Disaggregation of Probabilistic Ground-Motion Hazard in Italy

Probabilistic seismic hazard analysis is a process that integrates over
\naleatory uncertainties (e.g., future earthquake locations and magnitudes) to calculate
\nthe mean annual rate of exceedance (MRE) of given ground-motion parameter values
\nat a site. These rates reflect the contributions of all the sources whose seismic activity is
\ndeemed to affect the hazard at that site. Seismic hazard disaggregation provides insights
\ninto the earthquake scenarios driving the hazard at a given ground-motion level. This
\nwork presents the disaggregation at each grid point of the Italian rock ground-motion
\nhazard maps developed by Gruppo di Lavoro MPS (2004), Meletti and Montaldo
\n(2007), and Montaldo and Meletti (2007). Disaggregation is used here to compute
\nthe contributions to the MRE of peak ground horizontal acceleration (PGA) and
\n5%-damped 0.2, 1.0, and 2.0 sec spectral acceleration values corresponding to different
\nmean return periods (MRPs of 475 and 2475 yr) from different scenarios. These sce-
\nnarios are characterized by bins of magnitude, M, source-to-site distance, R, and
\nnumber, ε, of standard deviations that the ground-motion parameter is away from its
\nmedian value for that M R pair as estimated by a prediction equation. Maps showing
\nthe geographical distribution of the mean and modal values of M, R, and ε are presented
\nfor the first time for all of Italy. Complete joint M–R–ε distributions are also presented
\nfor selected cities. Except for sites where the earthquake activity is characterized by
\nsporadic low-magnitude events, the hazard is generally dominated by local seismicity.
\nMoreover, as expected, the MRE of long-period spectral accelerations is generally con-
\ntrolled by large magnitude earthquakes at long distances while smaller events at shorter
\ndistances dominate the PGA and short-period spectral acceleration hazard. Finally, for a
\ngiven site, as the MRP increases the dominant earthquakes tend to become larger and to
\noccur closer to the site investigated.