Seismic Swarm Offshore Atacama, Chile: September 2020 Analysis

The offshore Atacama region of northern Chile forms part of the Peru-Chile Trench, where the Nazca Plate subducts beneath the South American Plate at a rate of approximately 6–7 cm per year. This convergent margin produces frequent seismic activity, including both large megathrust events and smaller earthquake swarms. The Atacama segment lies between roughly 25°S and 28°S latitude, characterized by intermediate-depth seismicity within the subducting slab and shallower activity along the plate interface.

Between 04:44 UTC on 1 September 2020 and 00:23 UTC on 5 September 2020, a seismic swarm comprising 90 earthquakes was recorded offshore Atacama. The sequence lasted 91 hours and 39 minutes. Magnitudes ranged from 2.5 to 4.9, with the largest event (M4.9) occurring at 05:21 UTC on 1 September at a depth of 23 km. Depths throughout the swarm varied between 15 km and 48 km, consistent with activity near the plate interface and within the upper portion of the subducting Nazca slab.

The swarm exhibited a typical pattern of clustered events with no single dominant mainshock followed by aftershocks. Instead, multiple events of comparable magnitude occurred in rapid succession during the first 24 hours, after which activity gradually declined. Notable events included four earthquakes of M4.5 or greater on 1 September and additional M4.4 and M4.7 events on 2 September. Depths remained predominantly in the 20–35 km range, suggesting a relatively shallow source zone.

Historical records indicate that earthquake swarms are uncommon in this segment of the margin. Since 1 January 2000, only two swarms have been identified in the offshore Atacama area: one in 2006 consisting of a single documented sequence and the present 2020 swarm. This low frequency aligns with the region’s overall seismic behavior, which is dominated by larger, isolated thrust earthquakes rather than prolonged swarm activity.

The 2020 swarm did not produce any reported damage or tsunami, consistent with its moderate magnitudes and offshore location. Such sequences provide valuable data on stress transfer and fluid migration along the subduction interface, potentially informing models of future seismic hazard in northern Chile.

References
SeismoSight internal classification records for Swarm S20200901.1
USGS Earthquake Catalog (regional seismicity summaries)
Chilean National Seismic Network (CSN) annual reports on subduction-zone activity