The 3D Crustal Structure in the Epicentral Region of the 1980, Mw 6.9, Southern Apennines Earthquake (Southern Italy): New Constraints From the Integration of Seismic Exploration Data, Deep Wells, and Local Earthquake Tomography

Abstract

We present the first 3D crustal model of the epicentral region of the 1980, M_w 6.9, normal-faulting Irpinia earthquake (southern Italy) determined by jointly interpreting the CROP-04 deep seismic profile, a grid of commercial seismic lines, deep exploration wells, and a high-resolution Local Earthquake Tomography. Despite numerous seismotectonic surveys and source studies of the background seismicity recorded by dense networks, a complete 3D geological model of the mid-upper crust was still lacking in the region. The architecture of the Neogene fold-and-thrust belt is also debated, with competing thin- and thick-skinned tectonic interpretations. We use the 3D geological model derived from subsurface exploration data to interpret the upper crustal tomographic velocities in terms of rock physical properties, while V_p and V_p/V_s anomalies provide inferences on the deep structural setting down to 12 km depth. We find that a thick-skinned deformation style allows explaining the geometry of Pliocene fold-and-thrust systems deforming the Apulian carbonates but also deeper Permo-Triassic metasediments and the Paleozoic crystalline femic basement. Inherited compressional structures and lithological heterogeneities control background seismicity occurring at two crustal levels. Fluid-driven shallow seismicity (<4–6 km) concentrates in a high-V_p/V_s wedge of fractured, brine-saturated Mesozoic stiff rocks delimited by the 1980 earthquake faults. Deep seismicity (9–14 km) clusters instead within the low-V_p/V_s crystalline basement underneath the Apulian carbonate ramp-anticlines. Commercial seismic data allow us to identify the Irpinia Fault, the main fault ruptured by the 1980 earthquake, reinforcing its previous interpretations as an immature structure with subtle geological and geophysical evidence.