Brune stress drop, fault slip distribution and high frequency radiation during the 2016-2018 Amatrice-Visso-Norcia seismic sequence

The predominantly normal-faulting sequence of earthquakes in the Appenines of central Italy during 2016-2018 included nine Mw > 5 earthquakes. Multiple researchers (e.g., Bindi et al., 2020; Morasca et al., 2019) have estimated Brune-style stress drops for earthquakes in this sequence, using different methods, providing an opportunity to compare systematic and random variability. In addition, Tinti et al. (2016) and Chiaraluce et al. (2017) calculated finite fault inversions for the three largest earthquakes: 24 August (Amatrice, Mw 6), 26 August (Visso, Mw 5.9) and 30 October (Norcia, Mw 6.3). Thus, we can compare estimates of Brune stress drop with the slip distribution (and its predicted stress release), including relative estimates for the similar-sized Amatrice and Visso earthquakes. We use two different approaches to calculate the source spectra, corner frequency, and Brune stress drop of 30 of the larger events. One involves correcting the observed amplitudes for independently calculated path and site terms. The other uses the empirical Green’s function (EGF) approach, in which the spectral ratio between target and EGF events yields the source spectrum of the target event. We compare our results with those from previous published studies using spectral approaches to calculate the Brune stress drop. The inter-study variation is significantly larger than the standard errors produced by any single study, and the (apparent?) magnitude dependence varies. All approaches find that the M6 Amatrice earthquake had a higher stress drop than the similar-sized M5.9 Visso earthquake. In contrast, the finite fault inversions of these two earthquakes (Chiaraluce et al., 2017) found that the Visso earthquake had the larger region of concentrated, higher slip, whereas the Amatrice earthquake had multiple, lower slip, sub-events. The spectra of the moment-rate functions are consistent with the spectral analyses; the spectrum of the Amatrice earthquake contains more high frequency energy than that of the Visso earthquake. This comparison of two similar-sized earthquakes suggests that consistent measurement of a higher corner frequency and Brune stress drop indicates greater high-frequency ground motion, but may correspond to greater rupture complexity rather than higher stress drop (and higher slip)