Site-dependent response spectra at the town of Sellano for the 1997-1998 Umbria –Marche seismic sequence

In the period between 3rd September and 14th October 1997, seven main seismic events occurred in the regions of Umbria and Marche in Italy. The events have been recorded by more than 30 stations of the Servizio Sismico Nazionale (S.S.N. 1999) located in this area. Because of the earthquakes, that struck the Umbro-Marchigiano area of Italy from September 1997 to April 1998, a lot of small, old centres suffered great damage to their buildings, including the towns of Fabriano, Nocera Umbra and Sellano. The main shocks that affected Sellano took place on 26 September 1997 and on 14 October 1997. On 26 September at 00:33 the epicentral area was located in the town of Colfiorito, about 10 km to the north of Sellano. The shock of 14 October was located only 3 km away from Sellano. The seismic sequence, which occurred in the Umbria and Marche Regions in 1997-1998, even if it was characterised by quite moderate magnitudes, caused significant damage to buildings and determined the failure of many slopes. An explanation of this phenomenon can certainly be looked for in the geotechnical characteristics of the soil involved; even if the high vulnerability of many masonry buildings and the cumulative effect due to the long earthquake repetition also play an important role. On the basis of the site characterisation of the Sellano area, the soil response was analysed, and a grade-2 microzonation was performed by Massimino et al. 2001. Furthermore a grade-3 microzonation was performed by Capilleri et al. 2001 including microzonation maps in terms of acceleration and amplification ratio. The aim of this work is to compare acceleration spectra obtained on the basis of the acceleration records (S.S.N.1999) with those provided for the last of EC8, (2002) which considers seven subsoil classes instead of three classes