Unveiling the hidden source of major historical earthquakes: A multi-scale, trans-disciplinary approach to the 1456 and 1688 Sannio earthquakes (Mw 7.0, southern Italian Apennines)

The southern Apennine chain ranks among Europe's regions with the highest historical seismicity, yet its seismogenic structures remain poorly defined or completely unknown, including those of highly destructive Mw ∼7.0, 1456 and 1688 Sannio earthquakes, which are examined in this study. Using a multi-scale, interdisciplinary approach - combining detailed field investigations (stratigraphic, geomorphological, structural, and paleoseismological analyses), tephrochronological and OSL dating, and reassessment of macroseismic intensity distribution from archival sources - this study identifies a possible source for these earthquakes. It corresponds to a ∼45 km normal fault system composed by two main branches (Eastern Calore Fault and Western Calore Fault) with primary segments trending E-W to ESE-WNW and dipping N- to NNE. Segments extend along the southern border of both eastern and western Calore River sub-basins and are partially connected by NNW-SSE to N-S trending, east-dipping, transfer zone fault splays. Although the morphostructural evidence of Quaternary tectonic activity is unevenly expressed in both sub-basins, they share a similar Middle Pleistocene-to-Holocene morpho-sedimentary evolution, suggesting a common driving factor, ascribable to the sub-coeval activity of the fault segments delimiting both sub-basins. On a short-time scale, this study provides the first evidence of a post-14 ka occurrence of a paleoearthquake with a surface displacement ≳0.7 m, as well as of a decametric off-set affecting post-9 ka sediments. These coseismic surface ruptures have an estimated recurrence time of approximately 1400 years, with the two more recent events likely corresponding to the Mw ∼7.0, 1456 and 1688 Sannio earthquakes. Further detailed paleoseismological investigations are recommended to uncover direct evidence of co-seismic displacement linked to the 1456, 1688, and earlier earthquakes. • The Quaternary sedimentary infill of the Calore River basin exhibits an asymmetric geometry. • A 45-km long fault system led the morpho-sedimentary evolution of the Calore basin. • A ∼14 ka coseismic surface faulting (≥70 cm) occurred along the Calore fault. • Over the past 14 kyr, the same fault likely caused ten similar surface displacements. • The last two faulting events likely caused the Mw 7, 1456 and 1688 Sannio earthquakes.