Title:

Seafloor geodesy reveals a depth-dependent interplate coupling controlled by seamount subduction

Abstract:
Shallow interplate coupling controls tsunami potential but remains poorly constrained because onshore geodesy cannot resolve near-trench deformation. Here we present the first seafloor geodetic observations from Chilean subduction, using five self-calibrating ocean-bottom pressure gauges deployed for 22 months offshore northern Chile. Continuous records reveal coherent near-trench subsidence (~2 cm/year) transitioning to coastal uplift, indicating shallow megathrust locking at 60-70 mm/year extending within 20 km of the trench. This spatially confined locked zone cannot explain broad continental deformation patterns, requiring additional deeper coupling. Integration with high-resolution bathymetry, seismic reflection profiles, and local earthquake tomography reveals that subducting seamount topography spatially correlates with the coupling distribution and intermediate-depth seismicity clustering. We interpret that seamount subduction creates structural and rheological heterogeneity producing shallow locking, mid-depth (30-40 km) decoupling through fracturing and fluid circulation, and resumed deeper coupling. This depth-dependent architecture demonstrates how rough bathymetry controls megathrust segmentation with fundamental implications for tsunami hazard assessment at erosional margins globally.