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 concentrated in the Colfiorito intermountain basin. These effects included surface faulting, ground cracks and settlements, rock falls, slides, hydrological and gas anomalies. The distribution and size of ground e€ects has proved useful for (1) defining the epicentral area and the location of the causative fault; (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 better evaluation of the magnitude from intensity data. Of special interest was 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 earthquakes are not common, being easily missed or misinterpreted; however, in this paper we emphasize that surface faulting due to the 1997 event can be used to infer the threshold magnitude for surface faulting in Central Apennines, allowing to calibrate palaeoearthquake size from fault offsets as seen in trench investigations.