Detection of Spatial and Temporal Stress Changes During the 2016 Central Italy Seismic Sequence by Monitoring the Evolution of the Energy Index.

We consider approximately 23,000 microearthquakes that occurred between 2005 and 2016 in central Italy to investigate the crustal strength before and after the three largest earthquakes of the 2016 seismic sequence (i.e., the Mw 6.2, 24 August 2016 Amatrice, the Mw 6.1, 26 October 2016 Visso, and the Mw 6.5, 30 October 2016 Norcia earthquakes). We monitor the spatiotemporal deviations of observed radiated energy, ES, with respect to theoretical values, ESt, derived from a scaling model between ES and M0 calibrated for background seismicity in central Italy. These deviations, defined here as Energy Index (EI), allow us to identify in the years following the Mw 6.1, 2009 L'Aquila earthquake a progressive evolution of the dynamic properties of microearthquakes and the existence of high EI patches close to the Amatrice earthquake hypocenter. We show the existence of a crustal volume with high EI even before the Mw 6.5 Norcia earthquake. Our results agree with the previously suggested hypothesis that the Norcia earthquake nucleated at the boundary of a large patch, highly stressed by the two previous mainshocks of the sequence. We highlight the mainshocks interaction both in terms of EI and of the mean loading shear stress associated to microearthquakes occurring within the crustal volumes comprising the mainshock hypocenters. Our study shows that the dynamic characteristics of microearthquakes can be exploited as beacons of stress change in the crust and thus be exploited to monitor the seismic hazard of a region and help to intercept the preparation phase of large earthquakes. Plain Language Summary: Modern seismic networks provide large volumes of data, thereby allowing us to track the changing properties of the Earth's crust. Here, we analyze ∼23,000 microearthquakes related to the 2016 seismic sequence in central Italy. By comparing the seismic moment, which measures an earthquake's size, and the radiated energy, which measures the dynamic and frictional properties of the rupture, we can characterize the stress acting during the rupture process. We use a parameter that, for a given earthquake size, measures the difference between the theoretically expected radiated energy and the experimentally observed value. Hence, we study how such parameter changes over time and space. We identify areas characterized by high stress around which the Mw 6.2 Amatrice and Mw 6.5 Norcia earthquakes have nucleated. We also show that when one of these large earthquakes occurred, the crustal volume around the other was affected by a significant microseismic activity, suggesting that the two earthquakes interacted. Our study shows that by analyzing the characteristics and spatiotemporal variability of microearthquake source parameters, we can better understand how large earthquakes are generated and how they affect the host rock. Key Points: ∼23,000 microearthquakes of the 2016 central Italy seismic sequence are characterized in terms of their dynamic rupture propertiesDeviations of radiated energy versus seismic moment from a reference model highlight high stress areas close to future mainshock hypocentersMicroearthquakes provide clues for an interplay between the Mw 6.2 Amatrice and Mw 6.5 Norcia earthquakes [ABSTRACT FROM AUTHOR]