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).