Temporal and spatial evolution of radiated energy to seismic moment scaling during the preparatory phase of the Mw 6.1, 2009 L’Aquila earthquake (Italy) and the 2016 Central Italy Seismic Sequence.

We consider approximately 23,000 microearthquakes 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 spatio-temporal deviations of the observed radiated energy, E_S, with respect to theoretical values, E_St, derived from a scaling model between E_S and M₀ calibrated for background seismicity in central Italy. These deviations, defined here as Energy Index (EI), allow us to identify the onset of the activation phase one week before the mainshock. We show that foreshocks are characterized by a progressive increase in slip per unit stress, in agreement with the diffusion of highly pressurized fluids before the L’Aquila earthquake proposed by previous studies. Our results suggest that the largest events occur where EI is highest, in agreement with the existing link between EI and the mean loading stress.

Furthermore, our results show a progressive evolution of the dynamic properties of microearthquakes in the years following the Mw 6.1, 2009 L’Aquila earthquake, 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 seen 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.