Time-Independent Annual Occurence Rates Computed for Seismic Hazard Assessment in Italy

In 2015 the Italian Department of Civil Protection, started a project for upgrading the Italian seismic hazard map released between 2004 and 2006 (MPS04-S1; zonesismiche.mi.ingv.it and esse1.mi.ingv.it) inviting the Italian scientific community to participate in a joint effort for its realization. Each participating research group was requested to provide spatially variable time-independent (Poisson) long-term occurrence rates of seismic events from an Mw4.5 minimum magnitude, considering bins of 0.1 magnitude units in geographical cells of 0.1° x 0.1° on the entire Italian territory. In our model, for each of the three considered data sets (recent versions of the historical and instrumental seismic catalogs, and the data base of the individual seismogenic sources) we determined a distinct annual rate value for each cell. The following criterion was adopted: if the cell falls within one of the recognized seismogenic faults, we adopted the respective value of occurrence rate, only referred to the magnitude bin of the characteristic event. This value was divided by the number of grid cells that fall on the horizontal projection of the source. If, instead, the cell falls outside of any seismic source, we considered the rates of earthquakes exceeding an Mw4.5 minimum magnitude obtained from the historical and the instrumental catalog, using the smoothing method introduced by Frankel (1995). In order to give an average rate, the two rates were first normalized to the same period of time and then merged together by a weighted average based on the temporal duration of the catalogs. The obtained value was then scaled in magnitude by the Gutenberg-Richter relationship in order to give the average rate of background seismicity for each magnitude bin from M4.5 up to higher values. Here we present the final results of our study to be used for the new Italian seismic hazard map.