Seismic Swarm PS20131125.1 in the South Atlantic Ocean

On 25 November 2013, a seismic swarm designated PS20131125.1 was recorded in the South Atlantic Ocean near the Falkland Islands region. The sequence began at 05:10 UTC and concluded at 07:21 UTC, encompassing seven earthquakes within a span of two hours and eleven minutes. This cluster occurred in a tectonically active portion of the ocean basin where the South American Plate interacts with surrounding oceanic structures.

The individual events exhibited the following parameters: at 05:10:07 UTC a magnitude 5.4 earthquake at 11 km depth; at 05:10:12 UTC another magnitude 5.4 event at 30 km depth; at 06:27:08 UTC a magnitude 5.6 earthquake at 10 km depth; at 06:27:33 UTC the largest event of magnitude 7.0 at 11 km depth; at 06:37:35 UTC a magnitude 4.9 earthquake at 12 km depth; at 06:41:50 UTC a magnitude 5.4 event at 15 km depth; and at 07:21:18 UTC a magnitude 6.0 earthquake at 14 km depth. The swarm center lay approximately 33 km from the epicenter of the magnitude 7.0 mainshock.

The Falkland Islands region occupies the Falkland Plateau, a submerged continental fragment extending eastward from the South American continental margin. This plateau formed during the Mesozoic breakup of Gondwana and remains part of the South American Plate. Seismic activity in the vicinity arises from distributed deformation along fracture zones and minor faults associated with the transition between the South American and Scotia plates. Historical records indicate infrequent but notable moderate-to-large earthquakes in this setting, consistent with low strain rates across the broad plate boundary zone.

The November 2013 swarm represents a classic example of clustered seismicity in an intraplate oceanic environment. Such swarms often reflect fluid migration or stress triggering along pre-existing weaknesses rather than a single through-going fault rupture. Depths ranging from 10 km to 30 km place the events within the brittle upper lithosphere, where frictional failure can propagate rapidly once initiated. The progression from smaller foreshock-like events to the magnitude 7.0 mainshock and subsequent aftershocks illustrates typical swarm dynamics observed in similar settings worldwide.

Post-2000 instrumental catalogs show that the magnitude 7.0 event on 25 November 2013 remains the largest recorded in the immediate Falkland Islands region. No comparable swarms have been documented in the same locale since that date, underscoring the episodic nature of seismicity here. Ongoing monitoring by global networks continues to refine understanding of stress accumulation along the plateau margins.

References

USGS Earthquake Catalog (events 2000–present)
Global CMT Project focal mechanism database
SeismoSight internal swarm classification PS20131125.1