Paleoseismic evidence for the Mw∼7 1857 earthquake along the Caggiano fault system (southern Italian Apennines)

The 1857 Campania-Basilicata earthquake in the southern Italian Apennines ranks among the most catastrophic events in Mediterranean history (Mw 7.1), with an estimated death toll ranging between 11,000 and 19,000. It consisted of a pair of mainshocks occurring within minutes of each other, affecting a vast region between the northern Vallo di Diano and the northern Agri Valley, two Quaternary intramontane basins separated by the Maddalena Range. The earthquakes were likely triggered by the cascading rupture of two adjacent, aligned normal faults, whose exact length and location are uncertain and debated, especially concerning the southern one. This study focuses on the northern normal fault, here named Caggiano fault system, which extends NW-SE for approximately 32 km across the Meso-Cenozoic carbonate of the Maddalena Range. After mapping the fault scarp using 1950s aerial photos and LiDAR-derived DTM, we conducted geological field survey of the entire fault traces, performing electrical resistivity tomography preparatory to paleoseismic trenching. Subsequently, we excavated trenches and pits, collecting numerous samples for radiocarbon dating. Results provide conclusive evidence of surface faulting to the post-Last Glacial Maximum-Holocene, indicating the Caggiano fault system as a reliable candidate for the 1857 earthquake, as well as earlier events, such as the one in 1561 and previously undocumented earthquakes in the Middle Age and Roman times. These findings represent a robust contribution to future seismic hazard assessments, which require reliable identification and seismogenic characterization of active fault systems.