4.2 3-Month Viscoelastic Relaxation Models with Different
Assumed Viscosity Structures
In order to isolate afterslip, we simulate the potential viscoelastic
relaxation contribution to the signal by testing different viscosity
structures. We find that the viscoelastic relaxation predictions are not
sensitive to the downdip distribution of the coseismic slip, because our
models with different fault widths all give very similar viscoelastic
model predictions (Figure S5). Based on this, we show here only the
viscoelastic relaxation models for our preferred 10km coseismic model.
The viscoelastic relaxation signal is smaller than the contribution of
afterslip over this short time window, but it is not negligible for some
models.
Using the method and model geometry described in section 3.2.2, we
varied the value of Maxwell viscosity of the mantle wedge and the
lithosphere thickness within the ranges given in section 3.2.2. We also
varied whether or not there is a cold nose. For each viscoelastic model,
we computed the displacements due to viscoelastic relaxation over the
first three months after the earthquake. The geometry of our model can
be found n Figure 4a, b.
Based on the three variations in the viscosity structure discussed
above, we identified the viscosity structure that results in the maximum
predicted displacements. That model has a Maxwell viscosity of the
mantle wedge of 1019 Pa-s, a lithosphere thickness of
40km, and there is no cold nose. We also identified the viscosity
structure that results in the minimum predicted displacements. That
model has a Maxwell viscosity of the mantle wedge of
5*1019 Pa-s, a lithosphere thickness of 50km, and a
cold nose that extends to 80km depth. In the latter case, the prediction
of viscoelastic relaxation is very small and can be ignored, so the
post-seismic signals could be regarded as being due only to afterslip in
that case. Figure 4 shows the geometry and viscosity structure of these
two upper and lower bound viscoelastic relaxation models and their
corresponding surface displacement predictions.