Abstract We present a high‐resolution local earthquake tomography model that constrains the distribution of hydration and dehydration processes within the subducting Nazca slab beneath Northern Chile. We image a distinct downdip transition from high Vp/Vs (∼1.75) over low Vp/Vs (∼1.65) to a homogeneously high Vp/Vs slab at depths of ≥ ${ge} $85 km. This transition corresponds to the transition between a double seismic zone and a broad, highly seismogenic volume. The latter is overlain by the highest Vp/Vs values (1.8–1.87) in the model volume, confined to the portion of the mantle wedge just above the slab top. Taken together, these observations imply the presence of slab hydration and the occurrence of dehydration reactions down to depths of ∼30 km below the slab top. While these processes are visible via their Vp/Vs signature where seismicity rates are high, we speculate that lower dehydration rates further updip are insufficient to create a similar fluid signature.