Evidence of Strong Upper Oceanic Crustal Hydration Outboard the Alaskan and Sumatran Subduction Zones

The hydration state of subducting oceanic crust has been proposed to influence subduction zone processes like seismic coupling at the megathrust interface and arc magmatism downdip. Plate bending in the outer rise region is thought to help rehydrate the incoming oceanic lithosphere before subduction. Although numerous seismic refraction studies provide constraints on the amount of water stored in the lower crust and uppermost mantle, little information exists about how much free water is present in the upper 1–2 km of oceanic crust. Here, we present results from the application of advanced techniques to long‐offset multi‐channel seismic data acquired outboard the Alaskan and South Sumatran subduction zones. Our results show that the incoming upper crustal seismic layer, layer 2A, is significantly hydrated in both areas. Favorable conditions offshore the Alaska Peninsula promote the creation of a dense system of bending‐related faults, facilitating the infiltration of fluids that increases average water estimates to ∼3.9 wt.% H2O in the outer rise. As the crust subducts and temperature increases, some free water may react with the host rocks to form mineral‐bound water that is carried to greater depths, in agreement with elevated water contents found in arc lavas of Shumagin Gap volcanoes. Offshore Sumatra, we propose that similar layer 2A water estimates (∼3.2 wt.% H2O average) and heterogeneous hydration within 2B are associated with the ongoing, slow, complex deformation occurring in the Wharton Basin and thus potentially contribute to the presence of a long‐lived slow slip event recently inferred there at seismogenic depths. Plain Language Summary Convergent plate boundaries, where one plate subducts beneath the other, produce large and destructive earthquakes and tsunamis. The amount of water carried at a range of depths in the incoming plate is suspected to strongly influence processes that occur downdip, including the generation of earthquakes. However, little information is available on how the uppermost crust is hydrated and the amount of water present there before it subducts. To address this question, we utilize high‐quality seismic data to provide some of the first estimates of the amount of free water in the upper ∼1.2–2.3 km of oceanic crust outboard the Alaskan and Sumatran subduction zones. We find that the crust in these areas contains significant amounts of water in the pores and that rehydration of the crust before subduction may occur in