5.2 Vertical Postseismic Signals over the first 3 months
Although we didn’t use the vertical signal in the previous discussion due to the larger uncertainties and the relatively small signal, the vertical signal further supports our conclusion of a narrower coseismic rupture, and provides some information about the viscosity structure. We do not fully explore the tradeoffs between parameters in the viscoelastic model here, because the timespan of the data is very short. However, we can use the vertical data to find a reasonable combination of lithospheric thickness, mantle wedge viscosity, and cold nose structure that can serve as a reference model for future modeling.
Figure 13 a, b shows the observed post-seismic vertical displacement, the vertical displacement due to an afterslip-only model and the vertical displacement due to a viscoelastic relaxation-only model. The predicted vertical displacement due to the afterslip-only model at all Alaska Peninsula sites is subsidence. The viscoelastic relaxation-only model predicts uplift at those same sites. In both cases, the model predictions are substantially larger than the observed displacements at Peninsula sites AB13 and AC21, so matching the observations requires a contribution from both mechanisms. The two Peninsula sites AB13 and AC21 are close to each other, but we observe subsidence at the site AC21uplift at the site AB13. The signals are small amplitude, indicating the subsidence displacement predicted by the afterslip and the uplift displacement predicted by the viscoelastic relaxation must share a similar absolute value at those two sites, but of opposite sign. This requires a non-negligible viscoelastic relaxation contribution to the observed signal. Therefore, we explore the range of our test models to find a reasonable model that might explain the vertical data.
(a) (b)