ITACA station classification: EC8 site classification based on 1:100.000 geological map

Abstract

This activity was requested by the project coordinators since it was necessary to have an early version of the recording stations classification in order to interface the ITACA database with the REXEL software for the selection of natural accelerograms compatible with NTC2008 proposed reponse spectra. Since the compilation of the monographs is ongoing and, at the same time, a cataloging for all the 616 ITACA stations was immediately needed, a preliminary classification based solely on geological data available at a homogeneous level for all the sites was undertaken. Using a lithological map by INGV (unpublished), at a national scale (1:100,000), this goal has been achieved. This map derives from the Geological Map of Italy at 1:100,000 scale, by merging different geological formations, based on lithological and geological age criteria, in several lithological units. Each unit was attributed an EC8 subsoil class. The limits of this type of “geological” classification are well known, as it allows only a approximate level of knowledge, but, as already mentioned, it was dictated by the urgency to set up a first classification. This classification based on lithological map was subsequently "corrected" using: a) detailed geological data; b) a geological "expert" evaluation; c) H/V microtremors measurements; d) photos of the sites; e) Vs30 values from down-hole and cross-hole tests. Main problems have been encountered where the site studied is near a geological limit between two different soil categories detached from two classes, the thickness of cover, rested on bedrock, is less than 20 m or landslides of some importance are present. VS30 values are available for very few stations which are denoted without asterisk in the “subsoil category” column.