Structure and geometry of the Chilean subduction zone near Copiapó (~27°S) based on an amphibious seismic refraction experiment

The subduction of the oceanic Nazca plate beneath the continental South American plate shapes the Chilean margin and is known to generate large megathrust earthquakes. Our study focuses on the region defined by the pre-collision and subduction of the Copiapó Ridge with the Chilean margin at ~27°S. This area has been a seismic gap since 1922, and little is known about the geometry and deep structures of the incoming plate, the overriding plate, and the processes related to the subduction of the Copiapó Ridge.  We model the seismic structure in the region by using wide angle seismic data from a recent amphibious seismic refraction experiment. Thereby, we utilize seismic signals from both offshore airgun-shots and onshore mining blasts. Overall, we use 36 Ocean-Bottom-Seismometers and 10 temporal seismic land stations along an approximately 420 km long profile ranging from more than 300 km offshore up to more than 100 km landwards. Our P-wave velocity model images the geometry and velocity structure of the incoming oceanic plate, including three seamounts belonging to the Copiapó Ridge, the marine and continental forearc, and the upper part of the downgoing slab. The model shows an oceanic crust with hardly any sediment cover (generally less than 10 m) and an average oceanic Moho depth of about 6.2 – 6.9 km below the seafloor, which increases to over 10 km below the seamounts of the Copiapó Ridge. The velocities beneath the seamounts are similar or slightly slower compared to the adjacent upper oceanic crust (Vp ranging from 3.5 to 6 km/s). This suggests that the Copiapó Ridge was predominantly formed by extrusive processes. In addition, the velocity model reveals a significant thinning (to less than 4 km) of the oceanic crust landwards of the trench axis. Together with recently acquired bathymetry data, we will compare our findings to other studies north and south of the Copiapó region and discuss the structural and geometric along-strike variations of the northern Chilean subduction zone.