Figure 5. Best fit afterslip only models for published coseismic models
(a) Elliott et al. (2022)model. (b) Liu et al.(2022) model (c) Ye et al.
(2022) model. The error ellipse show 95% confidence. The region of the
rupture areas is > 1m slip. Dashed light grey lines
outlines the depth contours from the Slab2 model (Hayes et al., 2018).
The white barbed line shows the plate boundary between the Pacific plate
and the North American plate.
the goodness of the post-seismic fit, we the utlilize near-field GPS
continuous sites except for site AC13, while focus on three peninsula
sites (AB13, AC21, and AC40), for these three sites have large
displacements and are most sensitive to the downdip distribution of
afterslip. We are not evaluating the fit to site AC13 for two reasons.
One is that it is not sensitive to the downdip afterslip (see the
downdiponly afterslip model, Figure S2 a), and the other is that it is
located at the edge of the rupture area, and its fast seaward motion
might also include a contribution from other post-seismic mechanisms
such as poroelastic rebound, which is beyond the scope of this paper.
The sites the Shumagin islands (AC41, AB07, AC28, and AC12) have
relatively small displacements and might be affected by the postseismic
processes of the July 21, 2020, Mw7.8 Simeonof earthquake, or by
alternative assumptions about the distribution of velocity-strengthening
material, so we will consider multiple misfit metrics to determine the
best model.
The Ye et al. (2022) model differs from the other models in ways. They
assumed a deeper than the Slab2.0 geometry used by others, based on a
seismic reflection study (Kuehn, 2019). Also, their slip model is more
compact in the downdip direction, as a result of an assumption they made
about the maximum possible depth of slip. Additionally, they added a
patch of shallow slip near Chirikof Island in order to better explain
the tsunami. This added slip patch, which was added to the slip model by
those authors after their initial slip inversion, also makes their model
predict the AC13 postseismic data better.
We estimated a new coseismic model by shifting the Elliott et al.,
(2022) model fault plane to be 6.5 km deeper, similar to the Ye et al.
(2022) model geometry, but found that the coseismic slip (Figure S3i)
and post-seismic displacement patterns (Figure S4) did not change much.
However, when we tested a narrower fault model, by restricting the
downdip extent of the model fault plane, we found that the fit to the
postseismic data improved dramatically while the fit to the coseismic
observations was nearly unchanged (Figure S3 b, Figure 6 b). These tests
indicated that it is the narrower downdip width of the slip distribution
that makes the Ye et al. (2022) model be a better of the postseismic
afterslip. A narrower fault model with a more abrupt decrease of the
coseismic slip at the downdip end of the rupture plane results in
afterslip being located farther offshore.
(a) (b)