GNGTS 2022 - Atti del 40° Convegno Nazionale

10 GNGTS 2022 Sessione 1.1 UNVEILING TRANS-RIDGE EN-ECHELON FAULT PATTERNS POSSIBLY RESPONSIBLE FOR THE 1857 BASILICATA EARTHQUAKE (Mw 7.1) S. Bello 1,2 , G. Lavecchia 1,2 , C. Andrenacci 1,2 , M. Ercoli 2,3 , D. Cirillo 1,2 , F. Carboni 2,3 , M.R. Barchi 2,3 , F. Brozzetti 1,2 1 DiSPUTer, University G. d’Annunzio, via dei Vestini 31, Chieti 66100, Italy 2 CRUST - Centro InteRUniversitario per l’analisi Sismotettonica Tridimensionale, Chieti, Italy 3 Dipartimento di Fisica e Geologia, Università degli Studi di Perugia, Perugia, Italy 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 (Imax = XI MCS; 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; Branno et al. , 1983). There are many open questions regarding this earthquake, such as the causative fault (one or more seismogenic sources?), the possibility of a hidden source fault, and the lack of surface faulting events during the earthquake. Further questions regard the size and depth of the seismogenic fault segments, the dip-angle and dip direction (NE rather than SW) within the Val d’Agri (Benedetti et al., 1998; Burrato and Valensise 2008). 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. Further, it should be especially considered that the Val d’Agri hosts a significant oil treatment center above the largest onshore hydrocarbons field in Europe. Through an initial phase of satellite image interpretation, we have delineated an alignment of morphostructural elements, which we consider clues to guide a series of geological-structural field surveys. About 370 punctual structural data (fault plane and striation) were subsequently acquired over an area of ~900 km 2 and used to delineate a ~65 km-long SSW-dipping en-echelon 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. It is identified onsite as an alignment of rather continuous fresh scarps, bounding several narrow intramountain basins filled by late Quaternary clastic deposits. Most commonly, active normal faults bound major Quaternary basinal depressions, thus geologists tend to limit the search for seismogenic faults in those contexts. However, the strong 1980 Irpinia earthquake (Mw 6.9) had already proven that faults capable of generating strong earthquakes can run for long distances in the highest portions of the massifs, generating small along-strike elongated intramountain basins ( i.e. , a few tens or hundreds of meters) Pantosti and Valensise, 1990; Galli and Peronace, 2014; Bello et al. , 2021a). The similarities between the structural style of the fault-controlled 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. , 2021a). These results highlight an evident deviation of the intra-Apennine tensional axis froma regional SW-NE trend to a WSW-ENE trend in the Campania-Lucania sector. By applying commonly used along-strike segmentation criteria (see e.g. , Biasi and Wesnousky, 2016; Brozzetti et al. , 2019; Bello et al. , 2022), we subdivided the CMF into four levels of segmentation, all arranged in a right-lateral en-echelon pattern. In particular, we consider the CMF as a first-order master fault, subdivided into two ~30 km-long second- order “segments”, seven ~10-15 km-long third order “sections” and fourteen ~5-7 km-long fourth order “sub-sections”. The fault evidences disappear at surface when crossing the Val d’Agri basin. To constrain the fault structures in this key-area, we have operated a seismic