Investigating Spectral Estimates of Stress Drop for Small to Moderate Earthquakes With Heterogeneous Slip Distribution: Examples From the 2016-2017 Amatrice Earthquake Sequence

Estimates of spectral stress drop are fundamental to understanding the factors controlling earthquake rupture and high frequency ground motion, but are known to include large, poorly-constrained uncertainties. We use earthquakes from the 2016-2017 sequence in the Italian Appenines (largest event at Norcia, M-w 6.3) to investigate these uncertainties and their causes. The similarly-sized events near Amatrice (M-w 6.0) and Visso (M-w 5.9) enable better constrained relative analysis. We calculate S wave source spectra, corner frequencies, and spectral stress drop for 30 of the larger events. We compare both empirical and modeling approaches to isolate the source spectra and calculate source parameters; we also compare our results with those from published studies. Both random and systematic inter-study variations are larger than the standard errors reported by any individual study. The reported magnitude dependence of stress drop varies between studies, being largest for generalized inversions and smallest for more individual event based approaches. The relative spectral estimates of inter-event stress drop are more consistent; all approaches estimated higher stress drop in the Amatrice earthquake than the similar-sized Visso earthquake. In contrast, finite fault inversions of these two earthquakes found that the Visso earthquake had the larger region of concentrated, higher slip, whereas the Amatrice earthquake had multiple, lower slip, subevents. The Amatrice spectra contain more high frequency energy than those of the Visso earthquake. This comparison suggests that consistent measurement of a higher spectral stress drop indicates greater high-frequency ground motion but may correspond to greater rupture complexity rather than higher stress drop.