The Umbria-Marche case: some suggestions for the Italian seismic zonation

Probabilistic seismic hazard maps, in terms of spectral acceleration and un iform hazard response spectra at given sites, considering local soil condit ions, represent a much more complete estimate of the seismic hazard than th e traditional maps in terms of peak ground acceleration or macroseismic int ensity. This is particularly true when the requests of urban planners and e ngineers have to be met. The present analysis shows how some hazard paramet ers, such as the effective peak acceleration and the spectrum intensity, ca n well synthesise the overall information available from traditional probab ilistic studies, but also suggests that soil condition is a first-order ing redient for effective seismic hazard mapping at national level. Three Itali an towns, damaged by the 1997 Umbria-Marche earthquake sequence, are consid ered as example to demonstrate that: (1) soil condition dependent uniform h azard spectra well approximate actual spectra recorded during some events o f the seismic sequence; (2) for these localities, the design spectrum of th e present Italian seismic code does not seem adequate. These considerations have induced the Italian scientific community to propo se an updating of the national seismic zonation on the basis of several haz ard parameters, that are described in this paper. (C) 2001 Elsevier Science Ltd. All rights reserved.