Resolving focal mechanisms and stress field from microseismic events with short-term dense monitoring in the Southern Apennines

Microseismicity continuously occurs within active seismogenic faults, where major earthquakes might be generated. These small events offer critical insights into the geometry and mechanical state of faults. To enhance the detection of low-magnitude events, often obscured by seismic noise, 200 seismic stations were deployed from September 2021 to August 2022 across the complex normal-faulting environment of the Southern Apennines, organized into 20 sub-kilometric arrays, as part of the DETECT experiment. Using this dense network, an enriched seismic catalog was generated by integrating machine learning and template matching techniques, which has allowed to identify ~3,600 earthquakes with magnitudes -1.5 < M < 2.8.

Here, we resolved focal mechanisms for 289 earthquakes in this catalog. Our analysis is based on the inversion, with the software FPFIT, of the P-wave onset polarities determined by leveraging a convolutional neural network, incorporating a tailored weighting scheme. Dense monitoring allows to increase the number of focal mechanisms by a factor of ~2 compared to six years microseismicity observed with ordinary seismic network. The retrieved fault parameters align with the orientation and normal kinematics of the primary fault segments associated with the 1980 M6.9 Irpinia earthquake, but they also reveal minor occurrences of inverse and oblique faulting. Fault plane solutions are used to constrain the orientation and relative magnitudes of the stress field components, iteratively discriminating between the principal and auxiliary nodal planes by introducing fault plane instability. Our analysis reveals a stress field characterized by a near-vertical maximum compressive stress (σ₁) and quasi-horizontal intermediate (σ₂) and least compressive (σ₃) stress components. The azimuth of σ₃ aligns with the anti-Apenninic direction of the extensional regional stress field, consistent with previous estimates derived from long-term microseismic observations. In the central sector, the stress field orientation supports the presence of a kinked structure identified through earthquake relocations. Moreover, the high number and spatial distribution of resolved fault planes enable the investigation of potential small-scale stress field variations. By inverting focal mechanisms within the Northern, Central, and Southern sectors of the Irpinia region, we retrieve individual stress tensors, which reveal spatially coherent stress orientations and relative magnitudes of stress components across the region. These findings demonstrate the feasibility of accurately resolving stress fields from short-term array monitoring, even in the absence of major earthquakes, highlighting the potential for detailed exploration of the stress field in tectonically complex regions.