Evaluation of Probabilistic Site-Specific Seismic-Hazard Methods and Associated Uncertainties, with Applications in the Po Plain, Northern Italy

Site-specific ground motion hazard assessed by a PSHA one-step approach that handles a single-site sigma and its uncertainties and uses a simple logic tree, is compared with a two-step approach that includes bedrock motion evaluation and wave propagation through a local soil profile with consideration of the main epistemic uncertainties. The one-step analysis relies on accelerometer data from the Po Plain, a sedimentary basin in Northern Italy where an earthquake sequence with two Mw ~6.0 events occurring in 2012 was extensively recorded. UH spectra on soil and exposed bedrock are evaluated at three deep-soil accelerometer sites (MRN, NVL and T0821), using residual measures available from other studies, by which uncertainties in site terms (S2S) and single-site sigma (ss,s) are estimated. Despite similarity in geologic conditions, at least one out of three sites displays in the one-step analysis substantial differences in mean level (T0821) or in the (84-16) percentile spread (NVL), depending on differences in site terms and single-site sigma, possibly caused by source-to site propagation effects. The two-step approach was applied to the single MRN site using as excitation carefully selected and broadband-matched acceleration signals and linear, equivalent-linear and nonlinear approaches in propagation analyses, finding that assumptions on soil degradation curves dominate the variability of results. The linear approach provided the best results, based on: (i) the similarity of the one-step non-ergodic UHS with the two-step result based on the linear approach; (ii) the comparison with observed records at MRN during the 2012 sequence mainshocks, showing PGA and short period spectral levels well beyond those predicted by different non-linear assumptions; (iii) a similar evidence from a set of 21 stations at deep soil sites of the Japanese Kik-net.