Abstract The amount of water entering subduction zones and how it is stored within the slab is debated. This limits our understanding of where subducted fluids are released and therefore how pore pressure influences slip behavior along megathrusts. Here we present 3‐D compressional and shear‐wave velocity models, and their ratio Vp/Vs, of the Alaska Peninsula subduction zone using local earthquake tomography. We investigate the hydration of, and fluid release from, the downgoing plate, and how this impacts recent megathrust ruptures. First, we identify a wide‐spread oceanic crust and upper mantle water reservoir, resulting from fluid‐filled porosity. Second, slab dehydration is inferred from velocity anomalies along the plate interface interpreted as high pore fluid pressure regions. These regions bound the 2020 Mw7.8 Simeonof, 2021 Mw8.2 Chignik, and 2023 Mw7.2 earthquake rupture zones, demonstrating how locations of fluid release and elevated pore pressure impact megathrust frictional properties and act as rupture barriers.

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