Seismic Swarm in the Samoa Islands Region: Analysis of the 2009 Event Sequence

The Samoa Islands region lies within a tectonically active zone of the southwestern Pacific Ocean, where the Pacific Plate subducts beneath the Australian Plate along the northern Tonga Trench. This convergence drives frequent seismicity, with the area characterized by shallow to intermediate-depth earthquakes and occasional large-magnitude events capable of generating tsunamis. The islands themselves formed through hotspot volcanism superimposed on the subduction setting, resulting in a complex crustal structure prone to both volcanic and tectonic stress release.

Between 18:13 UTC on 29 September 2009 and 07:16 UTC on 3 October 2009, a seismic swarm comprising 28 earthquakes was recorded in this region. The sequence unfolded over approximately 85 hours, with the majority of events occurring at shallow depths of around 10 km. Magnitudes ranged primarily between 4.5 and 6.1, featuring an initial cluster of five events above magnitude 5.0 within the first hour, followed by sustained activity over subsequent days. Notable events included a magnitude 6.1 earthquake on 2 October at a depth of 8 km and several magnitude 5.5 shocks distributed across the period.

This swarm represents the sole such episode documented in the region since 2000. Its temporal distribution showed peak intensity during the first 24 hours, with events becoming more sporadic toward the conclusion. Depths remained consistently shallow except for isolated deeper occurrences, such as one at 36 km on 2 October. The pattern aligns with typical swarm behavior in subduction margins, where fluid migration or stress transfer along the plate interface can trigger clustered seismicity without a single dominant mainshock.

Geological context indicates that the Samoa region experiences elevated seismic hazard due to its position near the Tonga-Kermadec subduction system. Historical records document major earthquakes in 1917 and 1977, underscoring recurring activity. The 2009 swarm coincided with a period of heightened regional strain, consistent with the broader tectonic regime of oblique convergence and back-arc spreading influences.

Analysis of event timing reveals a rapid onset followed by gradual decay, with 18 events recorded in the initial 48 hours. Magnitudes clustered around 5.0–5.5 for most shocks, interspersed with stronger releases. Such sequences provide insight into localized stress accumulation along fault segments within the subduction zone.

References

USGS Earthquake Catalog Pacific Tsunami Warning Center historical reports Global CMT Project database