First evidence of shallow crustal seismicity around the Cariño Botado fault system: Implications for diffuse deformation at the West Andean Front, southern Chilean flat-slab

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Abstract We report the first evidence of very shallow crustal seismicity (2–6 km depth) along the West Andean Front at ~32.8°S, central Chile, in the southernmost part of the Chilean flat-slab segment. Earthquakes cluster along the trace of the Cariño Botado fault system, an active west-vergent reverse fault system previously inferred from geomorphic and paleoseismic observations. Seismicity coincides with density and resistivity contrasts imaged by gravity and magnetotelluric data, suggesting fault-controlled uplift of the forearc basement. This finding challenges the long-standing view of aseismic upper crust at the West Andean Front, where the nearby San Ramón fault (~33.5°S) remains largely aseismic at shallow depths despite evidence of Quaternary ruptures. We suggest that the distribution of shallow crustal earthquakes may reflect the influence of subduction of the Juan Fernández Ridge and the resulting flat-slab configuration of the segment, which enhances interplate coupling and localizes shortening in forearc structures. Our results indicate that active faults north of Santiago are accommodating present-day strain and may represent a previously unrecognized seismic hazard for nearby urban centers.

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