Abstract
In 1968, six earthquakes with magnitude between 5.1 and 6.4 destroyed or heavily damaged several towns in the Valle del Belice (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 Belice 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 M 0 .
Highlights
The 1968 earthquakes in Western Sicily represent one of the most important sequences in the seismic history of Italy in the second half of the 20th century
We re-analyse the primary sources to reconstruct the evolution of the damage scenario during the sequence, assess intensity by using the European Macroseismic Scale [EMS-98, Grünthal, 1998] and we compare the macroseismic parameters calculated by these new data with the ones reported in the CPTI15 earthquake catalogue [Rovida et al, 2016]
De Panfilis and Marcelli [1968] – the main source of information for the subsequent studies by Barbano et al [1980], Cosentino and Mulone [1985], Guidoboni et al [2018] – report generic information on damage produced by the individual shocks in several localities, admitting that the maximum intensities of the various shocks are calculated from the instrumental magnitudes; they do not provide any intensity data points
Summary
The 1968 earthquakes in Western Sicily represent one of the most important sequences in the seismic history of Italy in the second half of the 20th century. The impact on the territory and society, mostly in terms of resilience, was enormous, so much so as to become, together with the case of Irpinia after the 1980 earthquakes, a symbol of a problematic and very lengthy reconstruction [Guidoboni and Valensise, 2011] This can be ascribed to a number of factors, the main ones being the poor quality of buildings, the number of shocks occurring in nearby areas during a very short timespan and, last but not least, the condition of poverty in this underdeveloped and rather withdrawn part of Sicily [Traina, 1978]. In Sicily, only three seismic stations were operating with almost ‘primitive’ typology of instruments – mechanical Wiechert type seismometers – located at Palermo, Messina and Catania Another two seismometers were installed in Trapani and Agrigento after January 25, effectively when all the strong events had already occurred; the instrumental analysis was affected by large uncertainties with rather unstable earthquake parameters [De Panfilis and Marcelli, 1968; Bottari, 1973]. We re-analyse the primary sources to reconstruct the evolution of the damage scenario during the sequence, assess intensity by using the European Macroseismic Scale [EMS-98, Grünthal, 1998] and we compare the macroseismic parameters calculated by these new data with the ones reported in the CPTI15 earthquake catalogue [Rovida et al, 2016]
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