 at 09:40 GMT) rocked Central Italy causing 12 casualties and heavy damage to old historical towns and monuments in the Umbria-Marche region such as the celebrated San Francesco Monastery in Assisi. These seismic events were part of a sequence started on September 4 with a M1 4.4 shock, which continued with strong activity until mid October including two other mainshocks on October 3 (Mw = 5.5) and 14 (Mw = 5.7), and is still in progress when we revised this note in May 1998. We conducted a field survey from early in the morning of September 26 in order to describe earthquake ground effects in the epicentral area. In particular, we searched for surface faulting features by using as reference the map of capable faults at 1:25000 scale produced by the Camerino University. Right after the September 26 mainshocks, we observed systematic reactivations of mapped capable normal faults, NW to NNW trending and W dipping, with offsets in the order of centimetres and discontinuous rupture lengths in the order of kilometres. These faults belong to the tectonic system controlling the late Quaternary evolution of the Colfiorito basin, a seismically-active extensional structure similar to many other intramontane basins in the Apennines. In this note we provide a first-hand report of our field observations and briefly outline the implications of the observed surface faulting for seismic hazard assessment in Italy.