3.10 Andreanof section
The Andreanof section (Figure 5) extends ~350 km and is centered along the 1,230 km-long aftershock zone of the 1957 rupture (Tape & Lomax, 2022), a region of apparently little to no coseismic slip in 1957 (Johnson & Satake, 1993). The Amlia fracture zone intersects roughly the center of this section and has been hypothesized to have a major influence on upper plate structure and interface coupling (Ryan et al., 2012). Whether the Amlia fracture zone modulated slip in the 1957 earthquake is unclear, but it did not arrest rupture (Sykes, 1971). The Amlia fracture zone roughly corresponds to the eastern edge of the aftershock extent of the 1986 Mw 7.9 Andreanof Islands earthquake (Figure 2), which ruptured mainly the neighboring Adak section (Tape & Lomax, 2022).
At present, no paleoseismic records are available from the Andreanof Islands westward along the Alaska-Aleutian subduction zone, and so recurrence values are estimated from geodetic data alone for these sections. Geodetic observations in the western Aleutians came primarily from a geodetic campaign network started in the 1990s by Avé Lallemant & Oldow (2000) and by the USGS Alaska Volcano Observatory, with results summarized by Freymueller et al. (2008).
Geodetic data from the Andreanof section show that this section is, on average, poorly coupled based on sparse observations (Cross & Freymueller, 2008). We generalize the results of Cross and Freymueller (2008) by modeling 25% coupling along a polygon spanning approximately 20-35 km depth on the interface (Figure 5).