Site amplification in the epicentral area of the 31/10/2002 earthquake (Molise, Italy): comparison between damage data, microtremors, weak- and strong-motions

On October 31st and November 1st 2002 the Basso Molise area (Southern Italy) was struck by two moderate magnitude earthquakes (M = 5.3 and 5.4). The epicentral area showed a high level of damage, attributable both to the high vulnerability of existing buildings and to site effects due to the geo-morphological setting. In order to detect the main cause of the severe building loss, damage and vulnerability distributions were analyzed, together with microtremors and weak motion monitoring, in several municipalities located in the epicentral area (Bonefro, S. Croce di Magliano, S. Giuliano di Puglia and Colletorto). A limited number of strong motion recordings from the most severe aftershocks were also available for Bonefro and S. Giuliano. We initially evaluated the site response by H/V ratios performed on microtremors. The low reliability of the Nakamura method in detecting the absolute amplification level drove us to the adoption of other techniques. With the purpose of understanding the site amplification influence on the damage caused by the main shock, we installed a local network to record weak motions and perform standard spectral ratios with a reference site and single station H/V ratios. Finally, the damage and vulnerability distributions in the building stock were derived from the database of the post-event survey Also the normalized damage was calculated to better understand the separate contribution of vulnerability and site amplification to the actual macroseismic intensity observed in each municipality, and to identify some still open questions.