The 1857 Basilicata earthquake (Mw 7.2): is the trans-ridge Caggiano-Montemurro en-echelon normal fault the responsible?

Identifying still unrecognized surface evidence of faults capable of releasing moderate-to-strong earthquakes is especially relevant for seismotectonic and seismic hazard purposes. This is the case of the Basilicata 1857 earthquake (Mw 7.2), which, in a time-lapse of ~3 minutes, released two distinct subevents generating devastation and victims (10.000 to 19.000 casualties) in a large area of the southern Apennines, between the Vallo di Diano and the Val d’Agri (Mallet, 1862). There are many open questions regarding this earthquake, such as the causative fault, the possibility of a hidden source fault, the lack of surface faulting events during the earthquake, the size and depth of the seismogenic fault segments and the dip-angle and dip direction (NE rather than SW) within the Val d’Agri. Answering these open questions is crucial due to the study area’s social and economic structure and heritage and the related seismic and anthropogenic hazards. Through an initial phase of satellite image interpretation, we delineated an alignment of morphostructural elements, which we considered clues to guide field surveys. About 370 punctual structural data (fault plane and striation) were acquired over an area of ∼900 km2 and used to delineate a ~65 km-long SSW-dipping enechelon fault system connecting the Auletta, Vallo di Diano, and Val d’Agri basins between the Caggiano and Montemurro villages. The fault system develops transversally across a portion of the Monti della Maddalena NNW-SSE striking ridge; therefore, it is referred to as Caggiano-Montemurro (CMF) trans-ridge fault (Bello et al., 2022). Most commonly, normal faults bound major Quaternary basinal depressions. However, the strong 1980 Irpinia earthquake (Mw 6.9) had already proven that faults capable of generating strong earthquakes can run in the highest portions of the massifs, generating small along-strike elongated intramountain basins (i.e., a few tens or hundreds of meters; Bello et al., 2021). The similarities between the structural style of the faultcontrolled small basins in the highest portion of the Monti della Maddalena massifs in the study area and the small basins along the Irpinia fault, can then be interpreted as similar evidence of recent fault activity. Furthermore, formal stress field inversion of fault/slip data surveyed along the CMF provides a N032-trending near-horizontal s3-axis, which is coaxial to that computed for the neighboring Irpinia area (Bello et al., 2021), highlighting a deviation of the intra-Apennine tensional axis from a regional SW-NE trend to a WSW-ENE trend in the Campania-Lucania sector. Considering the continuity of the CMF, we hypothesize that the whole system may represent the master fault of the area and we speculate on its maximum expected magnitude (between 7 and 7.3), making the CMF a possible responsible for the 1857 earthquake. In the CMF, we find the simplest possible explanation, supported by literature data (Galli et al., 2006).