3.6 Semidi section
The Semidi section (Figure 4) extends ~250 km from
approximately the southwestern edge of the 1964 CE rupture near Sitkinak
Island to ~70 km east of the Shumagin Islands (Nishenko
& Jacob, 1990). The Semidi section is differentiated from the
neighboring Kodiak and Shumagin sections based on historical and
paleoseismic earthquake history: this portion of the AASZ hosted great
historical ruptures in 1938 (Mw 8.3) and 2021
(Mw 8.2) (Elliott et al., 2022; Freymueller et al.,
2021), although these appear to be much smaller than the 1788 rupture
from paleoseismic records (Briggs et al., 2014; Nelson et al., 2015). We
draw the boundary between the Semidi and the adjacent Shumagin segment
based on the presumed western edge of the 2021 Mw 8.2
Chignik earthquake (Elliott et al., 2022), which also corresponded to a
segment boundary in the interseismic model of Drooff and Freymueller
(2021). von Huene et al. (1999) infer that the Patton-Murray hot spot
swell may influence rupture character of the Semidi section, which
exhibits much higher interseismic coupling than the Shumagin section to
the southwest; alternatively, von Huene et al. (2012) also argue that
the subduction of the head of the Zodiac fan may influence rupture
behavior in this section. The Semidi section was recognized as a
potential source for a Pacific basin-wide tsunami with risk implications
for the western coast of the United States (Ross et al., 2013).
The geologic record of subduction earthquakes for the Semidi section is
derived from geologic studies on Chirikof and Sitkinak Islands. On
Chirikof Island, stratigraphic evidence of 13 paleotsunamis since
~3.5 ka, including the major historical1788 rupture,
corresponds to a paleotsunami recurrence of 180-270 years (Nelson et
al., 2015). Sitkinak is at the westernmost edge of the neighboring
Kodiak section, and so is not strictly in the Semidi section; however,
marshes at Sitkinak record a series of land level changes that we infer
record ruptures of the Semidi section. Five land-level changes at
Sitkinak (Briggs et al., 2014), between ~1050 BP and
1788 CE indicate a recurrence interval of ~222 years, in
agreement with the paleotsunami record from Chirikof (Table 1).
Geodetic observations in the Semidi section consistently show a highly
coupled region with a lower coupling toward the Shumagin islands to the
west (Drooff & Freymueller, 2021; Li & Freymueller, 2018). Li &
Freymueller (2018) estimated strong coupling in their ‘Semidi segment’
(~70%), with much lower coupling to the west in their
‘Shumagin segment’ (~40%). However, few data were used
to constrain the location of the boundary, so the location was quite
uncertain. Drooff and Freymueller (2021) revised the segment boundaries
of Li and Freymueller (2018), incorporating additional data from
Veniaminof volcano on the Alaska Peninsula, which had been excluded in
the earlier study due to the presence of volcanic deformation. Drooff
and Freymueller (2021) shifted the western boundary of the Semidi
section to the east, leaving strong coupling in their segment 2, and
broke the Shumagin region into two segments (their segments 3 and 4).
Our location of the Semidi-Shumagin boundary corresponds to the boundary
between the Drooff and Freymueller (2021) segments 2 and 3, which also
corresponds closely to the southwestern edge of the 2021 rupture. For
hazard estimates, we represent 70% coupling ~125 km
from the deformation front, corresponding to a locking depth on the
Slab2 interface of ~20 km (Figure 4). The slip deficit
at shallow depth near the trench is highly uncertain due to poor model
resolution, and depends strongly on the assumed model regularization
(Xiao et al., 2021), but the total integrated moment accumulation rate
does not vary much even where the appearance of the slip deficit
distribution with depth varies a lot. Because we lack concrete
information about whether the shallow part of the interface is locked or
creeping, we adopt the estimates based on models that assume locking to
the trench.