Location:

Period:

27 Aug 2021 13:47:35 - 28 Aug 2021 20:10:12 (1 day 6 hours 22 minutes)

Volcanoes in 100km radius:

Southern Thule(65km), Bristol Island(66km)

Earthquakes:

9 swarms found nearby.

2015

PS20150526.1(109.1km)

26 May

24 minutes

5 earthquakes

2018

PS20180905.1(152.9km)

4 Sep

15 hours

7 earthquakes

2019

PS20190828.1(135.3km)

27 Aug

20 hours

8 earthquakes

2021

PS20210812.2(41.9km)

12 Aug

2 days 7 hours

41 earthquakes

PS20210816.1(39.2km)

15 Aug

4 days 8 hours

20 earthquakes

PS20210823.1(121.7km)

22 Aug

19 hours

8 earthquakes

PS20210826.1(91.8km)

25 Aug

9 hours

5 earthquakes

PS20210905.1(86.2km)

4 Sep

1 day 16 hours

6 earthquakes

PS20210909.1(64.3km)

9 Sep

11 hours

5 earthquakes

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Seismic Swarm PS20210827.1 in the South Sandwich Islands Region

The South Sandwich Islands region lies in the southern Atlantic Ocean along a tectonically active plate boundary where the South American Plate subducts westward beneath the Scotia Plate. This subduction zone generates frequent earthquakes and sustains the volcanic arc that forms the islands themselves. The area experiences ongoing seismic activity due to the convergence of these plates at rates of several centimeters per year, with hypocenters commonly distributed between shallow crustal depths and intermediate levels around 50 km.

On 27 August 2021 at 13:47 UTC, a seismic swarm designated PS20210827.1 began in this region and continued until 20:10 UTC on 28 August 2021, spanning 30 hours and 22 minutes. During this period, seven earthquakes were recorded with magnitudes ranging from 4.8 to 5.3 and focal depths between 10 km and 51 km. The sequence included two events of magnitude 5.0 within the first minute, followed by additional shocks of magnitudes 5.1, 4.8, 5.2, 5.3, and a final 5.1. Depths generally clustered around 35 km, with outliers at shallower and deeper levels, consistent with activity along the subduction interface and within the overriding plate.

Earthquake swarms in subduction settings often reflect fluid migration, stress transfer along the megathrust, or minor slip events that do not culminate in a single mainshock. The PS20210827.1 swarm fits this pattern, with closely spaced events showing no clear foreshock-mainshock-aftershock progression. All events remained below the threshold for widespread felt shaking on the remote islands, and no tsunami was generated.

Historical records since 2000 indicate seven swarms have occurred in the South Sandwich Islands region. These include single swarms in 2015, 2018, and 2019, along with four swarms in 2021. The elevated frequency in 2021 suggests episodic increases in seismic productivity that may relate to transient changes in slab dehydration or plate coupling. Such swarms contribute to the long-term release of accumulated strain in a region that also hosts occasional great earthquakes exceeding magnitude 7.

The South Sandwich subduction zone forms part of the broader Scotia Sea tectonic framework, which developed following the separation of South America and Antarctica. Volcanism along the arc has built the islands over the past few million years, with several volcanoes remaining active today. Seismic monitoring in this remote location relies on global networks that provide rapid detection and location of events, supporting both scientific understanding and hazard awareness for any future larger earthquakes.

References

USGS Earthquake Catalog (events and depths)
SeismoSight internal swarm classification PS20210827.1