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 AveĢ 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).