AbstractIn 2011 we acquired an 11 × 55 km, 3‐D seismic reflection volume across the Costa Rica margin, NW of the Osa Peninsula, to accurately image the subduction thrust in 3‐D, to examine fault zone properties, and to infer the hydrogeology that controls fluid accumulation along the thrust. Following processing to remove water column multiples, noise, and acquisition artifacts, we constructed a 3‐D seismic velocity model for Kirchhoff prestack depth migration imaging. Images of the plate boundary thrust show high‐reflection amplitudes underneath the middle to lower slope that we attribute to fluid‐rich, poorly drained portions of the subduction thrust. At ~ 5 km subseafloor, beneath the upper slope, the plate interface abruptly becomes weakly reflective, which we interpret as a transition to a well‐drained subduction thrust. Mineral dehydration during diagenesis may also diminish at 5 km subseafloor to reduce fluid production and contribute to the downdip change from high to low amplitude. There is also a layered fabric and systems of both thrust and normal faults within the overriding plate that form a “plumbing system.” Faults commonly have fault plane reflections and are presumably fluid charged. The faults and layered fabric form three compartmentalized hydrogeologic zones: (1) a shallow NE dipping zone beneath the slope, (2) a steeply SW dipping zone beneath the shelf slope break, and (3) a NE dipping zone beneath the shelf. The more direct pathway in the middle zone drains the subduction thrust more efficiently and contributes to reduced fluid pressure, elevates effective stress, and creates greater potential for unstable coseismic slip.