Disaggregation of Probabilistic Ground-Motion Hazard in Italy

Probabilistic seismic hazard analysis is a process that integrates over aleatory uncertainties (e. g., future earthquake locations and magnitudes) to calculate the mean annual rate of exceedance (MRE) of given ground-motion parameter values at a site. These rates reflect the contributions of all the sources whose seismic activity is deemed to affect the hazard at that site. Seismic hazard disaggregation provides insights into the earthquake scenarios driving the hazard at a given ground-motion level. This work presents the disaggregation at each grid point of the Italian rock ground-motion hazard maps developed by Gruppo di Lavoro MPS (2004), Meletti and Montaldo (2007), and Montaldo and Meletti (2007). Disaggregation is used here to compute the contributions to the MRE of peak ground horizontal acceleration (PGA) and 5%-damped 0.2, 1.0, and 2.0 sec spectral acceleration values corresponding to different mean return periods (MRPs of 475 and 2475 yr) from different scenarios. These scenarios are characterized by bins of magnitude, M, source-to-site distance, R, and number, epsilon, of standard deviations that the ground-motion parameter is away from its median value for that M-R pair as estimated by a prediction equation. Maps showing the geographical distribution of the mean and modal values of M, R, and e are presented for the first time for all of Italy. Complete joint M-R-epsilon distributions are also presented for selected cities. Except for sites where the earthquake activity is characterized by sporadic low-magnitude events, the hazard is generally dominated by local seismicity. Moreover, as expected, the MRE of long-period spectral accelerations is generally controlled by large magnitude earthquakes at long distances while smaller events at shorter distances dominate the PGA and short-period spectral acceleration hazard. Finally, for a given site, as the MRP increases the dominant earthquakes tend to become larger and to occur closer to the site investigated.