3.11 Adak section
The Adak section extends ~315 km from Tagalak Island to
Amchitka Pass (Figure 5) and is the westernmost portion of the 1957
rupture and the location of the highest values of coseismic slip
(Johnson & Satake, 1993). The Adak section also encompasses the main
slip areas of two notable Mw 7.9 aftershocks of the 1957
rupture in 1986 and 1996 (Boyd & Nábělek, 1988; Tanioka & Gonzalez,
1998; Tape & Lomax, 2022) (Figure 1).
From the Adak section westward, a substantial component of arc-parallel
motion is observed in interseismic velocities at island Global
Navigation Satellite Systems (GNSS) sites (Cross & Freymueller, 2008).
These motions reflect oblique convergence between the Pacific and North
America plates and the transition of the subduction margin to a
composite transform-convergent plate boundary (Ryan & Coleman, 1992)
resulting in translations of arc slivers along strike-slip faults and
block rotations in the overriding plate (Geist et al., 1988; Avé
Lallemant, 1996).
Because plate convergence become increasingly oblique in the west AASZ,
we report plate convergence rates (Table 2) that reflect observed
Pacific-Arc trench-perpendicular rates, which account for translation of
arc slivers relative to the Bering plate (Cross & Freymueller, 2008).
Geodetic observations in the Adak section from Adak Island can be
modeled with 100% coupling of a small portion of the interface, while
those from the Delarof Islands indicate less updip coupling in the
western part of the section (Cross & Freymueller, 2008). Because of the
historical seismicity in the region, we give priority to the Adak Island
observations and infer 100% coupling extending to ~30
km on the plate interface (Figure 5).