Seismic Swarm PS20180905.1 in the South Sandwich Islands Region

A seismic swarm designated PS20180905.1 occurred in the South Sandwich Islands region between 13:32 UTC on 4 September 2018 and 05:09 UTC on 5 September 2018. Over 15 hours and 37 minutes, seven earthquakes were recorded with magnitudes ranging from 5.0 to 5.6. All events originated at shallow to moderate depths between 10 km and 35 km.

The sequence began with a magnitude 5.0 event at 35 km depth on 4 September at 13:32:29 UTC. Subsequent shocks included a magnitude 5.2 at the same depth two hours later, followed by a magnitude 5.1 at 10 km depth. Activity intensified overnight with a magnitude 5.6 at 10 km depth on 5 September at 00:36:23 UTC, accompanied by a magnitude 5.0 at identical depth minutes afterward. Additional events reached magnitudes 5.4 at 20 km and 5.3 at 35 km before the swarm concluded.

This swarm represents one of only four documented swarms in the region since 1 January 2000. Earlier episodes took place in 2008, 2014, 2015, and 2017. Swarm activity here typically involves clustered moderate-magnitude events without a single dominant mainshock, consistent with stress release along the complex plate boundary.

The South Sandwich Islands occupy the eastern margin of the Scotia Plate, where the South American Plate subducts westward beneath it along the South Sandwich Trench. This convergent boundary produces frequent intermediate-depth and shallow seismicity. The volcanic arc results from ongoing subduction, with the islands themselves formed by Quaternary volcanism. The tectonic setting features rapid plate convergence rates exceeding 7 cm per year, contributing to elevated seismic productivity.

Notable strong earthquakes have occurred nearby since 2000. The magnitude 8.1 event of 12 August 2021 struck 18 km from the 2018 swarm center at a depth of approximately 50 km. Two additional large events followed on the same day, including a magnitude 7.5 located 83 km away. A magnitude 7.1 earthquake occurred on 11 December 2018, 86 km from the swarm epicentral area. These events underscore the persistent seismic hazard along the subduction interface.

The 2018 swarm provides insight into localized stress accumulation and release within the overriding plate or along subsidiary faults. Such clusters may precede or follow larger ruptures by redistributing strain in the subduction zone. Continued monitoring remains essential given the region's history of both swarm-type activity and infrequent great earthquakes.