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.