GNGTS 2022 - Atti del 40° Convegno Nazionale

174 GNGTS 2022 Sessione 1.3 THE 1968 BELICE SEISMIC SEQUENCE (SOUTH ITALY): NEW RESULTS FROM DIGITIZED ANALOG DATA S. Scolaro 1 , B. Orecchio 1 , J. Batlló 2 , D. Stich 3,4 , G. Neri 1 , D. Presti 1 , C. Totaro 1 1 Department of Mathematics, Computer Sciences, Physics, and Earth Sciences, University of Messina, Messina, Italy 2 Institut Cartogràfic i Geològic de Catalunya, Barcelona, Spain 3 Departamento de Física Teórica y del Cosmos, Universidad de Granada, Granada, Spain 4 Instituto Andaluz de Geofísica, Universidad de Granada, Granada, Spain The 1968 Belice seismic sequence represents the strongest seismic activity recorded in historical times in western Sicily, along a segment of the Nubia-Eurasia convergent margin previously considered as seismically quiescent (see, e.g., Michetti et al. , 1995). The sequence lasted fromabout January to June 1968 and the primitive seismic network operating at thetime in Sicily recorded more than 350 earthquakes (Valle, 1969). Seismic activity mainly occurred in the month of January when, according to the Parametric Catalogue of Italian Earthquakes (CPTI15; Rovida et al. , 2020), six shocks with magnitude greater than 5 struck the Belice Valley. We present the first estimates of moment tensor solutions and probabilistic nonlinear hypocenter locations for the most energetic earthquakes occurred during the January-June 1968 Belice sequence (Orecchio et al. , 2021). Poorly constrained and often controversial focal mechanism solutions and hypocenter locations available from the literature have led to uncertainties on the possible geometry and kinematics of the seismogenic source of the sequence, also having primary effects on the regional seismotectonic modeling and seismic hazard evaluations. In particular, the two most shared models proposed in the literature indicate thrusting on an about WSW–ENE striking plane or right-lateral transpression on a NNW–SSE-striking plane (Mckenzie, 1972; Gasparini et al. , 1985; Anderson and Jackson, 1987). Aiming to solve this ambiguity, we focused on the starting and most energetic phase of the sequence and we computed the moment tensor solutions for three of the strongest earthquakes by using digitized original seismograms and a time-domain waveform inversion algorithm properly implemented for analog seismic data (Stich et al. , 2005). We collected and selected the original seismograms recorded by long-period seismographs available from the digital archives of the SISMOS project (Michelini et al. , 2005). Each seismogram was digitized manually by using the software GIMP (GNU Image Manipulation Program), then it was interpolated and corrected for geometrical distortions and first arrivals were accurately verified. Moreover, we collected the arrival times from original bulletins and catalogs to analyze, by means of a Bayesian hypocenter location method, the spatial distribution of most energetic earthquakes. Our results indicate that the 1968 Belice sequence was characterized by a mainly thrust faulting activity occurring on about E-to-NE trending structures, thus solving the dualism between models previously proposed in the literature. The main features of the causative source of the 1968 Belice seismic sequence in Sicily revealed by our analyses, well agree both with the geodynamic framework governed by the Nubia-Eurasia NNW-trending convergence and with the geological reconstructions of the regional thrust front in the western Sicily area (Monaco et al. , 1996; Lavecchia et al. , 2007). The time-domain moment tensor inversion also furnished the first direct estimate of moment magnitude for the analyzed events of the sequence, clearly showing the overall overestimation of the previous assessments coming from macroseismic data and/or conversion of instrumental estimates of Ms and mb. The results of this study, furnishing an improved knowledge of the most relevant seismic activity of western Sicily, have important consequences on local and regional seismotectonic modeling and seismic hazard evaluations of the study area.