Seismic Swarm PS20160822.1 in the South Sandwich Islands Region

A seismic swarm designated PS20160822.1 occurred in the South Sandwich Islands region from 04:52 on 22 August 2016 to 02:52 on 23 August 2016. Over this 21-hour-59-minute period, six earthquakes were recorded, with magnitudes ranging from 4.5 to 5.3 and focal depths between 10 and 14 km. The events clustered tightly in both space and time, characteristic of swarm activity rather than a typical mainshock-aftershock sequence.

The sequence began with a magnitude 5.0 event at 04:52:49 on 22 August, followed by a magnitude 5.2 shock at 08:32:38. Subsequent events included two additional magnitude 5.0 earthquakes at 11:11:15 and 20:43:59, a magnitude 4.5 event at 21:31:45, and a final magnitude 5.3 shock at 02:52:05 on 23 August. All but one event originated at 10 km depth, indicating shallow crustal processes within the overriding plate or near the plate interface.

The South Sandwich Islands occupy a tectonically active segment of the Scotia Sea, where the South American plate subducts westward beneath the Scotia plate along the South Sandwich Trench. This convergent margin produces frequent intermediate-depth and shallow seismicity, as well as arc volcanism that has built the island chain. The trench reaches depths exceeding 8,000 m, and the subduction rate is estimated at approximately 65–78 mm per year, sustaining one of the most seismically productive regions in the South Atlantic.

Historical records since 2000 show only three documented swarms in the immediate area: one in 2004, one in 2014, and the present 2016 sequence. This low frequency underscores that swarm-type activity is uncommon relative to isolated mainshock events. Notably, a magnitude 7.4 earthquake struck the South Georgia Island region on 19 August 2016, centered roughly 58 km from the swarm epicentral area. That large event may have altered local stress conditions, potentially contributing to the subsequent swarm initiation three days later.

Such shallow swarms in subduction settings often reflect fluid migration, aseismic slip transients, or stress redistribution following nearby large earthquakes. Continued monitoring of the South Sandwich subduction zone remains essential for understanding how these transient clusters relate to the long-term seismic cycle and volcanic processes along the arc.

References

USGS Earthquake Catalog
Global CMT Project
South Sandwich Islands tectonic summaries (NOAA/NGDC)