Revealing a complex blind fault structure by the analysis of 2019-2020 San Leucio del Sannio seismic sequence (Southern Italy)

Investigating the seismicity of a fault system and its geometric and kinematic characteristics is of paramount importance for mitigating the associated seismic risk. In particular, the characterization of microseismicity can both reveal the presence of unknown or blind fault segments and provide important insights into the evolution of seismicity during the occurrence of a strong seismic sequence.

In this work, we have studied a seismic sequence that recently occurred near Benevento (southern Italy), characterized by a main earthquake of magnitude 3.8, which took place in a region with the highest seismic hazard in Italy. This sequence, characterized by significant activity between November 2019 and January 2020, allowed the identification of a previously unknown fault segment. By installing eight 3-C velocimeter stations in a 12 km radius around the epicenter and applying a template matching technique, we have been able to detect a significant number of events that allowed us to generate an enhanced catalog as compared to those provided by the national and local permanent networks. The augmented catalog consists of hundreds of earthquakes with a minimum magnitude of -0.9. Earthquake relocations of the seismic sequence were achieved by computing differential P- and S-wave travel times and by using a double-difference algorithm.

Our results, combined with the estimation of focal mechanisms for the strongest earthquakes, allow us to identify a fine-scale fault structure consisting of several small segments with strike-slip kinematics between 10 and 15 km depth. Integrating these results with the calculation of source parameters and the analysis of the spatio-temporal distribution of the sequence will enhance our understanding of the mechanical and kinematic characteristics of this complex fault structure. Moreover, the approach followed in our work holds significant potential for analyzing microseismicity and defining complex fault geometries in high seismic risk regions.