Location:

Period:

6 Sep 2004 12:42:59 - 7 Sep 2004 02:55:29 (14 hours 12 minutes)

Volcanoes in 100km radius:

Protector Seamounts(83km)

Earthquakes:

6 swarms found nearby.

2014

PS20140629.1(34.4km)

29 Jun

9 hours

11 earthquakes

S20140629.1(47.6km)

29 Jun

2 days 22 hours

49 earthquakes

2016

PS20160819.1(169.5km)

19 Aug

1 day 20 hours

26 earthquakes

PS20160822.1(144.2km)

22 Aug

21 hours

6 earthquakes

PS20160902.1(150.5km)

1 Sep

17 hours

5 earthquakes

2024

PS20240910.1(172.3km)

10 Sep

2 hours

7 earthquakes

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Seismic Swarm in the South Sandwich Islands Region: September 2004

An earthquake swarm designated PS20040906.1 occurred in the South Sandwich Islands region of the South Atlantic Ocean. The sequence began at 12:42 UTC on 6 September 2004 and concluded at 02:55 UTC on 7 September 2004, spanning 14 hours and 12 minutes. Seven events were recorded during this interval, all at a focal depth of 10 km.

The swarm initiated with a magnitude 6.9 earthquake at 12:42:59 on 6 September. This was followed by events of magnitude 5.8 at 14:17:19, 5.7 at 14:41:00, 5.0 at 20:07:23, 5.6 at 21:17:36, 5.6 at 21:18:29, and a final magnitude 5.3 event at 02:55:29 on 7 September. Such clustered activity without a dominant mainshock-aftershock pattern is characteristic of swarms in subduction-related settings, where fluid migration or stress transfer along the plate interface can trigger multiple similar-magnitude events over short periods.

The South Sandwich Islands occupy the eastern segment of the Scotia Arc, a tectonically active zone formed by the eastward subduction of the South American Plate beneath the Scotia Plate. This convergent margin produces the South Sandwich Trench and an associated volcanic island arc. The subduction rate in the region exceeds 70 mm per year, contributing to frequent moderate-to-large earthquakes and ongoing volcanic activity. The islands themselves are composed primarily of basaltic to andesitic lavas erupted from submarine and subaerial centers.

Seismic records document persistent activity along this margin throughout the instrumental era. The trench has hosted great earthquakes exceeding magnitude 7.5, while the volcanic arc experiences both tectonic and volcanotectonic swarms. Updated global catalogs confirm that the South Sandwich region remains one of the most seismically active segments of the Atlantic basin, with shallow events concentrated near the plate interface at depths of 10–40 km.

The September 2004 swarm fits within this established tectonic framework. All events occurred at shallow crustal depths consistent with the seismogenic zone of the subduction interface. No significant surface deformation or tsunami generation was associated with the sequence, as expected for moderate-magnitude, shallow offshore events in this remote setting.

References

SeismoSight internal swarm classification PS20040906.1 USGS Earthquake Catalog (South Sandwich Islands region) Global CMT Project focal mechanism database