A reappraisal of the 1978 Ferruzzano earthquake (southern Italy) from new estimates of hypocenter location and moment tensor inversion

The March 11th, 1978 Ferruzzano earthquake is the most recent moderate-to-major earthquake occurred in the southern Calabrian region (southern Italy), one among the highest seismic risk areas of the whole Mediterranean. Previous information available from the literature on the 1978 earthquake is quite contrasting and not well framed in the regional seismotectonic scenario. In the present study we selected and digitized analog seismograms coming from stations of the Euro-Mediterranean region to invert for the deviatoric seismic moment tensor through a time-domain algorithm properly implemented to analyze data recorded before the advent of the digital era. Moreover, we estimated a new hypocentral location by using original bulletin data and a non-linear probabilistic earthquake location technique working with 3D velocity models. The quality and stability of the obtained results, both for hypocenter location and moment tensor inversion, were accurately checked by several inversion tests. Our results indicate that the 1978 earthquake (i) occurred westward and at a shallower depth respect to previous hypocenter locations, (ii) is characterized by a ca. N-S trending normal faulting mechanism and (iii) has a moment magnitude of 4.7, thus suggesting an overestimate of previous evaluations. This study furnishes new information on the 1978 Ferruzzano earthquake allowing to better frame it in the regional seismotectonic scenario and also proves that the time-domain waveform inversion algorithm applied to digitized old seismograms is capable to successfully invert also Mw < 5 earthquakes. The obtained results pave the way for future analyses of the early instrumental seismicity potentially capable to furnish new constraints to local and regional seismotectonic modeling.