INFLUENCE OF LOCAL SOIL CONDITIONS ON INTENSITY DISTRIBUTION IN CESI (ITALY) DURING THE UMBRIA EARTHQUAKES (SEPTEMBER 1997)

摘要

Umbria (Central Italy) was hit by a 5.8 (M_L) earthquake on 26 September 1997, the epicenter of which was located very close to the small village of Colfiorito. Several people were killed, while thousands of buildings in the broader area were severely damaged; among them, unfortunately, were invaluable historical monuments as the chapel of San Francisco di Assisi. Over eighty-five urban units were damaged, while the distribution of macroseismic intensity presented a selective pattern from one unit to another. This differentiation is mainly attributed to local geological and geotechnical conditions, given that building type is more or less uniform throughout the meizoseismal area. Differentiation in intensity distribution was observed on both regional, among urban units, and local, within the same unit, scale. Specifically, our recordings of damage showed that within one single village the intensity would range between two to three degrees of EMS-92. This observation was confirmed by instrumental recordings, which showed that within a single urban (Cesi) unit amplification values were locally four times greater than in the surroundings. In this village we conducted detailed damage recording, following the guidelines of EMS-1992, which incorporates building type in the evaluation of damage. The locations of significant intensity differentiation coincided with the outcrops of geotechnically distinct geological formations. The main outcome of our survey is that geological and geotechnical factors affect greatly the distribution of damage throughout the meizoseismal area, and are responsible for significant variations not only on regional, but also on local scale.