Seismic Swarm PS20091008.1: Analysis of Activity Near Lata, Solomon Islands

Seismic swarm PS20091008.1 occurred in the Santa Cruz Islands region of the Solomon Islands, centered approximately 128 km south of Lata. The sequence began at 22:29 on 7 October 2009 and concluded at 09:14 on 9 October 2009, encompassing 13 earthquakes over 34 hours and 45 minutes. All events were recorded at shallow to intermediate depths, predominantly 35 km, with two exceptions at 44 km and 80 km.

The swarm exhibited a rapid onset followed by sustained activity concentrated on 8 October. Magnitudes ranged from 5.0 to 6.1, with the strongest event (M6.1) occurring at 10:11 on 8 October at 44 km depth. Multiple events clustered within minutes of each other during the early morning hours of 8 October, including a sequence of M5.9, M5.3, and M5.8 shocks between 01:59 and 02:12. Later activity included an M6.0 at 08:34 and several M5.2 events in the afternoon and evening. The final M5.6 shock marked the swarm’s termination on 9 October.

This temporal clustering indicates a localized release of tectonic stress within a short window, characteristic of swarm behavior rather than a classic mainshock-aftershock sequence. Depths near 35 km align with the seismogenic zone associated with regional subduction processes, where the interaction between the Pacific and Australian plates generates frequent moderate-magnitude events.

The Santa Cruz Islands lie within the Pacific Ring of Fire, a tectonically complex area marked by subduction along the New Hebrides Trench system and associated microplate boundaries. This setting produces high background seismicity, with the potential for both interplate thrust events and intraplate normal or strike-slip faulting. Historical records since 2000 show five prior swarms in the vicinity (2000, 2004, 2006, 2007, and 2008), underscoring the region’s recurrent swarm activity.

Notable larger earthquakes since 2000 include an M7.2 event on 2 September 2007 located 97 km south of Lata (28 km from the 2009 swarm center), an M7.1 on 8 February 2013 situated 32 km southeast of Lata, and two M7.8 events on 7 October 2009 near the Santa Cruz Islands and northwest of Vanuatu (93–96 km from the swarm center). The 2009 M7.8 shocks occurred within hours of the swarm’s initiation, suggesting possible dynamic triggering or shared stress conditions along nearby fault segments.

Overall, swarm PS20091008.1 reflects the ongoing seismic hazard of this subduction-dominated margin, where moderate-magnitude clusters can occur alongside or following major ruptures.

References

USGS Earthquake Hazards Program regional tectonics summaries
SeismoSight internal swarm classification database