Seismic Swarm S20200911.1 in Tarapacá, Chile

A notable seismic swarm occurred in the Tarapacá region of northern Chile between 7:35 UTC on 11 September 2020 and 19:03 UTC on 13 September 2020. Over 59 hours and 27 minutes, 79 earthquakes were recorded, with magnitudes ranging from 2.5 to 6.2 and focal depths between 41 and 54 km. The sequence began with a magnitude 6.2 event and featured several events above magnitude 4.0, followed by a dense cluster of smaller shocks.

Tarapacá lies within the Andean subduction zone, where the Nazca plate converges with the South American plate at approximately 6–7 cm per year. This tectonic setting produces frequent seismicity at intermediate depths corresponding to the descending slab. The region’s crust and upper mantle exhibit complex faulting influenced by both slab dehydration and inherited crustal structures from the Andean orogeny.

Earthquake swarms in subduction zones often reflect transient processes such as fluid migration or localized stress perturbations rather than a classic mainshock–aftershock sequence. The 2020 Tarapacá swarm displayed a relatively high event rate without a single dominant trigger after the initial magnitude 6.2 shock, consistent with swarm characteristics observed elsewhere along the Chilean margin. Depths clustered around 45–53 km suggest activity within the subducting plate, possibly linked to phase changes in the oceanic crust.

Historically, northern Chile has experienced large megathrust earthquakes, including the 1877 Iquique event (estimated Mw 8.5–9.0) and the 2014 Iquique earthquake (Mw 8.2). Intermediate-depth seismicity, such as the 2005 Tarapacá earthquake (Mw 7.8 at ~95 km depth), underscores the persistent hazard from intraslab events. The 2020 swarm, while moderate in size, illustrates ongoing strain release within the same tectonic framework.

No significant surface damage or tsunami was reported from the swarm, owing to its moderate magnitudes and depth. Continued monitoring by Chilean and international networks remains essential for understanding swarm evolution and potential links to larger subduction-zone ruptures.

References

USGS Earthquake Catalog Servicio Sismológico Nacional de Chile Global CMT Catalog Oncken et al., 2022, Tectonics of the Andean Subduction Zone