Analysis of seismological and geological observations for moderate-size earthquakes: The Colfiorito Fault System (Central Apennines, italy)

To contribute to the understanding of the relationships between moderate earthquakes and the faults that are recognizable in the geological record, we analysed seismological and geological data related to the 1997-1998 Umbria-Marche (Central Italy) earthquake swarm. The seismological recordings, collected by local networks, allowed accurate location of about 1000 events, whereas the geological field observations provided a picture of the structural features and the ground-surface deformations. We also re-examined and used some published data and results, mostly about the fault plane solutions and the geology. On the basis of earthquake locations, fault plane solutions, and geological mapping we explored the possible correlation between the earthquake causative fault planes and the normal faults exposed in the area. Our results show that the two main shocks that occurred on 1997 September 26 (M-W = 5.7 and M-W = 6.0) originated on the same structure, reactivating at depth the Colfiorito normal faults. Neither rupture propagated up to the ground surface, but both triggered gravitational sliding that occurred along pre-existing fault scarps. The earthquake that occurred on 1997 October 14 (M-W = 5.6) originated on another fault branch at a much shallower depth. In spite of its lower magnitude, this earthquake produced tectonic ruptures where the fault plane projects to the surface in an area where no faults were previously mapped. By comparing the palaeostress reconstruction, based on slickenside lineation analysis, and the focal mechanism solutions, we suggest a possible correlation between the long-term (Early Middle Pleistocene) cumulative effects of the Colfiorito Fault System and the short-term behaviour of the fault planes observed during this earthquake swarm, favouring the idea of a seismogenic source producing clustered moderate-size earthquakes rather than large events scattered in time.