We consider approximately 32,000 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 spatio-temporal deviations of the scaling between the seismic moment, M0, and the radiated energy, ES, with respect to a model calibrated for background seismicity. 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 the 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 suggested hypothesis that the Norcia earthquake nucleated at the boundary of a large patch that was highly stressed by the two previous mainshocks of the sequence. Furthermore, 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.

Although the non-uniqueness of the solution is commonly mentioned in the context of studies that perform spectral decompositions to separate source and propagation effects, its impact on the interpretation of the results is often overlooked. The purpose of this study is to raise awareness on this important subject for modelers and users of the models and to evaluate the impact of strategies commonly applied to constrain the solution. In the first part, we study the connection between the source-station geometry of an actual data set and the properties of the design matrix that defines the spectral decomposition. We exemplify the analyses by considering a geometry extracted from the data set prepared for the benchmark Community Stress Drop Validation Study (Baltay et al., 2021). In the second part, we analyze two different strategies followed to constrain the solutions. The first strategy assumes a reference site condition where the average site amplification for a set of stations is constrained to values fixed a-priori. The second strategy consists in correcting the decomposed source spectra for unresolved global propagation effects. Using numerical analysis, we evaluate the impact on source scaling relationships of constraining the corner frequency of magnitude 2 events to 30 Hz when the true scaling deviates from this assumption. We show that the assumption can not only shift the overall seismic moment versus corner frequency scaling but can also affect the source parameters of larger events and modify their spectral shape.