Ground motion modelling including finite fault and 1D site effects in north-eastern Italy

The accurate prediction of the ground motion is of utmost importance in the seismic hazard assessment. 3D or 2D modelling is desirable to obtain realistic estimates of ground motion, but often the shortage of information about source and crustal structures compels us to adopt simplified techniques. In this paper we propose and test an expeditious methodology to compute near field ground motion including source and 1D site effects. Our methodology is based on a stochastic finite-fault modelling technique together with a 1D code to generate the soil response. As study area we choose the Friuli region in north-eastern Italy, seen as optimal since it was possible to identify a zone, located in the Friuli plain, and some sites at short distance with different soil conditions where past earthquakes produced severe damage. The 1936 Cansiglio earthquake (M_S=5.8) and the May 6, 1976 (M_S=6.5) Friuli earthquake, together with three aftershocks of the 1976 seismic sequence with ML ranging between 4.3 and 5.8, were chosen as target events to validate our procedure in a wide frequency contents. We compare peak ground motions or response spectra derived from the synthetic signals with those obtained from the recordings at bedrock or soil stations. When none (1936 Cansiglio earthquake) or few recordings are available (1976 Friuli earthquake), we compare the intensities derived from response spectra with the macroseismic observations. As a final step, we compute the expected ground motion generated by two scenario earthquakes of M=6.7, that is the maximum magnitude expected for the area under investigation.