Fault fragment control in the 1997 Umbria-Marche, central Italy, earthquake sequence

The Umbria-Marche region in central Italy experienced a sequence of shallow earthquakes in late 1997, including three mainshocks on September 26(th) ( Ms 5.5 and 5.9) and October 14(th) (Ms 5.5). This seismic sequence illustra tes the relationships between small-scale active faults and moderate-magnit ude earthquakes. We suggest that a small-scale active fault corresponds to a "fault fragment" and that it refers to the fault area required for produc ing a coseismic deformation at the ground surface. The seismic activity and related three mainshocks occurred along three fault fragments which total similar to 25 km in length and show a listric geometry at depth. Fault frag ments are laterally controlled by pre-existing transverse fold-and-thrust s tructures and may constitute a major component of the seismic strain releas e in active continental regions.