Seismic Swarm PS20040516.1: Analysis of the 2004 Solomon Islands Event

On 16 May 2004, a seismic swarm designated PS20040516.1 was recorded 153 km south of Lata in the Solomon Islands. The sequence began at 07:30 and concluded at 10:58 local time, encompassing eight earthquakes over 3 hours and 28 minutes. All events occurred at a shallow depth of 10 km, characteristic of crustal activity in this tectonically dynamic region.

The individual events unfolded as follows: the initial shock at 07:30:25 registered magnitude 5.4; a magnitude 4.1 event followed at 10:27:00; magnitude 5.0 at 10:31:52; magnitude 5.6 at 10:32:10; magnitude 5.3 at 10:37:08; magnitude 5.0 at 10:41:59; magnitude 5.8 at 10:55:49; and the largest at magnitude 5.9 at 10:58:38. These closely spaced occurrences without a dominant mainshock define the swarm pattern, reflecting fluid migration or stress redistribution along local fault networks rather than a single rupture.

The Solomon Islands lie along the convergent boundary between the Pacific and Australian plates, where the Pacific plate subducts westward beneath the Australian plate at rates exceeding 10 cm per year. This subduction drives the formation of the Solomon Islands arc and generates frequent shallow to intermediate-depth seismicity. The swarm location aligns with the eastern segment of this arc, near the Santa Cruz Islands, an area marked by complex plate interactions including possible slab tears and back-arc spreading influences.

Historical records since 2000 indicate this as the sole swarm in the immediate vicinity, with the prior swarm occurring in 2000. The region has also experienced several strong earthquakes in the same period, including a magnitude 7.2 event 97 km south of Lata on 2 September 2007, located 66 km from the swarm center, and two magnitude 7.8 events on 7 October 2009—one 196 km northwest of Sola, Vanuatu (64 km from the swarm center) and another in the Santa Cruz Islands (70 km from the swarm center). These larger events underscore the persistent seismic hazard driven by ongoing plate convergence.

Such swarms provide insights into preparatory stress conditions that may precede or accompany major ruptures in subduction zones. Continued monitoring remains essential for understanding temporal patterns in this highly active margin.

References

USGS Earthquake Catalog
SeismoSight Internal Database