Influence of ridge subduction on seismicity in South Peru

The South Peru subduction zone is a complex, highly active region, where the flat slab associated with the Nazca Ridge subduction in the North transitions to a much steeper subduction in the South. This transition not only causes the slab to contort, but affects seismicity patterns in the region. Here we use data from 26 seismic stations active from March 2022 to December 2024 as part of the DEEPTrigger project, along with 16 permanent Peruvian stations and 15 permanent Chilean stations, to create a 3-year seismicity catalogue of South Peru. Using PhaseNet for phase picking and PyOcto for phase association, we obtain a total of 166 971 events. These earthquakes are located with NonLinLoc-SSST using a new 3-D P and S-wave velocity model of the region obtained from full-waveform inversion (Kan et al., 2025), then relocated using double difference methods with cross-correlation times to obtain precise locations. We thus obtain the first dense and precisely-located earthquake catalog of the region.

With this new catalog, we are able to demonstrate the influence of the Nazca Ridge on seismicity patterns. We find numerous shallow seismic swarms where the ridge enters subduction, while they are absent from the rest of the margin. In combination with GPS records of nearby stations, they hint at the likely presence of slow slip. We also find that the edge of the Nazca Ridge is particularly active, down to depths below 80 km. This same edge was activated by the Mw 7.2 Acari earthquake which occurred on June 28th 2024 at the plate interface, and was preceded by a Mw 6.0 intraslab foreshock on June 16th 2024. The Acari mainshock triggered a large aftershock expansion towards the northwest where the Nazca Ridge subducts, and a triggered swarm and possible SSE in that region. It also caused an increase of intraplate seismicity directly downdip along the Nazca Ridge edge, demonstrating the ridge’s ability to concentrate stress.