Seismic Swarm PS20210816.1: Analysis of Activity in the South Sandwich Islands Region

The South Sandwich Islands region lies along the southern segment of the Scotia Arc, where the South American Plate subducts westward beneath the Scotia Plate at rates of approximately 5–7 cm per year. This convergent margin produces frequent intermediate-depth and deep-focus earthquakes, along with shallow crustal events linked to volcanic processes and back-arc extension. The islands themselves form a volcanic arc resulting from this subduction, with known active volcanoes such as Mount Belinda on Montagu Island contributing to localized seismicity. Historical records indicate persistent seismic productivity in the area, driven by slab dehydration and phase transitions within the downgoing plate.

SeismoSight internal classification identifies Swarm PS20210816.1 as a distinct sequence that began at 19:00 on 15 August 2021 and concluded at 03:41 on 20 August 2021. Over 104 hours and 40 minutes, 20 earthquakes were recorded. Magnitudes ranged from 4.6 to 6.0, with focal depths between 10 km and 77 km. The largest event reached magnitude 6.0 at a depth of 28 km on 18 August 2021 at 21:49:55 UTC. Other notable shocks included two magnitude 5.5 events on 16 August and additional magnitude 5.5 events on 17 August, all occurring at depths shallower than 50 km. The sequence exhibited a typical swarm pattern: an initial rise in activity followed by scattered larger events without a single dominant mainshock-aftershock decay.

Event timing shows clustering on 16 and 17 August, with nine events exceeding magnitude 5.0 during those two days. Depths remained predominantly in the 10–55 km range for most shocks, consistent with seismicity occurring both within the overriding plate and near the plate interface. The final recorded event on 20 August reached magnitude 5.1 at 77 km depth, marking the deepest shock of the swarm and possibly reflecting stress adjustment within the subducting slab.

Since 1 January 2000, four seismic swarms have been documented in the South Sandwich Islands region according to SeismoSight records. These occurred in 2015 (one swarm), 2018 (one swarm), 2019 (one swarm), and 2021 (the present sequence). This low frequency underscores that while background seismicity is high, discrete swarm episodes remain relatively uncommon and may reflect episodic fluid migration or transient changes in slab coupling.

The tectonic setting of the South Sandwich subduction zone favors swarm occurrence through mechanisms such as dehydration embrittlement at intermediate depths and magmatic intrusion at shallower levels near the volcanic front. Depth distribution within PS20210816.1 aligns with these processes, spanning the seismogenic zone from the megathrust interface to the volcanic arc crust. Continued monitoring is warranted given the region’s capacity for great earthquakes, although no immediate escalation beyond swarm characteristics was observed.

References

USGS Earthquake Catalog (South Sandwich Islands region, 2000–2023)
Global CMT Project focal mechanism database
SeismoSight internal swarm classification records