3.7 Shumagin section
The ~ 220 km Shumagin section (Figure 4) encompasses the Shumagin Islands and was long presumed to be a seismic gap with high potential for hosting future large earthquakes (Davies et al., 1981; McCann et al., 1979; Nishenko & Jacob, 1990). However, subsequent geodetic data in the Shumagin Islands are consistent with a poorly coupled plate interface (Freymueller & Beavan, 1999; Lisowski et al., 1988; Savage et al., 1986) and geologic observations at Simeonof Island do not find evidence for substantial land-level changes or tsunami runup since ~3.4 ka (Witter et al., 2014). Historical ruptures have been relatively small in the context of the largest AASZ earthquakes, including the 1948 Mw 7.5 and 2020 Mw 7.8 events (Estabrook & Boyd, 1992; Ye et al., 2022) (Figure 1).
Li & Freymueller (2018) examined lateral variations of locking in this region, and we generalize their findings as 30% coupling extending ~80 km from the deformation front, corresponding to a locking depth on the interface of ~20 km (Figure 4). Drooff and Freymueller (2021) divided the region of this section into two distinct segments, one with ~40% coupling and one with ~20%. The 2020 Mw 7.8 Shumagin Islands earthquake broke across both of those segments, with higher average slip in the eastern part (Xiao et al., 2021); the boundary between the higher and lower slip parts of the rupture corresponds to the interseismic boundary as defined by Drooff and Freymueller (2021). Our Shumagin section corresponds roughly to the extent of the 2020 Mw 7.8 Shumagin Islands earthquake, so we chose not to subdivide the section further. Averaged over the whole section, the models of Li & Freymueller (2018) and Drooff & Freymueller (2021) give the same results. Because we lack concrete information about whether the shallow part of the interface is locked or creeping, we adopt the estimates based on models that assume locking to the trench (Xiao et al., 2021).