Earthquake Swarm PS20041112.1: Seismic Activity Near Kirakira, Solomon Islands

The Solomon Islands lie within one of the most seismically active regions on Earth, situated along the convergent boundary between the Pacific and Australian plates. This tectonic setting produces frequent earthquakes through subduction processes, where the Australian Plate descends beneath the Pacific Plate, generating both shallow crustal events and deeper Wadati-Benioff zone activity. The island arc's complex geology, involving multiple microplates and transform faults, contributes to clustered seismic swarms alongside major thrust earthquakes.

Swarm PS20041112.1 occurred 75 km SSW of Kirakira on Makira Island. It began at 17:33 on 11 November 2004 and concluded at 19:28 on 12 November 2004, spanning 25 hours and 55 minutes. During this interval, seven earthquakes were recorded. The sequence opened with events of magnitude 5.9 and 6.7 at 10 km depth, followed by a magnitude 5.5 shock minutes later. Activity resumed the next day with a magnitude 4.4 event, then a magnitude 5.8, and closed with three final shocks of magnitudes 5.0, 5.1, and another at 80 km depth.

Such swarms reflect episodic stress release along subduction-related faults rather than a single mainshock-aftershock sequence. The predominance of shallow foci around 10 km aligns with brittle failure in the upper plate or accretionary wedge, while the deeper event indicates minor involvement of the subducting slab. Historical records since 2000 show only two comparable swarms in the region, with the earliest occurring in 2002, underscoring the relative rarity of this swarm pattern amid background seismicity.

The area has also experienced several strong earthquakes since 2000. A magnitude 7.8 event struck 69 km WSW of Kirakira on 8 December 2016, approximately 62 km from the swarm center. In April 2014, two events of magnitudes 7.4 and 7.6 occurred 55 km and 51 km from the center, respectively, both south and southeast of Kirakira. These larger shocks illustrate the potential for great earthquakes in the same tectonic corridor.

Seismic swarms like PS20041112.1 provide insight into transient fault behavior within an otherwise high-strain subduction zone. Continued monitoring supports improved understanding of precursory patterns and regional hazard assessment.

References

USGS Earthquake Catalog
SeismoSight internal swarm classification data