From flat to steep subduction: the South Peru margin imaged by a new high-density seismic catalog

Summary The South Peru subduction zone is a complex, highly active region, which has hosted four Mw 8 + earthquakes over the last 100 years. It marks the transition between the flat slab associated with the Nazca Ridge subduction in the North and more steeply dipping subduction in the South, causing the slab to contort and affecting seismicity patterns in the region. In this study, we present the first high-density, high-quality seismic catalog of the region between the arc and the trench, totaling 166 825 events between January 1st 2022 and December 31st 2024, including 125 467 well-located ones. We first picked and associated phases using PhaseNet and PyOcto, then located the resulting events with NonLinLoc-SSST and GrowClust3D. Finally, we derived a new slab model from the seismicity, allowing us to classify the earthquakes as upper plate (16 per cent), interface (12 per cent), lower plate (68 per cent), outer rise (0.20 per cent) and human-related (3.1 per cent). The region is broadly divided into four subregions with different seismicity patterns and slab geometries: the flat slab, with intense interface and intraslab activity, the slab transition zone, where the plate contorts to accommodate its change in geometry, the Arequipa region, with intense upper plate seismicity but very low intraslab and interface seismicity, and the North Chile region, with a large band of dense intraslab seismicity. We find that in the flat slab region, the Nazca Ridge is linked to the presence of dense seismicity close to the trench, and seismic swarms hinting at the presence of slow slip. Meanwhile, the intraslab seismicity in that region is organized in trench-parallel bands which are likely related to slab bending. In the slab transition region, we image multiple orthogonal faults just south of the slab contortion, suggesting a damaged slab. Further south, in the Arequipa region, upper plate seismicity forms a large, trenchward-dipping structure seemingly connected to the Incapuquio fault at the surface. Finally, in North Chile, the deep band of intraslab seismicity appears to locate further downdip as we move to the north, perhaps reflecting changes in slab properties.