The comparison of the NDSHA, PSHA seismic hazard maps and real seismicity for the Italian territory

Abstract

Rigorous and objective testing of seismic hazard assessments against the real seismic activity must become the necessary precondition for any responsible seismic risk estimation. Because seismic hazard maps seek to predict the shaking that would actually occur, the reference hazard maps for the Italian seismic code, obtained by probabilistic seismic hazard assessment (PSHA), and the alternative ground shaking maps based on the neo-deterministic approach (NDSHA), are cross-compared and tested against the real seismicity for the territory of Italy. The comparison between predicted intensities and those reported for past earthquakes shows that models generally provide rather conservative estimates, except for PGA with 10 % probability of being exceeded in 50 years, which underestimates the largest earthquakes. In terms of efficiency in predicting ground shaking, measured accounting for the rate of underestimated events and for the territorial extent of areas characterized by high seismic hazard, the NDSHA maps appear to outscore the PSHA ones

Metrics for Assessing Earthquake‐Hazard Map Performance

“…There is increasing interest in this issue, and a variety of approaches have recently been used (Stirling and Peterson, 2006;Albarello and D'Amico, 2008;Mucciarelli et al, 2008;Miyazawa and Mori, 2009;Stirling and Gerstenberger, 2010;Kossobokov and Nekrasova, 2012;Wyss et al, 2012;Mak et al, 2014;Nekrasova et al, 2014) and are being developed under the auspices of the Global Earthquake Model project.…”

Earthquake recurrence and seismic hazard assessment: a comparative analysis over the Italian territory

“…By forecasting the expected value of shaking to be observed over a specified time interval, maps with fixed return period underestimate the actual shaking if earthquakes with longer recurrence times occur. To carry out the formal comparison between PSHA and NDSHA (Nekrasova et al [6]), we consider the values of peak ground acceleration (PGA) in the seismic hazard maps for Italy, obtained by probabilistic seismic hazard assessment [22], sampled at the same grid points of the NDSHA map. The first comparison between NDSHA and PSHA maps is made for T=475 years (PGA10%).…”

“…As a case study, in Peresan and Panza [27] the predictions of NDSHA are compared to the PHSA ones for the Emilia earthquake area, as shown by maps published before the earthquake [6,22]. The PSHA map, forming the basis for the Italian building code, predicts PGA to be less than 0.175 times the acceleration of gravity (g), whereas the NDSHA map predicts values in the range 0.20-0.35g, in good agreement with the observed motion that exceeded 0.25g.…”

“…Comparison between PSHA and NDSHA estimates in terms of macroseismic intensity in Zuccolo et al [7] indicates that the epicentral area of the Emilia earthquake is in a zone where PSHA predicted an intensity (as low as VIII on the modified Mercalli scale) at least one unit less than the NDSHA prediction, the latter of which is closer to the actual intensity of the earthquake. A single case study obviously cannot be considered a rigorous testing of the two methods, therefore a more systematic test is performed in Nekrasova et al [6]. The seismic hazard maps seek to predict the shaking that would actually occur, therefore the reference hazard maps for the Italian seismic code, obtained PSHA, and the ground motion maps based on NDSHA are tested against the real seismicity for the territory of Italy.…”

“…The reference hazard maps for the Italian seismic code, obtained by PSHA, and the alternative ground motion maps based on NDSHA are cross-compared and tested against the real seismicity for the territory of Italy (for details see Nekrasova et al [6]). The comparative analysis evidences the very severe dependency of PSHA expected ground shaking on earthquakes recurrence (i.e.…”