3D modelling of the fault-bounded Castelluccio basin (2016 Mw 6.5 Norcia earthquake, central Italy) using deep resistivity Full Waver survey

The Castelluccio basin in the central Apennines (Italy) is the main Quaternary depocenter in the hangingwall of the Vettore-Bove normal fault system (VBFS), which was responsible for the 2016-2017 seismic sequence culminated with the Mw 6.5 Norcia earthquake. The tectonic nature of this basin and its relation with the VBFS is still debated since the infill architecture is poorly constrained by available borehole and geophysical data. We carried out an extensive 3D survey using the recent Fullwaver (FW) technology conceived to perform deep electrical resistivity tomography (DERT). The experiment covered a 23 km² area (which we subdivided into two subset areas according to logistics) reaching an exploration depth of ~1 km. A total of 24 independent 2-channels receivers connected each to 3 grounded steel electrodes, 200 m spaced (MN), were used to record the electrical field for each subset area. Current was injected in the ground through an induced polarization transmitter. This enabled transmissions (AB) up to 4 Amps using an external 5 kW generator and voltage recordings of about 0.2 V in the most distant receivers. A total of 25 AB acquisitions were performed for each subset region with a maximum AB electrodes distance of ~6 km. The spacing between transmitters and receivers was planned to ~400 m and then adjusted due to the presence of rough topography and extensive cultivations. Data were duly processed and then modelled via a regularized inversion with smoothness constraints to cope with the strong resistivity changes expected in the basin filled by fine/coarse deposits above an articulated carbonate substratum. This innovative technology allowed us to characterize the buried expression of VBFS fault splays and to constrain the geometry of the basin. Main resistivity structures show two trends, NNW and NNE, pointing to a poly-phase evolution of the basin. The basin infill is well resolved as a N-trending conductive structure (< 150 Ωm) that deepens down to 500 m and delineates two depocenters. NNW-striking structures in the NE sector can be related to blind splays of the VBFS that offset the carbonate substratum and locally control the architecture of the infill. Our results document that the FW system is an effective tool to acquire 3D DERT over a large area and to obtain high-resolution images of fault-related complex basins