Bimodal seismic-aseismic behavior of a weakly coupled megathrust segment revealed by kinematic analysis of a seismic swarm and slow slip event offshore Copiapó, Chile

A seismic swarm occurred in Chile in 2023, in a region well known for hosting this type of seismicity, the Atacama region, and more precisely within the low coupling zone of Barranquilla. It started on August 26th and lasts about 6~weeks, ending early October. Thanks to our high-density small-scale GNSS network, we recorded the spatio-temporal evolution of surface deformation generated by a slow slip event associated with the swarm. The slow slip event lasts as long as the seismic swarm but takes place in two distinct phases, located on two distinct patches of slip. Overall, the cumulative slip distribution extends over about 100 km (between 27°S and 28°S), reaching an equivalent Mw 6.1. The two patches of slip exhibit a quite different interplay between aseismic slip and seismicity, revealing bimodal seismic and aseismic behaviors. In the northern patch, which hosted almost all known swarms and where higher coupling was found, slip is collocated with seismicity, exhibiting important brittle deformation. In contrast, in the southern patch where coupling is low, microseismicity is not collocated with slip and is most likely driven by Coulomb stress transfer, suggesting a dominant aseismic behavior. We therefore demonstrate that the SSE associated with the 2023 swarm reveals the details of the low coupling zone structure that slow, interseismic deformation could not entirely decipher. Investigations on the long-term seismicity suggest that the aseismic behavior of the low coupling zone southern part has persisted at least since seismicity was cataloged and plausibly at the timescale of the regional seismic cycle.