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).