Seismic hazard estimate in the Alps and Apennines (Italy) using smoothed historical seismicity and regionalized predictive ground-motion relationships

The aim of this study is to conduct a new probabilistic seismic hazard assessment for central and northern Italy, using the most recently developed predictive relationships for ground motion, along with updated seismic catalogs. Seismic hazard is evaluated over three regions (western Alps, eastern Alps, and Apennines) using a new methodology that follows the procedure originally described by Frankel (1995). The approach of using spatially-smoothed historical seismicity is different from the one used previously by Slejko et al. (1998) and Romeo et al. (2000) for Italy, in which source zones were drawn around the seismicity and the tectonic provinces. We analyze the declustered historical seismicity, compute the rate of earthquakes on a grid, and smooth these rates to account for uncertainty in the spatial distribution of future earthquakes. In our study, the smoothed seismicity is obtained by counting the number of earthquakes with magnitude greater than 4 (accounting for completeness variations) in each cell of a grid with spacing 0.1° in latitude and 0.1° in longitude. We incorporate new regionalized predictive ground-motion relationships into the hazard calculation, and compare results with alternative models derived by Ambraseys et al. (1996) and Sabetta and Pugliese (1996). We generate maps of peak ground acceleration with 10% probability of exceedance in 50 years (475-year return period) for a rock site condition. These are not intended to be the final seismic hazard maps for the regions; instead, we wish to present the usage of a new methodology for Italy, and examine the effect of the new attenuation relationships on the hazard. We estimate the seismic hazard from the historical seismicity only, and not from faults with recurrence rates obtained from geologic data (this will be taken into account in future work). The maps obtained in this study are based on the assumption that the process of earthquake occurrence is inherently Poissonian, so that the probabilistic ground motions are time-independent.