Seismic Swarm PS20041121.1: Insights from Guadeloupe, November 2004

The Lesser Antilles arc formed through subduction of the North American plate beneath the Caribbean plate at rates of approximately 2 cm per year. This tectonic setting produces frequent seismic activity and andesitic volcanism across the islands, including Guadeloupe. The island chain lies along a seismically active margin where intermediate-depth earthquakes commonly occur due to slab dehydration and brittle failure within the downgoing plate.

On 21 November 2004, seismic swarm PS20041121.1 was recorded 12 km south of Petites Anses, Guadeloupe. The sequence began at 11:41 UTC and concluded at 18:53 UTC, spanning 7 hours and 11 minutes. Six events were registered, with the largest reaching magnitude 6.3 at 14 km depth. Subsequent events included magnitudes 5.1, 5.0, 5.4, 5.2, and a final 5.4, mostly at depths near 10 km, with one deeper shock at 33 km. Such swarms typically reflect fluid migration or localized stress adjustments rather than a single dominant rupture, a pattern consistent with the region’s volcanic and hydrothermal systems.

Guadeloupe’s volcanic centers, particularly La Soufrière, have produced historical unrest episodes that coincide with elevated microseismicity. The 2004 swarm occurred within this framework, highlighting how subduction-related fluids can trigger clustered events without generating a classic foreshock-mainshock-aftershock sequence. Depths between 10 km and 33 km align with the expected range for both crustal and upper-slab seismicity in the arc.

Since 2000, the broader region has experienced additional strong events, including a magnitude 7.4 earthquake on 29 November 2007 located 18 km west-northwest of Basse-Pointe, Martinique, approximately 87 km from the 2004 swarm centroid. This larger shock further illustrates the persistent seismic hazard along the plate boundary.

Seismic swarms such as PS20041121.1 provide valuable data for refining regional hazard models. Their rapid onset and short duration underscore the need for dense monitoring networks capable of distinguishing swarm behavior from isolated mainshock sequences. Continued integration of geodetic, geochemical, and seismic observations remains essential for assessing future activity in this tectonically active setting.

References

SeismoSight internal swarm classification records
USGS Earthquake Catalog (events since 2000)
Caribbean tectonic framework summaries from peer-reviewed geophysical literature