Ground effects and surface faulting in the September-October 1997 Umbria-Marche (Central Italy) seismic sequence

The September-October 1997 seismic sequence in the Umbria-Marche regions of Central Italy (main shocks on September 26, Mw 5.7 and 6.0, and on October 14, Mw 5.6) left significant ground effects, which were mainly concentrate d in the Colfiorito intermountain basin. These effects included surface fau lting, ground cracks and settlements, rock falls, slides, hydrological and gas anomalies. The distribution and size of ground effects has proved usefu l for (1) defining the epicentral area and the location of the causative fa ult; (2) complementing the intensity pattern from damage distribution (this can be very useful in poorly inhabited zones); (3) integrating or testing the intensity assessment of many historical events, in order to obtain a be tter evaluation of the-magnitude from intensity data. Of special interest w as the observation of surface ruptures generated along segments of a system of normal faults already mapped, as capable, with end-to-end lengths of 12 km and maximum displacements of 8 cm, Many pieces of evidence confirm that coseismic slip was not a secondary, gravity-induced, phenomenon, but had a tectonic origin. Detailed descriptions of surface faulting for moderate ea rthquakes are not common, being easily missed or misinterpreted; however, i n this paper we emphasize that surface faulting due to the 1997 event can b e used to infer the threshold magnitude for surface faulting in Central Ape nnines. allowing to calibrate palaeoearthquake size from fault offsets as s een in trench investigations. (C) 1999 Elsevier Science Ltd. All rights res erved.