SITE AMPLIFICATION IN THE EPICENTRAL AREA OF THE 31/10/2002EARTHQUAKE (MOLISE, ITALY): COMPARISON BETWEENDAMAGE DATA, MICROTREMORS, WEAK- AND STRONG-MOTIONS

摘要

On October 31st and November 1st 2002 the Basso Molise area (Southern Italy) was struck by twomoderate 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 geomorphologicalsetting. In order to detect the main cause of the severe building loss, damage andvulnerability distributions were analyzed, together with microtremors and weak motion monitoring, inseveral municipalities located in the epicentral area (Bonefro, S. Croce di Magliano, S. Giuliano di Pugliaand Colletorto). A limited number of strong motion recordings from the most severe aftershocks werealso available for Bonefro and S. Giuliano. We initially evaluated the site response by H/V ratiosperformed on microtremors. The low reliability of the Nakamura method in detecting the absoluteamplification level drove us to the adoption of other techniques. With the purpose of understanding thesite amplification influence on the damage caused by the main shock, we installed a local network torecord weak motions and perform standard spectral ratios with a reference site and single station H/Vratios. Finally, the damage and vulnerability distributions in the building stock were derived from thedatabase of the post-event survey Also the normalized damage was calculated to better understand theseparate contribution of vulnerability and site amplification to the actual macroseismic intensity observedin each municipality, and to identify some still open questions.