Seismic Swarm PS20130206.1: Santa Cruz Islands Earthquake Sequence

The seismic swarm designated PS20130206.1 occurred in the Santa Cruz Islands region of the Solomon Islands, commencing at 19:39 on 5 February 2013 and concluding at 20:09 on 11 February 2013. Over this 144-hour-and-30-minute period, 144 earthquakes were recorded. The swarm was centered near the epicenter of the mainshock of the 2013 Santa Cruz Islands Earthquake, a magnitude 8.0 event that struck on 6 February 2013 at a depth of 24 km.

The first 100 events began with a magnitude 5.1 earthquake at 24 km depth on 5 February at 19:39:36 UTC. Activity intensified rapidly, featuring a magnitude 6.0 at 11 km depth shortly after midnight on 6 February, followed by the main magnitude 8.0 shock at 01:12:25 UTC. Subsequent events included multiple magnitude 5+ aftershocks clustered at shallow depths around 10 km, such as a magnitude 7.1 at 10 km and a magnitude 5.9 at 10 km within the first hour. Depths predominantly ranged from 7 km to 24 km, with the majority at or near 10 km, indicating shallow crustal faulting. Magnitudes showed a typical decay pattern, with frequent events between 5.0 and 5.6 interspersed with larger shocks, including a magnitude 6.1 at 10 km depth and a magnitude 6.7 at 11 km later on 7 February. By 8 February, activity included a magnitude 6.8 at 12 km depth amid continuing moderate events.

This sequence reflects the highly active tectonic environment of the Santa Cruz Islands, situated at the boundary between the Pacific Plate and the Australian Plate. The region forms part of the Pacific Ring of Fire, where subduction along the New Hebrides Trench drives frequent seismicity. Historical records since 2000 document eight prior swarms in 2000, 2004, 2007, 2008, 2009, 2012, and two in 2013, underscoring recurrent swarm behavior linked to plate convergence rates exceeding 80 mm per year.

Strong earthquakes since 2000 further illustrate the area's hazard potential. Notable events include the magnitude 8.0 mainshock of 6 February 2013 (11 km from swarm center), a magnitude 7.1 on the same day (51 km distant), another magnitude 7.1 on 8 February 2013 (88 km distant), a magnitude 7.0 on 6 February 2013 (57 km distant), and a magnitude 7.0 on 18 July 2015 (50 km distant). These occurrences align with the swarm's timing and location, highlighting aftershock triggering and stress redistribution along regional faults.

The swarm's concentration of shallow events near the mainshock epicenter provides insight into post-seismic relaxation processes typical of subduction zone sequences in this arc setting. Such activity contributes to ongoing seismic hazard assessment for nearby populations in the Solomon Islands.

References

USGS Earthquake Catalog SeismoSight internal swarm classification records