4.3 3-Month AfterslipOnly Models with Different Fault Width
Assumed
Because even the viscoelastic models with maximum displacement have
relatively small amplitudes compared to the data, it is useful to start
by considering afterslip-only models. The models with the minimum
potential viscoelastic relaxation contribution subtracted are equivalent
to aftersliponly models, because the viscoelastic relaxation
contribution for that viscoelastic model is negligible.
The first important question to address for the afterslip-only models is
where the afterslip allowed to occur The edge of the west portion of the
Chignik earthquake rupture plane is adjacent to the coseismic rupture
area of the Mw7.8 Simeonof earthquake. If that part of the fault has
velocity-weakening friction, there should be no afterslip allowed within
the coseismic rupture region of the S earthquake. However, we find that
allowing afterslip there or not does not significantly affect our
predictions the sites outside of the Shumagin Islands (Figure S6). Thus,
we first considered three scenarios of afterslip forward models:
allowing the afterslip to occur up-dip only, down-dip only, and fully
surrounding the coseismic rupture zone, using the coseismic model of
Elliott et al. (2022), and we focus on the predicted displacements on
the Alaska Peninsula. None of these models predict the displacement of
site AC13 well, although the Ye et al. (2022) model with its large patch
of shallow slip very close to AC13 does fairly well
Figure S2a shows that the downdip-only afterslip model can easily
explain the displacements along the Alaska Peninsula. Downdip afterslip
contributes almost nothing to the signal at AC13, which must be
explained by some combination of updip afterslip and perhaps poroelastic
relaxation. Figure S2b shows that the updip-only afterslip model
predicts displacements at Peninsula sites AB13 and AC40 that are much
smaller than the observations, because the total stress change is not
able to generate enough afterslip to match the observed displacements
there. Thus, for simplicity our preferred model is that afterslip is
allowed to fully surround the coseismic rupture zone. Changing the updip
frictional parameters would have only a minimal impact on our model
predictions, except at AC13.
We then use the coseismic rupture models with different assumed fault
widths that we obtained from section 4.1 as input for a series of
afterslip-only models. We find that using the coseismic model with a
fault width of 10km significantly reduces the azimuthal misfit of the
two peninsula sites AB13 and AC40 with the best fit model having a
minimum reduced \(\chi^{2}\) of 16.27 when considering all data (Figure
8). The same model has the minimum misfit whether we consider only the
Peninsula sites or include the Shumagin sites as well. A slightly
narrower model (110 km width) minimizes the angular misfit of the
displacements for AB13 and AC21. The frictional parameters we find for
this preferred model are a \(\sigma\) =0.60Ma, \(V_{0}\) =
0.8m/yr. Figure shows the best fit afterslip only models for coseismic
models with different fault widths. The postseismic fit is hugely
improved by reducing the fault widths, so that afterslip occurs further
offshore (Figure ). This indicates that the postseismic give new
constraints to the coseismic slip model.