Seismic Swarm Activity in the South Sandwich Islands Region, November 2018

The South Sandwich Islands region experienced a notable seismic swarm event designated PS20181116.1. This swarm commenced at 16:49 on 15 November 2018 and concluded at 05:25 on 16 November 2018, spanning 12 hours and 35 minutes. During this interval, six earthquakes were recorded, with magnitudes ranging from 4.8 to 6.4 and focal depths primarily around 10–15 km.

The sequence began with a magnitude 5.5 event at 16:49:41 on 15 November, followed by a magnitude 6.4 earthquake at 20:02:22. Subsequent events included magnitudes of 5.2 at 20:07:21, 5.4 at 04:18:14 on 16 November, 4.8 at 05:01:06, and 5.1 at 05:25:24. All events occurred within the tectonically active South Sandwich Islands region.

This activity aligns with the area's established pattern of seismic swarms. Historical records since 1 January 2000 indicate four such swarms in total, occurring in 2010, 2014, 2017, and 2018. These episodes reflect episodic clustering of events rather than isolated mainshock-aftershock sequences, providing insight into underlying stress redistribution along regional fault systems.

The South Sandwich Islands form a volcanic arc situated above the South Sandwich subduction zone, where the South American Plate descends beneath the Scotia Plate. This convergent boundary generates frequent intermediate-depth and shallow seismicity, accompanied by active volcanism on the islands themselves. The trench reaches depths exceeding 8,000 meters, contributing to the region's high seismic productivity. Geological evolution of the arc dates to the Miocene, driven by continuous subduction that has shaped both the island chain and associated back-arc spreading in the Scotia Sea.

Seismic swarms in subduction settings like this often indicate fluid migration or aseismic slip processes that modulate stress without producing a dominant mainshock. The 2018 event, with its compact timeframe and moderate magnitudes, exemplifies such behavior, consistent with prior swarms in the catalog.

• References
  USGS Earthquake Catalog (earthquake.usgs.gov)
  Global Volcanism Program, Smithsonian Institution (volcano.si.edu)
  Bird, P. (2003). An updated digital model of plate boundaries. Geochemistry, Geophysics, Geosystems.