Site effect in Messina and Reggio Calabria Area using HVSR tecnique

Abstract

The Messina Strait is one of the most seismically active areas of the Southern Italy; in fact, since 1783, 7 earthquakes with M ranging between 6.0 and 7.2, are reported with extensive damages and induced coseismic environmental effects well described. The most remarkable phenomena were massive landslides (with consequent obstruction of the valleys), small lakes formation, watercourses deviation, slumps, soil fracturing and liquefaction; in the urban area many of the coesismic effects were without doubt due to the settlement of loose sediments and artificial filling (e.g., Messina and Reggio Calabria dock areas). The aim of this work is to understand whether the site effect (generated by resonance phenomena in the upper sediments layers) could affect, at least partially, the distribution of urban damage after the 1908 earthquake. In fact the existence of detailed damage information for this earthquake, collected by one of the most relevant seismologist of that time (BARATTA, 1910), provides the opportunity of making a comparison between observed damage and the natural frequency of the foundation soil. For this purpose a geophysical survey was carried out in the two main cities of the Straits, and the HVSR method was adopted to identify areas subject to amplification effects and delineate the geometry of sedimentary bodies. The microtremor HVSR method (originally proposed by NOGOSHI AND IGARASHI (1970) and later diffused by NAKAMURA (1989) widely used for site effect study provides a direct estimate of the resonance frequencies of sedimentary cover, which tend to amplify soil motion in the case of earthquake.