In Southeast Alaska, glacial isostatic adjustment (GIA) is the dominant process causing present-day vertical crustal motions. Since the early 2000s, GPS observations have highlighted two major uplift areas centered at the Yakutat (YK) and Glacier Bay (GB) ice fields, exceeding 30 mm/yr , as shown in Figure 1. Past studies, beginning with Larsen et al. (2005), have shown that these uplift rates are the result of the Earth's viscoelastic response to the decline of glaciers after the Little Ice Age (LIA), and elastic rebound to present-day ice melt (PDIM). Raised shorelines indicate that uplift started at the end of

Plain Language Summary Rapid uplift in Southeast Alaska is caused by past and current melting of glaciers-which is termed "Glacial Isostatic Adjustment" (GIA). Traditionally, GIA models have considered a vertically stratified Earth model. However, the Earth's structure below Southeast Alaska is more complex and laterally variable due to a long history of subduction and tectonism. We aim to reconstruct the mantle viscosity below Southeast Alaska to best match the present-day GPS observations. Viscosity describes how readily the mantle flows when subjected to forces. We prescribe our models with an ice load history derived from ice-mass changes during and after the Little Ice Age. We test several models that predict the 3D viscosity using seismic velocity models and search for a parameter that determines how much of those variations are caused by temperature as opposed to other effects. We show that, the lateral variations in the Earth do not significantly impact the modeled uplift predictions in comparison to a laterally averaged uniform structure. This may be due to limitations of the seismic model. We find that, the viscosity variations are slightly influenced by thermal effects and are more likely due to hydration of the mantle, which is consistent with the tectonic history of this region.

We aim to better understand the overriding plate deformation during the megathrust earthquake cycle. We estimate the spatial patterns of interseismic GNSS velocities in South America, Southeast Asia, and northern Japan and the associated uncertainties due to data gaps and velocity uncertainties. The interseismic velocities with respect to the overriding plate generally decrease with distance from the trench with a steep gradient up to a "hurdle", beyond which the gradient is distinctly lower and velocities are small. The hurdle is located 500-1000 km away from the trench, for the trench-perpendicular velocity component, and either at the same distance or closer for the trench-parallel component. Significant coseismic displacements were observed beyond these hurdles during the 2010 Maule, 2004 Sumatra-Andaman, and 2011 Tohoku earthquakes. We hypothesize that

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