3.1 Yakataga section
The Yakataga section (Figure 3) extends ~ 150 km (measured along the deformation front) from Yakutat in the east to Cape Yakataga in the west. Although not a classic subduction interface, this section encompasses the easternmost end of AASZ subduction-related deformation where the Yakutat microplate is colliding obliquely with North America (Eberhart-Phillips et al., 2006) and models of geodetic strain allow for strain accumulation along the Yakutat plate interface (Elliott et al., 2013). Based on velocity models from crustal reflection and refraction studies, Worthington et al. (2012) interpret flat-slab subduction of the Yakutat terrane extending beyond the termination of the AASZ trench, from Kayak to Yakutat Bay. The Mw 7.2 St. Elias earthquake may have ruptured a portion of a low-angle plate interface in this section (Estabrook et al., 1992). The western edge of this section is defined as the approximate location of a tear in subducting Yakutat crust interpreted from geophysical data and seismicity (Daly et al., 2021; Mann et al., 2022). The deformation front here is interpreted as coinciding with the Foreland Thrust system (Malaspina, Pamplona, and Esker Creek sections) (Worthington et al., 2008). The Wesson et al. (2007) model included a similar section (their Yakutat segment) to account for the possibility of a low-angle plate interface beneath the Yakataga fold-and-thrust belt.
It is unclear if coastal paleoseismic observations in the Yakataga section record slip on the Yakutat plate interface. Shennan et al. (2009, 2014) report paleogeodetic evidence for earthquakes at Yakutat Bay from the integration of marsh and landscape uplift at multiple sites spanning the inferred eastern edge of the 1964 rupture. Shennan et al. (2009) infer that the Yakataga section ruptured with the neighboring Prince William sound sections in ~870 BP and ~1440 BP, while also recognizing potential complications from upper plate faults such as the two > Mw 8.0 Yakataga ruptures in September 1899. It is difficult to calculate a recurrence interval from only the ~870 BP and ~1440 BP events alone for which there is a single closed interval of 570 years, and so we estimate a ≥ Mw 8.5 open-interval recurrence of ~757 ± 264 years.
Geodetic observations in the Yakataga section indicate a strong gradient in velocities between the coast and the Wrangell Mountains (Elliott & Freymueller, 2020; Elliott et al., 2013). Geodetic models place most of the strain in this area on upper-plate faults, except for a small patch of strain accumulation on the Yakutat décollement. In keeping with this interpretation, we depict a relatively small patch of coupled interface (~140 × 30 km polygon) far landward from the deformation front (30-75 km distance) that incorporates subsections of the Yakutat low-angle plate interface modeled by Elliott et al. (2013) and Elliott and Freymueller (2020), to which we assign 30% coupling (Figure 3, Table 2). Further work in this region would be beneficial to image coupling along the Yakutat plate interface and the interplay between upper plate and plate interface strain accumulation.