3.2 Prince William Sound section
The Prince William Sound section (Figure 3) extends ~320
km between Bering Glacier and Seward and encompasses the region of
maximum slip in 1964 (Ichinose et al., 2007; Suito & Freymueller,
2009). The plate interface is shallow here (~6°) (Hayes
et al., 2018; Worthington et al., 2012) reflecting subduction of the
relatively buoyant Yakutat microplate with the Pacific Plate
(Eberhart-Phillips et al., 2006) and leading to a wide seismogenic zone.
Deep (60-80 km) nonvolcanic tremor located downdip from the 1964 rupture
implies show, persistent slip along the interface between the Yakutat
microplate and North America (Wech, 2016).
The geologic record of subduction earthquakes along the Prince William
Sound section is well-established relative to the rest of the AASZ after
many decades of paleoseismic studies (Carver & Plafker, 2008; Hamilton
& Shennan, 2005a, 2005b; George Plafker et al., 1992). Marsh
stratigraphy and geomorphologic studies show evidence for between seven
(Shennan et al., 2014) and nine (Carver & Plafker, 2008) subduction
ruptures along the Prince William Sound section since ~4
to ~5 ka. Recurrence intervals between the seven
youngest ruptures range from ~420 to 880 years, with the
mean recurrence interval between the most recent six earthquakes of 594
-15/+18 years reported by Shennan et al. (2014) (Table 1).
Geodetic observations in the Prince William Sound section document a
major slip patch in 1964 and a complex mix of subsequent postseismic and
interseismic motions (Cohen & Freymueller, 1997; Li et al., 2016; Suito
& Freymueller, 2009; Elliott & Freymueller, 2020). Strong coupling is
attributed to shallow dip and geometric complexity at the easternmost
end of the AASZ (Christensen & Beck, 1994). We simplify the available
geodetic models and assume 100% interseismic coupling extending over
300 km inland from the trench, corresponding to a downdip limit of
approximately 30 km depth (Li et al., 2016) (Figure 3). The depth and
lateral extent of the segment are defined primarily from Li et al.
(2016), which identified the presence of multi-year slow slip events in
the Cook Inlet region. The Li et al. (2016) study distinguished between
regions that have been persistently locked over the entire time span of
geodetic observations, and regions of the interface that have
accumulated slip deficit over certain time intervals and then released
it in multi-year slow slip events. Other studies, such as Elliott &
Freymueller (2020), used a single set of velocities and thus represent
an average between the slow slip and non-slow slip intervals. The
Elliott & Freymueller (2020) model estimated a more detailed upper
plate block model than that assumed in Li et al. (2016), but this model
also excluded some data from western Prince William Sound that was hard
to fit. Some of the sites in southwest Prince William Sound move at
nearly Pacific plate velocity, and all models persistently underestimate
these observations unless the slip deficit rate is allowed to exceed the
plate convergence rate. For example, Savage et al. (1998) used a plate
convergence rate of 65 mm/yr, almost 20% too high, to model the
observed velocities of a profile in western Prince William Sound (that
study also did not account for postseismic deformation). In part due to
the exclusion of some of the data from this profile, the Elliott &
Freymueller (2020) model estimates a lower average slip deficit rate
within parts of the Prince William Sound section than other studies did,
but these variations all lie within the region of the massive Prince
William Sound asperity as defined by coseismic slip models (e.g.,
Ichinose et al., 2007; Suito & Freymueller, 2009). Rather than
subdivide the section further, we average the slip deficit over the
whole polygon.
There is a discrepancy between the geodetic slip deficit rate and the
observed geological recurrence rate for great earthquakes in Prince
William Sound, as noted and discussed by Freymueller et al. (2008). The
geologic recurrence interval (Table 1) is estimated to be
594±~20 years (Carver & Plafker, 2008; Shennan et al.,
2014), but given the observed plate convergence rate and 100% coupling
coefficient required to fit the interseismic geodetic velocities, the
geodetic estimate for Mw 8.85 to 9.05 earthquakes is
only ~200-300 years, depending on assumptions. In short,
a fully locked plate interface, which is clearly needed to fit the
interseismic geodetic observations, would result in even more frequent
great earthquakes than observed; Reducing the coupling coefficient to
~50%, to match the geologic recurrence rate, would
produce an enormous misfit to the geodetic data. The Elliott &
Freymueller (2020) model includes significant permanent shortening of
the upper plate, with the crustal block in Prince William Sound moving
rapidly northward, and implying significant permanent contraction within
the Chugach Mountains. However, that study excluded some of the data
from SW Prince William Sound that were difficult to fit with any model
(as noted above, those data were fit in earlier studies by allowing a
slip deficit rate that exceeded the rate of plate motion). Some
combination of permanent northward motion of the crustal block(s) in
Prince William Sound (Elliott & Freymueller, 2020), a reduced incoming
plate rate if the subducting crust is Yakutat Block rather than Pacific
plate (Freymueller et al., 2008), or perhaps the occurrence of slow slip
events to shallower depth than yet observed, or additional slip in
M<~8.5 earthquakes that would be invisible in
the geologic record would be required to explain the discrepancy. Future
work would be necessary to fully explain the apparent mismatch between
the geodetic and geologic record along the Prince William Sound section.