A Reappraisal of the 1968 Valle Del Belice Seismic Sequence (Western Sicily): A case study of Intensity Assessment with Cumulated Damage Effects

In 1968, six earthquakes with magnitude between 5.1 and 6.4 destroyed or heavily damaged several towns in the Valle del Belìce (western Sicily), causing some three hundred fatalities. There have been some critical issues in the intensity assessment however in the macroseismic studies produced over the years , since the MCS scale was used as an estimation of shaking rather than a representation of the damage scenario; in practise, intensity was assigned for each earthquake of the sequence disregarding the effect of the cumulative damage. The case-study of the 2016-17 earthquakes in Central Italy reflected the difficulty in estimating the intensity in localities repeatedly hit by strong shocks, producing macroseismic parameters (epicentre, magnitude) inconsistent with the instrumental ones. As for the 1968 Valle del Belìce sequence, the Parametric Catalogue of Italian Earthquakes CPTI15 reports the macroseismic parameters, owing to the inadequacy of the seismic network operating in Italy in that period. Aware of the issues related to epicentre locations and magnitudes calculated by the existing macroseismic datasets, here we propose a reappraisal of the 1968 earthquakes following a methodology tested during the 2016-17 seismic sequence in Central Italy. By a new analysis of the primary sources, we reconstruct the evolution of the damage scenario during the sequence and assess intensity by using the European Macroseismic Scale EMS-98; the new macroseismic parameters are finally compared with the ones of the CPTI15 catalogue. Lastly, we propose a procedure for minimizing the magnitude overestimation in case of seismic sequences, based on the computation of the total seismic moment M0.