Seismic Hazard Assessment (2003-2009) for the Italian Building Code

This paper describes the probabilistic assessment of seismic hazard (PSHA) of Italy in view of the building codes from 2003 to 2009. A code was issued in 2003 as a Prime Minister Ordinance, requiring that a PSHA for updating the seismic zoning would be performed in one year, in terms of horizontal peak ground acceleration (PGA) with 10% probability of exceedance in 50 years on hard ground. For the first time in Italy, a working group, established by the Istituto Nazionale di Geofisica e Vulcanologia, adopted a logic-tree approach to model the epistemic uncertainty in the completeness of the earthquake catalog, the assessment of the seismicity rates and M max, and the ground-motion prediction equations. The seismic hazard has been computed over a grid of more than 16,000 points for the median value (fiftieth percentile) and the eighty-fourth and sixteenth percentiles of the 16 branches of the logic tree. Using the same input model, PGA values and spectral accelerations for 10 spectral periods were computed for nine different probabilities of exceedance in 50 years. This wealth of data made it possible to base the design spectra of a new building code on point hazard data instead of being related to just four zones. The 2009 M w 6.3 L’Aquila earthquake has led many to attempt to test the reliability of this study. In this paper, we analyze suggestions coming from that event and conclude that significant changes to the design spectra are not to be recommended based just on evidence from the L’Aquila earthquake.