A NEW 3D INTERPRETATION OF THE IRPINIA 1980 EARTHQUAKE (MW 6.9, ITALY) FAULT SYSTEM: 40 YEARS LATER

The Irpinia multi-event earthquake (M_w 6.9 –Italy) on November 23^rd, 1980 induced destruction over a large portion of SW Italy (Campania - Basilicata) with 3 sub-events at 0, 20 and 40 s.

A new structural field-digital-survey over a ~2400 km² area allowed us to identify 3 segmented master faults. The inner and intermediate ones, e.g. Inner Irpinia (InIF) and Irpinia Faults (IF), dip eastward; while the outer Antithetic Fault (AF) dips westward. Both the InIF and the IF strike NW-SE along the northern and central segments and rotate to W-E along the southern segments. This arrangement may imply a control in the stress transfer during the 1980 earthquake ruptures.

After nearly 40 years, co-seismic fault scarps with vertical displacement up to ~1 m are still well preserved along the 3 master faults.

In light of the new field data integrated with revised seismological data, a detailed 3D seismotectonic model, extrapolated to the base of the seismogenic layer, was built with the Move 3D software (by PETEX Ltd, 2019). The fault and crustal models were derived from serial seismological sections across the hypocentral area (from 1980 seismological focal mechanisms and relocated 1980 aftershock data), from wells and from a geologic reinterpretation of the CROP 04 profile.

The 3D model we reconstructed is consistent with rheological models present in the literature and shows a fault system that has cut the Apennine fold-and thrust-belt since the late Pliocene, and which likely roots into an east-dipping low-angle normal fault, accommodating an overall long-term dip‐slip displacement of more than 6000 m.

The time-space evolution of the 1980 earthquake was controlled by the interconnected fault system. The multidisciplinary dataset indicates the activation of 4 segments of the 3 faults of the 1980 earthquake fault system. The 0s event ruptured along the two IF segments (geological and seismological constraints). The 20s activated both the InIF E-W segment and the AF southernmost segment (geological and seismological constraints). The 40s event was along the northern and central part of AF (seismological constraints). This graben like model fits best with available data and may have implications for seismic hazard evaluation.