A Microtremor Survey in the Area Shocked by the ML 5.2 Salò Earthquake (North Italy): An Empirical Approach to Determine the Effects of Ground Motions

摘要

In this work, the results of a quick microtremor survey performed in the municipalities situated inthe epicentre area of the Ml 5.2 2004 Salo` earthquake (North Italy) are presented. The aim of thisstudy is to understand if the large amount of damage caused by the event (about 215 million eurosonly in the areas near to the epicenter) is correlated more to the local surface geology conditions orto the vulnerability of ancient Italian historical centers.A preliminary seismic zonation was carried out in 5 villages including about 30 measurementsof microtremors analyzed by the Nakamura technique (hereinafter HVNR). The points of measurementwere carefully selected considering sites located both near damaged buildings and overdifferent local geology conditions (alluvium deposits, fluvial-glacial deposits, debris fans androck). In order to strengthen the HVNR results and to evaluate the reliability of the Nakamuraanalysis, a comparison with spectral ratios calculated on earthquakes (hereinafter HVSR) recordedat the strong motion station of Vobarno was made. In general, the outcomes of the survey highlighta possible correlation between local geology conditions and ground motion amplification fordifferent frequency bands. In order to check if this evidence is linked with the damage, a seriesof macroseismic intensity values were collected for different zones of the investigated area, and anonparametric correlation approach was used to establish a possible correlation between damageand ground motion amplification for selected frequency bands. The results show, from a statisticalpoint of view, that in the area surrounding the epicenter of the November 24, 2004 mainshock, thedamage pattern is not strongly dependent upon the local surface geology but more correlated to thelow quality of the civil structures present in the area, including old buildings of the last century.