Structure of the North-Central Chile Subduction Zone From Local Earthquake Tomography

Abstract

The fluid cycle in subduction zones prescribes large parts of its structure and seismogenic behavior. Background seismicity inside the downgoing slab is linked to fluid release from dehydration reactions, whereas fluid overpressure along the plate interface can alter interplate coupling, megathrust earthquakes, and the presence or absence of slow-slip events (SSEs) and tectonic tremor. We present a high-resolution seismic tomography model of the Atacama segment in northern Chile, the only region along the Chilean margin where SSEs have been observed. Using traveltimes from over 8,800 seismic events determined using state-of-the-art algorithms (EQTransformer, PyOcto), we followed a staggered workflow (VELEST, SIMUL2023) to derive consistent 1D, 2D and 3D models of P-wave velocity (Formula presented.) and (Formula presented.) ratios, achieving high spatial resolution in the upper continental crust, mantle, and downgoing slab. The final 3D model reveals key features interpreted as subsurface fluid processes. High (Formula presented.) ((Formula presented.) 1.80) appears along the plate interface, with localized anomalies in the mantle wedge and lower continental crust. Regions with deep seismicity (∼80–100 km depth), notably around the Copiapó Ridge, exhibit zones of higher (Formula presented.) ratios ((Formula presented.) 1.82) extending upward from the oceanic slab into the continental crust, which otherwise shows lower (Formula presented.) ratios ((Formula presented.) 1.76). These observations reflect along-strike variations in dehydration-driven fluid release accompanied by microseismicity. Liberated fluids ascend into the mantle wedge and updip along the slab surface, where they may influence SSEs. Our results provide new constraints on possible fluid pathways and crustal heterogeneity, highlighting the role of fluids in modulating seismogenic processes.