3.9 Fox Islands section
The 1957 Mw 8.6 rupture spanned ~1,230
km and three of the sections defined here (Fox Islands, Andreanof, and
Adak) (Johnson & Satake, 1993; Tape & Lomax, 2022). The
~425 km Fox Islands section (Figure 5) was the location
of the easternmost extent of slip in 1957 modeled by Johnson & Satake
(1993), although the amount and location of slip based on the
teleseismic data is uncertain, and the depiction of the easternmost
portion of the 1957 rupture varies substantially among studies (McCann
et al., 1979; Tape & Lomax, 2022).Tsunami models of Nicolsky et al.
(2016) show that shallow (5–15 km) rupture in the Fox Islands section
in 1957 most closely reproduces the 1957 Dutch Harbor tide gage
observations and nearby >18 m runup at Sedanka Island.
The geologic record of interface ruptures is inferred from paleotsunami
data at sites on Umnak and Sedanka Islands (Witter et al., 2016, 2019).
The two sites record four previous tsunamis similar to the 1957 event,
implying a 164- to 257-year recurrence interval of tsunamigenic ruptures
(Table 1) (Witter et al., 2019). The five tsunamis interpreted as coeval
at the Umnak and Sedanka Islands are remarkable for their water height
(up to 15-23 m above modern sea level) and inundation, and are
interpreted as representing subduction interface ruptures similar in
magnitude to 1957 of Mw 8.6 or larger. An important
finding of the geologic studies on the Fox Islands is that similar to
1957, past ruptures appear to have crossed an apparent transition
between regions with variable coupling (Witter et al., 2019). This
indicates that our assumption that coupling is constant for hazard
purposes may be an oversimplification. However, no geodetic data
constrain time-varying coupling in the region.
Similar to other locations with observations far from the deformation
front, geodetic data in the Fox Islands section can be fit by multiple
models, including those that place complete or nearly complete locking
over a narrow patch close to the trench (Xue & Freymueller, 2020) or
lower values of coupling on a deeper patch (Cross & Freymueller, 2008).
Because Nicolsky et al. (2016) found the best model fit to the 1957
tsunami required predominantly shallow slip on the interface, we draw
primarily from Xue and Freymueller (2020) and model 93% coupling
extending ~40 km arcward from the deformation front,
corresponding to a depth of ~15 km along the subduction
interface (Figure 5).