Seismotectonics of the Central-Southern Apennines Junction (Italy): New Insights from High-Quality Background Seismicity Data

The junction between the central and southern Apennines represents a high-seismic-hazard region in the Mediterranean. Its seismotectonic setting is characterized by a complex, poorly understood interplay between SW-NE regional extension along the range axis and E-W mid-to-lower crustal shear zones in the Adria plate to the east. Although the range axis hosted several M6-7 historical earthquakes, their causative faults remain mostly debated. Monitoring by the Italian National Seismic Network (Rete Sismica Nazionale, RSN), with a station spacing of 10-30 km and a detection threshold of about M_L1.2 in the region, has proved insufficient to pinpoint and fully characterize source faults for recent low-to-moderate magnitude (M < 4) sequences.

To address these limitations, we conducted the first comprehensive study of background seismicity as part of the MOSAICMO project, an inter-disciplinary initiative investigating tectonic evolution and seismogenesis of this region. This study integrates a 2-year passive seismic experiment (2023-2025) with a re-analysis of the 2016-2022 RSN seismicity. Our objectives were to improve knowledge of the active faults and relationship between seismogenesis and physical properties of the crustal rocks. The seismic experiment integrated 13 temporary stations with 20 permanent stations of the RSN over an area of 60x60 km², reducing station spacing to 4-12 km. Initial analysis of the first nine months of the new dataset using a standard STA/LTA algorithm identified 470 events (0.2 < M_L < 2.8), representing a 220% increase over the RSN catalog. For these earthquakes, P-and S-phases were manually picked. For the 2016-2022 seismicity, we revised and augmented the phase picks for 1,400 selected events and applied cross-correlation template matching to a prolonged swarm-like sequence (2016-2017; Mw 4.3) to produce a high-resolution catalog.

We utilized these phase picks to construct catalogs, through: i) absolute locations using the probabilistic location software NonLinLoc and a new optimized 1D velocity model, based on a non-linear approach ii) high-precision relative locations using the double-difference technique HypoDD; iii) absolute 3D re-locations alongside with Vp and Vp/Vs crustal models derived from Local Earthquake Tomography on a 3 × 3 × 2 km grid.

Our results show that seismicity deepens eastward, from 3–12 km beneath the inner range to 15–22 km under the outer range. While the upper crust exhibits mixed extensional and strike-slip focal mechanisms, deeper eastern events are almost exclusively strike-slip. Most seismicity occurs in small, short-lived clusters. Along the inner range, seismicity concentrates at 5-10 km depth within high-Vp (6.0-6.7 km/s), low-Vp/Vs (1.70-1.85) zones. Here, high-precision relocations reveal NW-striking, NE-dipping alignments consistent with known Quaternary normal faults. Integrating these results with a subsurface geological model based on seismic commercial profiles and exploration wells, and a 2D magnetotelluric tomography, we find that: (i) axial seismicity is mainly hosted within the high-velocity, high-resistivity Mesozoic carbonates of the Apulia Platform, (ii) the 2016–2017 swarm-like seismicity also clusters within the Apulian Platform but correlates with a low-resistivity anomaly, suggesting a fluid-driven seismogenic mechanism.