Comparison between VS30 and other estimates of site amplification in Italy

Abstract

During 5 microzonation projects in Italy, we had access to 46 estimates of Vs30 (30 obtained with down-hole or cross-hole measurements, plus 10 velocity profiles obtained with surface techniques, 6 sites were considered A without drilling, being rock sites). In all the sites we performed HVNR measurements to verify the presence of a resonant frequency and in 34 sites we installed seismic instrumentation to record earthquakes and estimate site amplification using HVSR. It is important to note that we did not pre-selected the sites, but just followed the requests of two Regional governments (Marche and Basilicata) to study a set of localities that were chosen for reasons other than geo-morphological ones (previous earthquakes, pilot studies, design of new infrastructures). The comparison between site seismic amplification and Vs30 showed that this last parameter is not a good proxy of observed site effects. The reason why in Italy Vs30 does not provide satisfactory estimates is linked to peculiar geological settings that are widespread in our country. The main problems encountered are underestimations by Vs30 at sites with velocity inversions and overestimations on deep basins. Vs30 seems to work fine only if a site has a strictly monotonic velocity profile increasing with depth and a strong impedance contrast in the first dozen meters. Further data will be available thanks to an ongoing national project funded by the Civil Defence Department that is focusing on Vs30 estimates in the presence of velocity inversions, fractured rock masses, landslides and karst areas.