Fault‐Induced Hydration and Serpentinization of the Incoming Lithosphere Enhances Intra‐Slab Seismicity Offshore Taltal (∼25°S), Northern Chile

The northern Chilean margin is one of the most seismically active subduction zones, with a high potential for future megathrust earthquakes. The tectonic structure and the state of hydration of the incoming Nazca Plate, and their control on seismogenesis are poorly known, as the offshore domain remains largely unexplored. Here, we present high‐resolution bathymetry and amphibious seismic refraction data to investigate the structure of the incoming Nazca Plate within the Taltal region (24°S–26°S). The incoming plate is characterized by E‐W, trench‐perpendicular oriented horst and graben structures cross‐cut by N‐S oriented, bending‐related normal faults. The E‐W orientation of the horst and graben structures is unique in northern Chile, unrelated to subduction, and we suggest they formed through N‐S extension within the last 10 Ma years. This fault network facilitates seawater penetration into the crust and upper mantle, leading to widespread hydration and serpentinization. Modeling the P‐ and S‐wave velocities and deriving the VP/VS ratio shows reduced seismic upper mantle velocities (VP ∼ 7.5 km/s; VS ∼ 3.8 km/s) and elevated VP/VS ratios (VP/VS > 1.9), suggesting up to 30% serpentinization within the upper oceanic mantle. Furthermore, we link the subsequent dehydration of the serpentinized mantle at 70–100 km depth to a pronounced cluster of intermediate‐depth, intra‐slab seismicity beneath the Taltal region. Our findings demonstrate that pre‐existing tectonic fabrics can strongly influence the hydration budget of the incoming plate, directly impacting slab rheology and seismicity in northern Chile.