Relation Between Oceanic Plate Structure, Patterns of Interplate Locking and Microseismicity in the 1922 Atacama Seismic Gap.

We deployed a dense geodetic and seismological network in the Atacama seismic gap in Chile. We derive a microseismicity catalog of >30,000 events, time series from 70 GNSS stations, and utilize a transdimensional Bayesian inversion to estimate interplate locking. We identify two highly locked regions of different sizes whose geometries appear to control seismicity patterns. Interface seismicity concentrates beneath the coastline, just downdip of the highest locking. A region with lower locking (27.5°S–27.7°S) coincides with higher seismicity levels, a high number of repeating earthquakes and events extending toward the trench. This area is situated where the Copiapó Ridge is subducted and has shown previous indications of both seismic and aseismic slip, including an earthquake sequence in 2020. While these findings suggest that the structure of the downgoing oceanic plate prescribes patterns of interplate locking and seismicity, we note that the Taltal Ridge further north lacks a similar signature. Plain Language Summary: Deformation along plate boundaries can occur seismically (i.e. through earthquakes) as well as aseismically (i.e. slipping slowly), and it is important to understand where each of these modes is dominant. Along the Chilean subduction contact, North‐Central Chile is the only place where aseismic deformation episodes have been observed so far. In order to study these processes in detail, we deployed and operated dense geodetic and seismological networks in this region. Analyzing the data collected by these networks, we find notable relationships between seismic and aseismic processes. Thousands of small earthquakes are found at the boundaries of locked regions, whereas no small earthquakes are found at their interior. Thus, implying such regions are mechanically coupled, that is, currently accumulating elastic deformation energy that will 1 day be released during a large earthquake. Along the North‐Central Chilean plate boundary, there is one region (around 27.5°S) that shows many signs of aseismic deformation. It is located where a chain of seamounts is being subducted, which is likely responsible for the different behavior of this segment. Key Points: Microseismicity catalog and map of interplate locking derived for the Atacama 1922 seismic gap in North‐Central ChileSeismicity in vicinity of plate interface coincides with downdip edge of high couplingSeismo‐geodetic signals due to the subduction of the Copiapó ridge are prominent but negligible for the subducting Taltal Ridge [ABSTRACT FROM AUTHOR]