The role of inherited structural anisotropies during co-seismic surface faulting: The Pescopagano fault case study (Irpinia seismogenic area, Southern Italy)

Abstract

Co-seismic failure can occur on newly formed or on inherited structures. However, understanding their surface pattern is challenging when pre-existing structural anisotropies control rupture propagation. We focus here on the Pescopagano Fault (PF), considered part of the extensional fault system that ruptured during the 1980, Mw 6.9 earthquake in Southern Italy. Although the mainshock fault (Irpinia Fault) produced ∼ 40 km of NW-SE trending ground ruptures, these were not observed on a buried antithetic fault located to the northeast of the main fault and defined solely by seismological data. The PF studied here is part of an exposed fault array that spatially coincides with the trace of the antithetic fault. To better assess existing seismotectonic models for the Irpinia fault system, we investigated: i) whether ground ruptures occurred on the PF during past earthquakes, and ii) to what degree surface and deep faults are linked. Paleo-seismological trenches document that the PF has not released surface-rupturing earthquakes during the last ∼13–20 ka. Structural data suggest that the PF developed to accommodate the relaxation of a Pliocene thrust-fold system after the demise of thrusting. Results of this work highlight that the PF may be an inherited Pliocene or Early Pleistocene structure that does not reach the ∼10–15 km seismogenic depth typical of this region. In this scenario, the upward propagation of the antithetic fault from seismogenic depths towards the surface during a 1980-type earthquake may be impeded by a mélange layer developed during the growth of the Southern Apennines thrust belt and interposed between the deep antithetic fault and the upper crustal faults. We cannot exclude, however, that the PF may be activated during very large but infrequent and non-characteristic earthquakes on the Irpinia fault system.