First Tomographic Imaging of Mid-Crustal Doubling at the Abruzzi Outer Thrust Front, Central-Southern Italy

Abstract. The geometry, deep structural style, and seismotectonic setting of the outer Abruzzi thrust system are less understood than those of other segments of Italy's Late Pliocene–Quaternary contractional belt. This knowledge gap arises from the region's complex surface geology, low seismicity rates, and the limited resolution of existing geophysical data. Here, we present a local earthquake tomography of a large and previously unexplored area that encompasses the Abruzzi thrust system and spans from the Apennine extensional province in the west to the foreland strike-slip province in the east. The model is based on the inversion of 42,176 P-wave and 29,045 S-wave arrival times from earthquakes with ML ranging from 0.2 to 5.5. Our results show low seismic velocities at upper crustal levels in the western sectors, correlating with continental basins of the extensional domain. In contrast, marked Vp inversions at mid- to lower-crustal depths in the eastern sector delineate a crustal doubling. We interpret the tomographic results in the context of geological, geophysical, and seismological data to construct a 3D conceptual model of the region. This includes the first geometric reconstruction of the Abruzzi Arc basal thrust, an eastward convex arcuate structure extending ~170 km and reaching depths of ~24 km. The model also incorporates strike-slip faults in the footwall and east-dipping normal faults to the west. The structural affinity between the Abruzzi Arc basal thrust and other seismogenic structures of the Padan–Adriatic belt located in the same structural position, suggests potential seismogenic behavior, although slow deformation rates and long recurrence intervals obscure its seismic expression. This conceptual model provides new insights into regional geodynamics and has significant implications for seismic hazard assessment in the central–southern Apennine transition zone.