SOILS MECHANIC CHARACTERIZATION AIMED AT THE SEISMIC RISK ASSESSMENT IN THE SULMONA BASIN

Intermountain basins are a peculiar physiographic feature of the Apennine range in Central Italy. With flat extensions up to hundreds of km(2), these basins are endoreic depressions of tectonic origin characterized by a high seismicity and their morphology facilitate the development of important urban settlements and productive activities into the Apennine chain. In this paper are presented data from multidisciplinary investigations carried out in the central-western sector of the Sulmona intermountain basin, where main productive activities are located, aimed at the assessment of safety conditions of industrial plants regarding natural risks, namely the seismic hazard. Geomorfologic investigations, geoelectrical (VES) and seismic refraction prospectings were carried out to obtain a general framework of the main geological characteristics of the area. Subsurface layers are characterized by lithological bodies with different resistivity values and with different deformability properties, in agreement with the presence of lateral and vertical facies heterotopias, typical of the basin depositional framework. Collected data allowed to localize sites for the execution of mechanical soundings in which further were carried out cross-hole (waves-P and S) investigations; informations were used to integrate and validate previous data stored in a geognostic database and to compute the ground dynamic modula E and mu and the Poisson ratio (nu). Local seismic response induced by seismic waves travelling from the bedrock was performed by software Shake (R). Data input of subsurface layers were provided both by geophysical prospectings and cross-hole investigations; diagrams for the computing of G/G(max) ratios and damping values were obtained from literature. Results showed that seismic shaking leads to a maximum amplification in the 2-3 Hz and 5 Hz frequencies range and is equal to a factor of 5.5; as the spectrum frequencies values increase, the amplification reduces until it drops by about a fifth in the 20-25 Hz range. These values indicate the presence of conditions for a significant amplification of seismic waves and also processes of potential liquefaction phenomena in soft and un-cohesive soils.