Shakemaps uncertainties and their effects in the post-seismic actions for the 2012 Emilia (Italy) earthquakes

The May 20, 2012, Emilia Ml 5.9 earthquake was followed by some major aftershocks,
well recorded by a large number of temporary stations that were installed to monitor
the sequence. These additional recordings allowed us a thorough testing of the performance of the ShakeMap—a procedure designed to provide rapid information on the experienced ground motion. We found that the shakemaps for the May 29, 2012, Ml 5.8 earthquake, obtained using the permanent stations only, underestimate significantly the ground motion computed with the highest station density, especially for PSA at long periods (T=3.0 s). This low-frequency motion is controlled primarily by the surface waves recorded in the Po plain: the observed site effects are likely not accounted properly by the site correction coefficient based on Vs30 as implemented in the ShakeMap procedure. The shakemaps determined during
the seismic sequence have been included in an Italian national law that was passed after
the 2012 earthquake. According to this law, the factories safety verifications were bound to the comparison between the shakemaps and the design acceleration required by the current national seismic code.We then decide to appraise the impact of the shakemap accuracy on the law provisions. Following the law ecommendations, we have estimated the possible errors resulting from the incomplete evaluation of the ground shaking: our results show that, if the complete dataset were available at the time of the law approval, the number of buildings for
which the safety check was required would have been significantly smaller.