Seismic Attenuation Tomography traces fluid pathways in the Northern Chile Subduction Zone

Summary

We present a high-resolution three-dimensional P-wave attenuation tomography model of the northern Chilean subduction zone (21°–22°S), derived using the coda-normalization approach implemented in the MuRAT algorithm and a dense local earthquake dataset. This region represents an important segment of the South American margin, where the Nazca Plate subducts beneath the South American Plate, generating frequent intermediate-depth seismicity and sustained volcanic activity along the Western Cordillera. Understanding the distribution of attenuation and its relation to seismicity and fluid pathways is essential for constraining the physical state of the subduction system and its role in arc magmatism and crustal deformation. The inversion incorporates 147,639 high-quality waveforms from 42,460 local earthquakes recorded by 76 broadband stations between 2007 and 2021. The inversion was carried out using a three-dimensional velocity model with 10 km node spacing, and the resulting attenuation grid was parameterized at 14 × 25 km horizontally and 10 km vertically. The attenuation model reveals two main low-Q anomalies. The first extends along and immediately above the top of the subducting Nazca slab between 50 and 90 km depth, interpreted as the locus of fluid release from slab dehydration. The second low-Q zone ascends from the mantle wedge towards the lower crust beneath the volcanic arc, indicating fluid migration. These features coincide with high-Vp/Vs regions from velocity tomography models. Low-Q regions are generally found above seismicity concentrations in the downgoing Nazca slab, reaffirming the association of intraslab earthquakes with fluid release processes. Resolution tests confirm the robustness of the imaged structures. The obtained anomalies trace subduction-related fluids from their source in the downgoing slab through the mantle wedge towards the magmatic arc.