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).