6. Conclusion
We generated a suite of coseismic slip models for the 29 July 2021, Mw
8.2 Chignik earthquake by inverting seismic and geodetic data, and
varying the assumed downdip end of coseismic plane. Models with a
narrower allowed downdip width have slip located farther offshore, and
have a more abrupt downdip termination of slip, while models that allow
a wider rupture have slip that tapers to zero more gradually with depth.
For each model, we computed stress driven afterslip models and
viscoelastic relaxation models, and compared the postseismic model
predictions to observed postseismic displacements over the first 3
months after the earthquake. As long as the frictional properties are
uniform, the spatial pattern and relative magnitudes of postseismic
afterslip displacements are determined entirely by the coseismic slip
distribution, while the rate of early postseismic slip and its time
decay depend on the values of the frictional parameters chosen. The
predicted afterslip displacement pattern is significantly different for
the different coseismic models, but the predicted viscoelastic
relaxation deformation is not.
We find that the coseismic data alone cannot resolve the details of slip
downdip end of the rupture, but the postseismic displacement provide
important new information. By limiting the model fault plane width 120km
downdip, the observed post-seismic signal is much better explained with
stress-driven afterslip for all reasonable viscoelastic relaxation
contributions considered. A model with a narrower downdip extent of
slip, and a more abrupt downdip termination of slip, produces afterslip
located farther offshore, and this is necessary to match the
orientations of the observed postseismic displacements. This finding
holds for all reasonable contributions from viscoelastic relaxation,
including models where the viscoelastic signal is negligible. However,
overall data fit is improved when a combination of stress-driven
afterslip and viscoelastic relaxation is included in the model, and this
combination makes it possible to explain the vertical displacements as
well as the horizontal.
The preferred coseismic rupture plane with a 10km fault width also has a
much better model-data fit for the 3-week coseismic + post-seismic
deformation at GPS campaign sites , in which time period the coseismic
signal should be the dominant, which further confirms that the spatial
pattern of stress-driven afterslip brings new information of the
coseismic rupture of the 29 July 2021, Mw 8.2 Chignik earthquake.
Our result indicate an abrupt instead of gradual downdip termination of
coseismic slip. The lack of deep aftershocks further supports this
conclusion and suggests that there was limited interseismic slip deficit
deeper than the coseismic rupture, even accounting for stress-shadowing
effects, in line with the Xiao et al. (2021) coupling model for this
region.