The Irpinia region is one of the most seismically active areas of Italy due to ongoing, late-orogenic extension in the axial zone of the Apennines mountain belt. However, the 3D architecture and the nature of the faults that drive this extension are still uncertain, posing challenges to seismic hazard assessment. Here, we address these uncertainties by integrating a new catalog of high-resolution micro-seismicity (ML<3.5) complemented by earthquake focal mechanisms, with existing 3D seismic velocity models and geological data. We found that micro-seismicity is primarily taking place along a segmented, approximately 60 km long, deep-seated, Mesozoic normal fault that was inverted during the shortening stages of the Apennine orogeny and then extensionally reactivated during the Quaternary. These findings suggest that multiple events of reactivation of long-lived faults can weaken their strength, making them prone to co-seismic remobilization under newly-imposed strain fields in active mountain belts.

Neglecting fault segmentation in hazard assessments leads to underestimated potential hazard. Moreover, integrating the temporal evolution of fault segments activations in hazard assessment improve scenario’s reliability. In this view, enhanced seismic catalogs have potential in revealing previously neglected fault complexities. Past efforts were restricted to the 2D view analysis without involving the segment temporal activation. Our work provides a comprehensive approach, reconstructing 3D fault fine-scale geometry and segments activation evolution. We analyzed the 2014 Northern Nagano (Japan) (Mw 6.2) earthquake sequence using high-resolution seismic catalogs. We automatically detected and located about 2500 events between October and December 2014. We refined the automatic picks, based on cross-correlation and hierarchical clustering, and we relocated the hypocenters with the double-difference in 3D velocity models optimized for the area. Moreover, we calculated the composite focal mechanisms of the main clusters, crucial to constrain the 3D geometry of the fault segments, and rupture directivity that we interpreted jointly with the seismicity and the fault slip. We found that the multi-segmented fault system, is comprised of, at least, 9 distinct segments, that ruptured during 3 successive phases. Different segments exhibit a different rupture mechanism based on their spatial and temporal occurrence, influencing seismicity evolution and rupture length. The presented analysis can be used to improve the reliability of probabilistic hazard assessment in the high seismic potential area of the Itoigawa-Shizuoka fault system. The possibility of fault segment interaction and mutual triggering processes should be considered when drawing reliable seismic hazard scenarios.

A primary task of a network-based, earthquake early warning system is the prompt event detection and location, needed to assess the magnitude of the event and its potential damage through the predicted peak ground shaking amplitude using empirical attenuation relationships. Most of real-time, automatic earthquake location methods ground on the progressive measurement of the first P-wave arrival time at stations located at increasing distances from the source but recent approaches showed the feasibility to improve the accuracy and rapidity of the earthquake location by using the additional information carried by the P-wave polarization or amplitude, especially unfavorable seismic network lay-outs. Here we propose an evolutionary, Bayesian method for the real-time earthquake location which combines the information derived from the differential P-wave arrival times, amplitude ratios and back-azimuths measured at a minimum of two stations. As more distant stations record the P-wave the posterior pdf is updated and new earthquake location parameters are determined along with their uncertainty. To validate the location method we performed a retrospective analysis of mainshocks (M>4.5) occurred during the 2016-2017 Central Italy earthquake sequence by simulating the typical acquisition layouts of in-land, coastal and linear array of stations. Results show that with the combined use of the three parameters, 2-4 sec after the first P-wave detection, the method converges to stable and accurate determinations of epicentral coordinates and depth even with a non-optimal coverage of stations. The proposed methodology can be generalized and adapted to the off-line analysis of seismic records collected by standard local networks.