Moment Tensor Inversion from Historical Seismograms: Case Studies in Southern Italy

The investigation of historical seismicity has increasingly demonstrated its pivotal role in advancing seismic hazard and risk assessment. This study presents an integrated methodological approach to recover and analyze analog seismograms, aiming to enhance our understanding of historical earthquakes and their implications for local and regional seismotectonic modeling. Our work focuses on three seismic events occurred in southern Italy: the 1947 Squillace Basin earthquake, the 1968 Belice sequence, and the 1978 Ferruzzano earthquake. These events, located within the geodynamically complex and high-seismic-risk Southern Italy region, represent significant case studies to test the potential of analog seismograms in providing past earthquake characterizations. For each event, we employed a systematic workflow encompassing the selection, digitization, and processing of analog seismograms. The instrument corrections were rigorously applied, and data quality was assessed to ensure reliable results. A time-domain waveform inversion algorithm specifically tailored for pre-digital data was utilized to compute moment tensor solutions. This approach allowed us to determine key seismic parameters, including fault mechanisms, hypocenter locations, and moment magnitudes, offering new insights into the seismotectonic framework of this region. The 1947 Squillace Basin earthquake was identified as a Mw 5.1 event with left-lateral kinematics on a WNW-ESE fault, consistent with STEP fault activity of the Northern Calabria subduction edge. Similarly, the 1968 Belice sequence revealed predominant reverse faulting on E-to-NE trending structures, resolving long-standing ambiguities in its causative mechanism. The 1978 Ferruzzano earthquake, previously characterized by conflicting interpretations, was redefined as a Mw 4.7 event with a NS normal faulting mechanism. Our findings underscore the invaluable role of analog seismograms in extending the seismic record, refining earthquake parameters, and constraining seismotectonic models. In addition, these results demonstrate the feasibility of applying modern techniques to historical data, paving the way for future investigations focused on early instrumental seismicity. By addressing challenges related to data preservation, digitization, and analysis, our work contributes to the ongoing efforts to compile comprehensive datasets for historical earthquakes. These datasets are essential for improving seismic hazard assessment and informing risk mitigation strategies, ultimately supporting the resilience of vulnerable communities to earthquake-related natural hazards.