The crustal fault structure responsible for the 1703 earthquake sequence of central Italy

An active crustal shear zone has been recognized in the central Apennines (Italy) by detailed investigations of the surface fault trace pattern of the axial sectors of the mountain belt. Our data suggest that a 16-18 km thick seismogenic layer rests above the ductile lower part of this shear zone and is affected by an interconnecting Late-Quaternary fault system which includes three main transcurrent (left-lateral) fault sets trending roughly north-south. Several lines of evidence indicate that the central fault set is the seismogenic structure responsible for generating the 1703 seismic sequence of central Italy. Fault slip data, morphotectonic and paleoseismological observations, together with fractal statistics, suggest that coseismic deformation within the epicentral area of the 1703 earthquake sequence is partitioned among a few linked faults, and that the ratio of vertical vs. horizontal slip rates within the active central fault set is about 1:3. Slip rate estimates also show that recurrence times for 1703 type earthquakes are in the range of a millennium. The seismogenic fault structure of the 1703 earthquake sequence may be viewed as a multiple rupture zone with an immature fractal geometry evolving within a crustal volume which deforms in response to transcurrent motion. The main results of this work support our view that the seismogenic potential of this sector of the Apennines can be evaluated by integrating, into a fractal model, the effects due to higher and lower-rank structural components of the Late Quaternary fault system of central Italy. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.