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

In the last years many studies were carried out in order to assess the corr elation between nearsurface geology and seismic response of downtown Rome d uring local and regional earthquakes. Nevertheless, only historical macrose ismic data and theoretical modelling could be used so far to estimate the g round motion in the historical part of the city. The occurrence of the Octo ber 14, 1997 Umbria Marche event (M-w=5.7), located about 115 km far from R ome 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 intervie ws: such a large amount of macroseismic information, for the first time ava ilable in Rome, was related to 669 observation points providing a picture o f 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 po inted 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 s ignificant deterioration of geomechanical properties. (C) 1999 Elsevier Sci ence Ltd. All rights reserved.