Seismic hazard in the Po Plain and the 2012 Emilia earthquakes

The Emilia earthquakes of May 20, 2012 (ML 5.9, INGV;
MW 6.11, http://www.bo.ingv.it/RCMT/) and May 29, 2012
(ML 5.8, INGV; MW 5.96, http://www.bo.ingv.it/RCMT/)
struck an area that in the national reference seismic hazard
model [MPS04; http://zonesismiche.mi.ingv.it, and Stucchi
et al. 2011] is characterized by expected horizontal peak
ground acceleration (PGA) with a 10% probability of exceedance
in 50 years that ranges between 0.10 g and 0.15 g
(Figure 1), which is a medium level of seismic hazard in Italy.
The strong impact of the earthquakes on a region that is not
included among the most hazardous areas of Italy, and the
ground motion data recorded by accelerometric networks,
have given the impression to the population and the media
that the current seismic hazard map is not correct, and thus
needs to be updated.
Since the MPS04 seismic hazard model was adopted
by the current Italian building code [Norme Tecniche per le
Costruzioni 2008, hereafter termed NTC08; http://www.cslp.
it/cslp/] as the basis to define seismic action (the design spectra),
any modification to the seismic hazard model would
also affect the building code.
The aim of this paper is to briefly present the data that
support the seismic hazard model in the area, and to perform
some comparisons between recorded ground motion with
seismic hazard estimates and design spectra. All of the comparisons
presented in this study are for the horizontal components
only, as the Italian hazard model did not perform
any estimates for the vertical component.