Seismic Structure and Tectonics of the North-Central Chilean Subduction Zone Along the Copiapó Ridge From Amphibious Seismic Refraction Tomography and Local Seismicity

The Chilean margin is one of the Earth's tectonically most active plate boundaries, and yet, some of its segments are still underexplored. Here, we present amphibious data from the Copiapó region at ∼27°S located within the mature Atacama seismic gap. Combined 2D seismic refraction, multibeam bathymetry, and local seismicity data show a typical oceanic crust thickness of 6–7 km and seismic P-wave velocities between 3.0 and 7.3 km/s with slightly lower velocities and increased thicknesses underneath the Copiapó Ridge seamounts. The latter is most likely due to predominantly extrusive formation. Elevated velocities underneath one of the seamounts indicate a local region of magmatic underplating, while bending-related faults visible in the bathymetry and reduced mantle velocities near the trench suggest mantle hydration. The subduction angle of the down-going Nazca plate smoothly increases from 12° below the marine forearc to 22° at greater depths (40–60 km) with no abrupt change in the dip angle as observed at ∼22°S. The local seismicity off- and onshore Copiapó shows three separated bands of earthquakes sub-parallel to the down-going plate, and are most likely related to the plate interface, the oceanic Moho and the Double Benioff Zone. The largest event (M_W 5.9) during our observation period (December 2022–June 2023) and its aftershocks occurred in the deepest band ∼20 km below the subduction interface. Along the interface, seismicity is most pronounced in areas of high locking offshore, whereas areas of low locking are characterized by previously observed slow slip events and sparse seismicity.