3-D Viscoelastic FEM Modeling of Postseismic Deformation Caused by the 1964 Alaska Earthquake, Southern Alaska

To complement previous modeling efforts to understand the postseismic deformation following the 1964 Alaska earthquake, we are implementing a three-dimensional viscoelastic model using the Finite Element Method. Rapid postseismic uplift, as much as 125 cm, occurred on the central part of the Kenai Peninsula over the 35 years following the earthquake. Recent precise GPS observations show heterogeneous plate coupling, and a vigorous ongoing postseismic response to the 1964 earthquake. Past models of the postseismic deformation have focused on afterslip models because earlier viscoelastic-only models did a poor job at predicting the cumulative uplift observations, but this might be due to unrealistic assumptions for the model geometry. We are working toward a realistic 3D viscoelastic model that, in concert with afterslip models, can be used to understand how much each component contributes to the total postseismic deformation. Our model suggests that viscoelastic relaxation contributes to the total 30 years postseismic uplift moderately in the Kodiak region (40 %), slightly in the western Kenai Peninsula (<20 %) and not significantly in the northern Kenai Peninsula (<10 %). Most of the observed total uplift in Kenai Peninsula must be explained by afterslip. The depth of afterslip (30-70 km) is deeper than that of coseismic rupture (10-25 km). Some recently observed trenchward horizontal deformation far inland from the trench is explained by the viscoelastic response of the 1964 earthquake. But transient slip is required to explain the large trenchward motion in the northern and western Kenai Peninsula.