Preliminary assesment of site stratigraphic amplification for shakemap processing

ShakeMap® is the most widespread software generating maps showing spatial distributions of several ground motion parameters. Non-linear relationships describing the amplification induced by site effects are fundamental ingredients to obtain reliable maps for emergency issues. In this study, site effects are evaluated as the amplification factor SS which can directly multiply the reference peak ground acceleration to obtain the peak acceleration at surface. The accelerometric records of the Italian national network have been selected and the subsoil profiles of the recording stations were classified according to the National Technical Code (NTC). A first set of experimental amplification factors has been computed selecting those stations on deformable soil for which a record of the same event exists also at a nearby station located on outcropping rock. A second set of semi-empirical data has been obtained by analyzing the accelerograms at sites where the subsoil is adequately characterized. The recorded ground motions have been de-convoluted to the bedrock, allowing to back-figure the reference motion. A third set of barely analytical data came from 1D numerical site response analyses performed on virtual stratigraphic profiles, consistent with the ground classification of the NTC, and subjected to the acceleration time histories recorded at those Italian stations classified as rock sites. The three data sets have been interpreted to obtain unique relationships between the stratigraphic amplification factor, SS and the ground motion amplitude at bedrock for each clas The empirical relationships were finally assessed by simulating the ground motion distribution associated to the 23.XI.1980 Irpinia earthquake (MW = 6.9).