The Colfiorito earthquake sequence of September-October 1997: Surface breaks and seismotectonic implications for the central Apennines (Italy)

Between September-October 1997, a seismic sequence with three main shocks (Mw = 5.6, 5.9, on September 26, and Ms = 5.5 on October 14) struck the Umbria-Marche region in central Italy between the villages of Nocera-Umbra and Sellano. Although earthquake-induced landslides were observed and mapped in the epicentral area along the main active normal faults, no surface faulting was associated with the two first shocks of September 26. In contrast, the third shock of October 14 produced discontinuous coseismic tectonic ruptures (60 m-500 m long) along a total length of 5 km. Surface ruptures strike in the NNW-SSE direction, dip to the SW and show locally en-echelon geometry (either right- or left-stepping). Coseismic ruptures and active faults are in good agreement with CMT focal-mechanism solutions that indicate normal faulting for the three main shocks. These results provide additional information on surface tectonic ruptures caused by moderate-sized earthquakes, yielding some constraints on seismotectonic processes.