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.