Geometry and mechanics of a complex fault system from deep analysis of seismic sequences within the Irpinia Near Fault Observatory

The Irpinia Near Fault Observatory is a dense instrumented infrastructure monitoring the normal fault system of the Irpinia region, in the Southern Apennines (Italy). The area is one of the highest seismic hazard regions in Italy; nevertheless, the background seismicity rate is low, with about 4000 earthquakes detected in the last 15 years of network operation, with a magnitude of completeness of 1.1. Thus, understanding the fault system mechanical state requires high quality data and advanced tools for seismicity location and characterization, along with new monitoring systems, able to catch the earthquake signals at or below the noise level.   In this study, we investigated the geometrical and mechanical properties that can be inferred from the (micro)seismic sequences occurred during the network operation. The seismic catalogs enhanced through the use of machine learning and similarity-based techniques, and double-difference event relocation show that the events define kilometric-scale structures, sub-parallel to the main faults that generated the M 6.9, 1980 Irpinia earthquake. We estimated the stress release and the rupture area of the events within the sequences, showing that the static stress transfer is the main mechanism of triggering of the events within the sequences. The seismicity delineates slip-driven alignments that can be associated with fault roughness. In the case of the major sequence, the event distribution might also indicate the occurrence of an aseismic deformation episode, too small to be detected by surface GNSS instruments.  Finally, we also analyzed the seismicity recorded along the 1-year DETECT experiment, during which 200 velocimetric stations were deployed as a constellation of seismic arrays, within Irpinia Near Fault Observatory region. We discovered that deep seismicity mainly occurs in sequences, with most events having a magnitude smaller than the unity. When jointly analyzed with the 3D P-wave tomographic model, these sequences illuminate a SW-dipping, previously unknown, segmented fault with a total length of more than 30 km. This structure could be causative for a future M6+ event, depending on the rupture ability to overcome the geometrical stepover on the fault.