An unusually productive microearthquake sequence brings new insights to the buried active thrust system of Montello (Southeastern Alps, Northern Italy)

Abstract

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.

The Montello Thrust and the Active Mountain Front of the Eastern Southern Alps (Northeast Italy)

“…RSC is a high‐resolution permanent network, consisting of a cluster of high‐sensitivity stations deployed right above the reservoir, fully operating since 2012 (Priolo et al., 2015). Over the years, RSC has provided a detailed picture of the Montello area background seismicity, so far poorly known, and has demonstrated that the underground storage activities are not related to such microseismic events (Peruzza et al., 2022; Romano et al., 2019). The RSC catalog used contains 1635 earthquakes (−0.8 ≤ M L ≤ 4.5) localized with an optimized 1D velocity model in the period January 2012 to October 2017.…”

“…After the strong seismic events in the Friuli area, east of the Montello hill, in 1976, the eastern southalpine mountain front was explored from a seismotectonic point of view and the seismic sources defined, based on the available geological constraints (e.g., Burrato et al., 2008; Galadini et al., 2005; Peruzza, Poli, et al., 2002; Peruzza, Romano, et al., 2002). Several scholars analyzed its geodetic deformation in order to assess the seismogenic potential of the Montello structure (e.g., Barba et al., 2013; Cheloni et al., 2014; D’Agostino et al., 2005; Danesi et al., 2015; Serpelloni et al., 2016).…”