Text S3. Methods for estimating vertical surface displacements from megathrust
slip
To explore fault slip scenarios that would produce a detectable vertical signal on
Simeonof Island, we use a three-dimensional model of the Alaska-Aleutian slab geometry
(Hayes et al. 2012) meshed with triangular dislocations in an elastic half space [Meade,
2007] in the modeling environment CUBIT (Sandia National Laboratories, 2013). We
impose continuous, uniform slip on the slab to produce forward models of vertical motion
on a trench-perpendicular profile that intersects Simeonof Island. We assess several
locking depths (20–40 km) and assume slip propagates to the trench, the condition most
necessary to produce a tsunami. We also explore incremental rupture depths between 1535 km, the potentially seismogenic portion of the megathrust directly beneath and
adjacent to Simeonof. We then tune the magnitude of slip until ± 30 cm (assumed to be
the resolution of geologic proxies) of subsidence or uplift is reached on Simeonof Island.
To calculate a moment magnitude equivalent for each depth range (rupture width)
and slip amount, we assume rupture lengths of 125 km and 280 km centered on Simeonof
Island. These rupture lengths are derived from the Shumagin region locked patches 3 and
4 defined by Fournier and Freymueller [2007] and roughly correspond to the 200–250 km
Shumagin Gap as interpreted from seismic data [e.g. Boyd et al., 1988].
To evaluate whether or not our earthquake scenarios were consistent with GPS
observations, we computed interseismic deformation for each scenario for models with
100% and 30% locking. Models using 100% locking show a variety of horizontal
displacement magnitudes: 4 models have magnitudes higher than the modern GPS
velocity; 4 have magnitudes similar to modern GPS velocities; and 2 have much smaller
magnitudes. All scenarios using the 30% locked model have magnitudes less than half the
magnitude of the modern GPS horizontal vector at Simeonof Island. It appears that 8 out
of the 10 models are consistent with GPS motion at Simeonof if horizontal displacement
for each scenario uses a locking value between 30% and 100%. However, even assuming
100% locking, two of the scenarios (20–35 km and 25–35 km scenarios) have horizontal
displacements that fall short of the magnitude of the modern GPS velocity.
This exercise illuminates two differences between the slip model of Fournier and
Freymueller [2007] and our scenarios. First, the four scenarios with similar magnitudes
to modern GPS require 100% locking, which is substantially higher than the 30% locking
for the eastern Shumagin segment modeled by Fournier and Freymueller [2007]. We note
one of these scenarios involves rupture of the updip part of the megathrust between 20
km and the trench, a region with poor resolution if modeled with GPS instruments in the
Shumagin Islands. Second, four scenarios that rupture to the trench require models with
less than 100% but more than 30% locking, which is moderately higher than the 30%
locking modeled for the eastern Shumagin segment by Fournier and Freymueller [2007].