Variability of ETAS Parameters in Global Subduction Zones and Applications to Mainshock–Aftershock Hazard Assessment

Megathrust earthquake sequences can impact buildings and infrastructure due to not only the mainshock but also the triggered aftershocks along the subduction interface and in the overriding crust. To give realistic ranges of aftershock simulations in regions with limited data and to provide time‐dependent seismic hazard information right after a future giant shock, we assess the variability of the epidemic‐type aftershock sequence (ETAS) model parameters in subduction zones that have experienced M≥7.5M≥7.5 earthquakes, comparing estimates from long time windows with those from individual sequences. Our results show that the ETAS parameters are more robust if estimated from a long catalog than from individual sequences, given individual sequences have fewer data including missing early aftershocks. Considering known biases of the parameters (due to model formulation, the isotropic spatial aftershock distribution, and finite size effects of catalogs), we conclude that the variability of the ETAS parameters that we observe from robust estimates is not significant, neither across different subduction‐zone regions nor as a function of maximum observed magnitudes. We also find that ETAS parameters do not change when multiple MM 8.0–9.0 events are included in a region, mainly because an MM 9.0 sequence dominates the number of events in the catalog. Based on the ETAS parameter estimates in the long time period window, we propose a set of ETAS parameters for future MM 9.0 sequences for aftershock hazard assessment (⁠K0=0.04±0.02K0=0.04±0.02⁠, α=2.3α=2.3⁠, c=0.03±0.01c=0.03±0.01⁠, p=1.21±0.08p=1.21±0.08⁠, γ=1.61±0.29γ=1.61±0.29⁠, d=23.48±18.17d=23.48±18.17⁠, and q=1.68±0.55q=1.68±0.55⁠). Synthetic catalogs created with the suggested ETAS parameters show good agreement with three observed MM 9.0 sequences since 1965 (the 2004 MM 9.1 Aceh–Andaman earthquake, the 2010 MM 8.8 Maule earthquake, and the 2011 MM 9.0 Tohoku earthquake).This project (M. J. W.) has received funding from the European Union's Horizon 2020 research and innovation program under Grant Agreement Number 821115, Real‐Time Earthquake Risk Reduction for a Resilient Europe (RISE). Lizhong Zhang and Maximilian J. Werner appreciate support from the London Mathematical Laboratory. This research was also supported by the Southern California Earthquake Center (Contribution Number 9081). The Southern California Earthquake Center (SCEC) is funded by the National Science Foundation (NSF) Cooperative Agreement EAR‐1600087 and the U.S. Geological Survey (USGS) Cooperative Agreement G17AC00047. This work was supported by the Canada Research Chair in Multi‐Hazard Risk Assessment program (950‐232015) for Katsuichiro Goda