Seismic Swarm Activity in the South Sandwich Islands Region, May 2015

The South Sandwich Islands region forms part of an active subduction zone where the South American Plate descends beneath the Scotia Plate, generating frequent seismic events along the South Sandwich Trench. This tectonic setting has produced a history of both shallow and intermediate-depth earthquakes, with the volcanic arc of the islands resulting from associated magma generation. The area remains one of the most seismically productive zones in the South Atlantic.

On 26 May 2015, a seismic swarm designated PS20150526.1 was recorded in the South Sandwich Islands region. The sequence began at 23:26 UTC and concluded at 23:51 UTC, encompassing five earthquakes within a 24-minute interval. The events included a magnitude 5.4 earthquake at 32 km depth at 23:26:58, followed immediately by a magnitude 5.3 event at 100 km depth at 23:27:06. Subsequent activity comprised a magnitude 4.5 earthquake at 35 km depth at 23:31:53, a magnitude 5.7 event at 35 km depth at 23:41:40, and a final magnitude 5.4 earthquake at 35 km depth at 23:51:30.

Historical records indicate limited swarm occurrences in the region since 2000, with only one prior swarm documented in 2008. The 2015 swarm therefore represents a rare clustered episode amid the more typical pattern of isolated events driven by ongoing plate convergence.

Notable larger earthquakes have occurred nearby in subsequent years, including the magnitude 8.1 South Sandwich Islands event of 12 August 2021, centered 35 km from the 2015 swarm location, and a magnitude 7.1 earthquake on 11 December 2018, located 69 km from the swarm center. These events underscore the persistent seismic hazard associated with the subduction interface.

Seismic swarms in subduction environments such as this can reflect fluid migration or stress transfer along the plate boundary, though detailed source mechanisms require further instrumental analysis. The 2015 sequence, dominated by events at depths around 35 km with one deeper outlier, aligns with typical intermediate-depth activity in the trench system. Continued monitoring contributes to improved understanding of regional tectonics and hazard assessment.