3D full-waveform geoelectrical imaging of the Pantano di San Gregorio Magno basin (Irpinia region, Italy): constraining fault geometry for surface-rupture seismic hazard assessment

The identification and characterization of active and capable faults are essential for subsurface modelling and seismic hazard assessment. In tectonically active areas such as the Southern Apennines, where large historical earthquakes have occurred (Mw ≥ 6.0), detailed fault investigations are critical.  Surface ruptures linked to the Monte Marzano Fault System were observed during the most significant earthquakes of the last century in this region, including the 1980 Ms 6.9 Irpinia earthquake. This study presents a geophysical investigation aimed at detecting fault segments crosscutting the Quaternary sediments that fill the Pantano di San Gregorio Magno (PSGM) intramountain basin, in the Irpinia region.

The geophysical survey targeted a depth range of 25–150 m to image the basin fill and underlying bedrock. The survey was conducted using the FullWaver System (IRIS® Instruments), marking the first time that a 3D FullWaver-based resistivity and induced-polarization survey has fully covered the PSGM basin. The equipment included wireless dual-channel digital receivers and a 5-kW time-domain induced-polarization transmitter, providing flexibility for data acquisition across rugged terrain and minimizing logistical constraints.

After an extensive statistical quality check, considering acquisition conditions and lithological responses, the data were filtered and a robust inversion was executed using ViewLab software. These processes produced a detailed 3D resistivity model of the basin, integrated with a geological model to deliver an accurate view of its architecture. The results enabled the detection of fault segments concealed beneath Quaternary deposits, in agreement with available reflection seismic data. Moreover, induced-polarization data confirmed earlier evidence of degasification anomalies along the surface rupture associated with the 1980 earthquake.

Our findings highlight the effectiveness of deep resistivity tomography performed with wireless acquisition systems as an effective approach for imaging intramountain basins. Beyond methodological advances, these results provide critical constraints for fault-based seismic hazard models, improving the characterization of fault geometry and potential rupture zones in carbonate-dominated settings.