Seismic Swarm PS20040828.1: Analysis of Activity Southeast of Gizo, Solomon Islands

The Solomon Islands lie along the convergent boundary between the Pacific Plate and the Indo-Australian Plate, forming part of the Pacific Ring of Fire. This tectonic setting produces frequent seismic activity through subduction processes, where the Pacific Plate descends beneath the Solomon Sea Plate. The region experiences both shallow crustal events and deeper intraslab earthquakes, contributing to a high level of seismicity documented over decades.

On 28 August 2004, a seismic swarm designated PS20040828.1 was recorded 97 km southeast of Gizo. The sequence began at 16:09 and concluded at 23:19, encompassing five earthquakes within a span of seven hours and ten minutes. All events occurred at shallow depths between 10 and 13 km, consistent with activity near the plate interface or overlying crust.

The individual events unfolded as follows: at 16:09:35 with magnitude 5.6 at 13 km depth; at 16:13:14 with magnitude 5.6 at 10 km depth; at 17:00:58 with magnitude 5.7 at 10 km depth; at 18:19:28 with magnitude 5.0 at 10 km depth; and at 23:19:39 with magnitude 4.6 at 10 km depth. These magnitudes remained moderate, with the largest reaching 5.7, and no significant escalation occurred after the initial cluster.

This swarm occurred in proximity to subsequent larger events, including the magnitude 8.1 earthquake of 1 April 2007 centered 48 km from the swarm location and the magnitude 7.1 event of 3 January 2010 situated at the swarm center itself. Such clustering highlights the persistent strain accumulation along this segment of the subduction zone.

Geological records indicate that the Solomon Islands have hosted multiple great earthquakes in the past century, often accompanied by tsunamis due to the shallow focal depths and proximity to coastal populations. The 2004 swarm aligns with patterns of foreshock or aftershock sequences observed in similar tectonic environments, though it remained isolated without triggering a larger mainshock on that date.

Ongoing monitoring by regional seismic networks continues to track activity in this area, aiding in the assessment of seismic hazards for nearby islands. The combination of frequent moderate swarms and occasional great earthquakes underscores the need for preparedness in this tectonically active zone.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
SeismoSight internal swarm classification data