Earthquake swarms along the Chilean subduction zone, 2003-2020

We search for earthquake swarms along the Chilean subduction, from -18 degrees to -39 degrees of latitude, 2003-2020 by developing an objective, model-based method that detects potential swarms as anomalous changes in earthquake rate. A set of 16 swarms is obtained after careful inspection of the candidate swarms; this assessment allows to reject cases for which model errors, rather than 'true' anomalous rate changes, are likely the cause of the detection. Averaging the activity over these 16 episodes, we find indirect evidence for a mostly aseismic driving mechanism, and a mean aseismic to seismic ratio estimated to range between 40 and 90 when using the seismicity rate as a proxy for slip. All the swarms are found in the 20-50 km depth range with the notable exception of one 60-100-km-deep swarm that occurs several days after the 2010 Maule earthquake and downdip of it. The dominant depth range (20-50 km) is in agreement with previous studies that suggest this range to be a transition zone from the shallower, locked part of the subduction, to the freely slipping interface at greater depth and intraslab earthquake activity. The swarms can be separated into three spatial groups, two of which being related to a subducting oceanic ridge. This structural control by fluid-rich geological features is modulated by stress control, that is swarms cluster in time with intermediate to large ruptures, both prior and following them, pointing to a close interplay between seismic slip and aseismic deformation in specific, well separated segments of the Chilean subduction.