Unrevealing Seismogenic Sources of Historical Earthquakes: New Insights from the Elusive 1706 Maiella Earthquake (Abruzzi Region, Central Italy)

The Inner Abruzzi region ranks among Italy's highest seismic hazard areas, hosting a system of SW-dipping normal faults historically active with moderate-to-large earthquakes (Mw ≥ 5.5). The L’Aquila earthquake in April 2009 (Mw = 6.3) intensified interest in assessing active fault seismogenic potential. Despite extensive studies, the Maiella Massif's historical seismicity, notably the 1706 and 1933 earthquakes (Mw ~6.8 and 5.9, respectively), causing severe damage, remains elusive. The investigation of historical seismic events, such as the 1706 earthquake within the Maiella area, aligns with the pressing need to identify and characterize potential seismogenic sources of future seismic activity. This task is often hindered by the scarcity of unambiguous evidence or quantitative data at both near-surface and seismogenic depths. Many source hypotheses, possibly related to the 1706 earthquake, are present in the literature. These structures, with different geometrical parameters, depths, and kinematics, characterized the tectonic setting of this region. This study offers a comprehensive overview of these hypotheses in assessing the 1706 earthquake’s source. Their 3D geometrical representation was modelled, considering the available geological and geophysical information. The resultant seismic scenarios were estimated in terms of macroseismic intensity by calculating peak ground motion values (i.e., PGV, PGA) for each point within the 1706 macroseismic field. This procedure incorporates site amplification effects into the synthetic ground motion calculations at each site within the macroseismic field, coupling a Vs,30 map of the Italian territory. Finally, the determination of the best source model involves assessing the misfit (residuals) between the simulated macroseismic intensities and the observed ones. The accuracy of the simulated macroseismic field in reproducing the real field is evaluated by calculating the residual mean and the root-mean-square error (RMSE). The study outcomes highlight the complexities in determining the exact source of the 1706 earthquake. Despite detailed modelling efforts, discrepancies in intensity distributions and the absence of recent seismic activity on specific faults pose uncertainties in understanding seismicity in the Inner Abruzzi area. While contributing to the ongoing debate, the research underscores the need for further investigations to better constrain the seismic hazard of this region.