Figure 13. (a) 3- month vertical prediction of an afterslip-only model.
The coseismic fault width is 120km. (b) 3-month vertical prediction of a
viscoelastic relaxation only model. The viscosity structure is 50km LAB,
having a cold nose to the wedge and the wedge Maxwell viscosity is
5*1018 Pa-s. 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.
Figure S7 show the predicted vertical signal of viscoelasticonly models
based on different assumed viscosity structures, and Figure S8 show the
predicted vertical signal of aftersliponly models based on different
fault widths. The observed displacement at site AC40 is about 1 cm
uplift, and all of our afterslip models with different fault widths
assumed predict about 1cm subsidence; this require a
~2cm uplift contribution from viscoelastic relaxation at
site AC40. Our maximum viscoelastic relaxation contribution model used
above (40km LAB + 1019 Pa-s wedge viscosity and no
cold nose) only predicts ~0.5cm uplift at site AC40, but
a different combination of model parameters might produce a larger
uplift.
Figure S7 and Figure S1 show that by increasing the LAB depth and adding
a cold nose to the wedge, the model predicts more uplift and a smaller
horizontal displacement at site AC40. Thus, a cold nose to the wedge and
a deeper LAB than used in our models is needed in order to get a
predicted ~2cm uplift from viscoelastic relaxation at
site AC40, although this also would require a lower mantle wedge
viscosity. Together with afterslip, such a model can predict the
observed vertical displacement with a Maxwell viscosity of the mantle
wedge of 5*10 Pa-s. Thus, we consider a reasonable model with a 50km
lithosphere thickness, a cold nose to the wedge. Because of a tradeoff
between the lithosphere thickness (and cold nose geometry) and
asthenospheric viscosity, a model with a lower viscosity can give about
the same horizontal displacements as the models discussed earlier,
although the vertical displacements and also far-field displacements
will differ.
We computed the horizontal displacement of the viscoelastic relaxation
based on this viscosity structure and subtract that from the GPS
observation. We use the residu to search for the best-fit afterslip
model based on the 10km coseismic fault. Finally, we add together the
vertical prediction of viscoelasticonly model and afterslip model and
compare that to the observation. Figure 14 shows the horizontal
viscoelastic relaxation only prediction, the afterslip horizontal fit
and the total vertical fit. The model is a reasonable fit to both the
vertical and horizontal observations, but needs to be tested with a
longer time span of data and data from sites farther from the rupture,
and the parameter tradeoffs explored more thoroughly. In particular,
there are strong tradeoffs between the geometry of the cold nose,
lithospheric thickness, and wedge viscosity. These will be easier to
explore at later times when the afterslip contribution is smaller.
(a) (b) (c)