Temporal Variability of Ground Shaking and Stress Drop in Central Italy: A Hint for Fault Healing?

Ground-motion prediction equations (GMPEs) are calibrated to predict the intensity of ground shaking at any given location, based on earthquake magnitude, source-to-site distance, local soil amplifications, and other parameters. GMPEs are generally assumed to be independent of time; however, evidence is increasing that large earthquakes modify the shallow soil conditions and those of the fault zone for months or years. These changes may affect the intensity of shaking and result in time-dependent effects that can potentially be resolved by analyzing between-event residuals (residuals between observed and predicted ground motion for individual earthquakes averaged over all stations). Here, we analyze a data set of about 65,000 recordings for about 1400 earthquakes in the moment magnitude range 2.5-6.5 that occuned in central Italy from 2008 to 2017 to capture the temporal variability of the ground shaking at high frequency. We first compute betweenevent residuals for each earthquake in the Fourier domain with respect to a GMPE developed ad hoc for the analyzed data set. The between-events show large changes after the occurrence of mainshocks such as the 2009 M-w 6.3 L'Aquila, the 2016 M-w 6.2 Amatrice, and M-w 6.5 Norcia earthquakes. Within the time span of a few months after the mainshocks, the between-event contribution to the ground shaking varies by a factor 7. In particular, we find a large drop in the between-events in the aftermath of the L'Aquila earthquake, followed by a slow positive trend that leads to a recovery interrupted by a new drop at the beginning of 2014. We also quantify the frequency-dependent correlation between the Brune stress drop Delta sigma and the between-events. We find that the temporal changes of Delta sigma resemble those of the between-event residuals; in particular, during the period when the between-events show the positive trend, the average logarithm of Delta sigma increases with an annual rate of 0.19 (i.e., the amplification factor Delta sigma for is 1.56 per year). Breakpoint analysis located a change in the linear trend coefficients of Delta sigma versus time in February 2014, although no large earthquakes occuned at that time. Finally, the temporal variability of Delta sigma mirrors the relative seismic-velocity variations observed in previous studies for the same area and period, suggesting that both crack healing along the main fault system and healing of microcracks distributed at shallow depths throughout the surrounding region might be necessary to explain the wider observations of postearthquake recovery.