Event-specific ground motion anomalies highlight the preparatory phase of earthquakes during the 2016-2017 Italian seismicity

Although physical models are improving our understanding of the crustal processes that lead to large earthquakes, observing their preparatory phases is still challenging. We show that the spatio-temporal evolution of the ground motion of small magnitude earthquakes can shed light on the preparatory phase of three main earthquakes that occurred in central Italy between 2016 and 2017. We analyze systematic deviations of peak ground accelerations generated by each earthquake from the values predicted by a reference ground motion model calibrated for background seismicity and refer to such deviations as event-specific ground motion anomalies (eGMAs). The eGMA temporal behavior indicates that during the activation phase of the main earthquakes, the ground shaking level deviates, positively or negatively, from the values expected for the background seismicity. eGMA can be exploited as beacons of stress change and help to monitor the mechanical state of the crust and the nucleation of large earthquakes. The spatio-temporal properties of ground shaking and foreshocks are distinguishable from typical background seismicity using an approach that can pinpoint crustal stress changes before the 2016-2017 seismic sequence in central Italy.