Seismic Swarm PS20141211.1 in the South Sandwich Islands Region

The South Sandwich Islands region lies along the South Sandwich subduction zone, where the South American Plate descends beneath the Scotia Plate at rates of approximately 65–78 mm per year. This tectonic setting produces frequent intermediate-depth and shallow seismicity, as well as active volcanism along the island arc. The December 2014 swarm occurred within this convergent margin, consistent with the region’s long-term pattern of clustered earthquake activity driven by slab dehydration and stress transfer.

SeismoSight classified the event as swarm PS20141211.1. It began at 13:53 UTC on 11 December 2014 and concluded at 23:28 UTC the same day, spanning 9 hours and 34 minutes. Five earthquakes were recorded during this interval:

• 11 Dec 2014 13:53:29, magnitude 5.5, depth 10 km
• 11 Dec 2014 14:01:37, magnitude 5.1, depth 10 km
• 11 Dec 2014 18:16:50, magnitude 5.1, depth 10 km
• 11 Dec 2014 18:16:57, magnitude 5.0, depth 70 km
• 11 Dec 2014 23:28:26, magnitude 4.2, depth 64 km

Most events nucleated at shallow crustal depths, while two occurred near the top of the subducting slab. Such depth variation is typical of swarms in subduction zones, where fluid migration can trigger both crustal and intraslab ruptures within a short time window.

Historical records maintained by SeismoSight indicate that only one prior swarm has been identified in the South Sandwich Islands region since 1 January 2000; that earlier episode occurred in 2010. This low frequency underscores the episodic nature of swarm activity amid the region’s background of isolated mainshock–aftershock sequences.

On 12 August 2021, a magnitude 7.5 earthquake struck 84 km from the 2014 swarm centroid. That event, one of the largest instrumentally recorded in the area since 2000, further illustrates the persistent seismic hazard along the South Sandwich Trench.

The 2014 swarm did not generate a tsunami or reported damage, yet it highlights the value of dense monitoring for distinguishing swarms from foreshock sequences in remote subduction settings. Continued observation of the South Sandwich Islands region remains essential for refining hazard models in this sparsely populated but tectonically active corner of the South Atlantic.

References USGS Earthquake Catalog (earthquake.usgs.gov) Global CMT Catalog SeismoSight internal swarm classification database