Distribution of effects in the urban area of Rome During the October 14, 1997 Umbria Marche earthquake

Abstract

In the last years many studies were carried out in order to assess the correlation between nearsurface geology and seismic response of downtown Rome during local and regional earthquakes. Nevertheless, only historical macroseismic data and theoretical modelling could be used so far to estimate the ground motion in the historical part of the city. The occurrence of the October 14, 1997 Umbria Marche event (Mw = 5.7), located about 115 km far from Rome but largely felt in the city, allowed us to verify our knowledge of the seismic response on the basis of the present urban setting of Rome.A prompt macroseismic survey in the urban area within the ring-road (G.R.A.) was performed. 949 questionnaires were compiled by means of 1842 interviews: such a large amount of macroseismic information, for the first time available in Rome, was related to 669 observation points providing a picture of local intensity variations. The areal distribution of collected intensity points is quite homogeneous.The data analysis points out a preferential distribution of larger effects (65%) in correspondence of Holocene alluvium outcrops. Furthermore, a tight correspondence between remarkable intensities and alluvial sediments is pointed out in suburban areas characterized by the minor hydrographic network of the Tiber River. Macroseismic anomalies were outlined in sectors of the urban area where local hydrogeological conditions can be responsible for a significant deterioration of geomechanical properties.