The 1997 Umbria-Marche earthquake sequence: an example of geological analysis for seismic hazard evaluation in the Apennines (Italy).

On september 26, 1997, at 2.30 h, a Mw=5.7 earthquake occurred in the axial zone of the Umbria-Marche Apennines of central Italy. At 11.40 h, a Mw=6.0 earthquake struck again the same epicentral area within the Colfiorito basin, a major intramontane basin infilled with Quaternary continental deposits. Field inspection of the mapped capable faults, carried out immediately after the main shocks, revealed that most of the fault segments known to occur in the area were locally reactivated (for lengths of several hundreds of metres and surface slip in the range of 2-8 centimetres) during the september 26, 1997 earthquakes.