New Results for the 1968 Belice, South Italy, Seismic Sequence: Solving the Long-Lasting Ambiguity on Causative Source

We present the first estimates of moment tensor solutions and probabilistic nonlinear hypocenter locations for the 1968 Belice earthquake sequence, which is the most relevant seismic activity occurred in western Sicily in historical times. This seismic phase, including six earthquakes with magnitude between 5 and 6.4, produced severe damages and fatalities in a sector of the Nubia–Eurasia plate margin, previously considered aseismic. Poorly constrained and often controversial hypocenter locations and focal mechanism solutions available from the literature have led to a long-lasting ambiguity on the possible causative source of the sequence, also having primary effects on the regional seismotectonic modeling and seismic hazard evaluations. The two main fault models proposed in the literature alternatively assigned a primary role to the north-northwest-trending Nubia–Eurasia plate convergence, causing thrust faulting on about east-trending structures or to the differential foreland retreat driving dextral strike-slip movements on about north–south-oriented faults. By focusing on the starting and the most energetic phase of the 1968 sequence, we computed the moment tensor solutions for three of the strongest earthquakes using digitized waveforms and a time-domain waveform inversion technique. Then, we also analyzed, by means of a Bayesian hypocenter location technique, the spatial distribution of the 1968 earthquakes. All the results indicate that the 1968 Belice sequence was characterized by predominant reverse faulting occurring on about east-to-northeast-trending structures, thus solving the dualism between models previously proposed in the literature. Our findings well agree both with the geodynamic framework governed by the Nubia–Eurasia north-northwest-trending convergence and with the geological reconstructions of the regional thrust front in the western Sicily area. The results of moment tensor estimations and nonlinear hypocenter locations furnishing an improved knowledge of the most relevant seismic activity of western Sicily also concur to better constraint the seismotectonic modeling of the region.