Seismotectonic insights into the Emilia Arc: high-resolution earthquake relocation and 3D characterization of the 2024 Langhirano sequence

Resolving and characterizing the geometry and kinematics of blind thrusts is a primary challenge in active tectonic settings, notably where seismicity nucleates at depths beyond the resolution of industrial seismic reflection profiles and borehole data. In this context, the Emilian Thrust System represents a significant case study. As one of the three arcuate thrust fronts constituting the fold and thrust belt of the Northern Apennines (Italy), it exemplifies the complex interplay between deep-seated thrusting and shallower extension that drives crustal shortening in Plio–Quaternary basins. Despite this active deformation, the lack of surface constraints and the occurrence of seismicity at depths where standard geophysical imaging fails (20 – 25 km) create a critical knowledge gap.

This work aims to overcome these observational limitations by employing high-resolution microseismicity to decipher the hidden structural architecture of the arc. To address this, we performed a critical re-evaluation of the crustal velocity structure, as existing 1D and 3D regional models often provide discordant depth estimates, introducing significant uncertainties in hypocentral locations. By optimizing these models through the Velest algorithm, we were able to minimize depth location artifacts and better constrain the seismogenic volumes. Our new velocity model provided a robust basis for high-precision relocation via the NonLinLoc code. In order to isolate significant spatio–temporal clusters from the 2008–2024 background seismicity (0.4 ≤ M_L ≤ 5.1), we utilized Kernel Density Estimation and β-statistics. The resulting dataset, together with the 2024 Langhirano sequence (comprising over 350 events), were relocated using the updated velocity model. In addition, to further enhance the kinematic framework, we improved the completeness of the existing dataset by computing new focal mechanism solutions for events with 2.5 ≤ M_L ≤ 3.9 using the FPFIT software. Relocated hypocentral depths are primarily concentrated between 15 and 30 km, and focal mechanisms indicate kinematics ranging from compressional to strike‑slip.

Our results reveal that current seismicity is predominantly accommodated by a system of antithetic structures to the basal thrusts, spanning depths between 15 and 25 km. While the basal thrust remains largely seismically silent at these depths, the high-resolution definition of these previously unrecognized antithetic faults provides a novel perspective on the structural partitioning of the arc. Stress inversion results support this framework, indicating a prevailing compressive regime with a sub-horizontal σ₁ reflecting ongoing crustal shortening. These findings suggest complex seismotectonic behavior where moderate-to-small magnitude events illuminate secondary structures, potentially acting as a release for internal deformation within the wedge. This complexity is further evidenced by the S_Hmax orientation, which rotates from a N–S trend to approximately NE–SW in the proximity of the intersection between the Emilia and Ferrara arcs.

This integrated approach allows for a refined 3D characterization of blind active faults while offering a critical perspective on deep crustal features. Such results contribute to a better definition of the seismotectonic potential of the region, providing fundamental insights for seismic risk assessment in this strategic industrial and residential area.