Search

Abstract High-hazard seismic zones can remain silent over centuries with meager seismicity rates challenging our understanding of seismic processes. We focus on the comprehensive analysis of cascading episodes of swarms and seismic sequences following the 2009 L’Aquila mainshock (MW 6.3) in the southern-central Apennine that previously experienced ~ M7 earthquakes. We enhance the seismic catalog, unmasking low-magnitude seismicity down to completeness magnitude ML ~ 0, and we unveil that the microseismicity might be secondarily triggered by the L’Aquila mainshock, influencing the frictional properties in the nearby fault zones or opening fault valves generating the intense seismic activity detected from 2009 to 2013. The diffusivity, observed in the most seismic episodes, and the high Vp/Vs values (> 1.88) indicate fluid circulation promoting multilayered extensional seismicity within 11–15 km and 16–23 km depth ranges. Mapping the 3D distribution of seismicity alongside geological data reveals an evident tectonic influence, unveiling unknown geometric aspects and providing the first evidence of a NNE-dipping deformation zone bounding at depths of 11–15 km the overlying fault system. Deeper seismicity suggests a mantellic CO2 ascending shape. These findings enrich the literature on tectonic seismic swarms in extensional domains, providing essential constraints on fluid involvement in the seismic processes and contributing to forthcoming discussions on the seismotectonic setting in high-seismic-risk areas of the Apennines of Italy.

AbstractIt is an open question whether machine-learning (ML) methods can be trusted in areas where dense and localized seismic networks are in operation, and prompt and accurate detection and location of earthquakes are essential to guide decision-making processes that contribute to seismic-risk-mitigation-strategies, even for very low-magnitude events. To address these concerns, we compare the performance of a widely-used ML phase picker, PhaseNet, integrated with several popular earthquake location methods (included in LOC-FLOW), with the results obtained by the workflow adopted since 2012 by the Collalto Seismic Network, installed to monitor natural and potentially induced microearthquakes nearby an underground gas storage. The tested dataset concerns the most populated microseismic sequence observed so far (374 events, ML⩽2.5, August 2021, Refrontolo, NE-Italy), as its unusual productivity raised some criticalities in the combination of automatic routines, and time-consuming manual revision of phase picks adopted by the standard workflow. LOC-FLOW is able to detect the majority of the events listed in the manually revised catalog, demonstrating its ability to efficiently and accurately build earthquake catalogs from continuous seismic data. We highlight both the advantages and limitations of the ML-picker and recommend the use of template-matching-techniques in the final stage of processing to increase the number of events.

In August 2021, an extremely productive seismic sequence took place in northeastern Italy. Within 1 month, we identified and localized 407 microearthquakes in the area monitored by the local Collalto Seismic Network. This is about 20 times the average monthly rate of events detected in the last decade; 92% of them are clustered near the village of Refrontolo at about 9 km depth (main event a ML2.5, MW2.4). None of the Refrontolo sequence earthquakes were felt by the population. The earthquakes, mostly identified by automatic procedures, were post-processed by manual picking of P, S-phases and polarities; they were localized by different techniques and define a small SE dipping volume that departs from the brittle surface previously enlightened by microseisms and assigned to the NNW dipping Montello Thrust. We interpret the sequence ruptured pre-stressed patches near to failure of sub-vertical, antithetically oriented faults. The unusual productivity rate and precursory foreshock activity support the hypothesis that the Montello system has a relevant aseismic component. We believe that this episode, of no importance from an energetic point of view, is of greatest interest for deciphering the seismic potential of this area and for strengthening automatic microearthquake detection and location procedures.