Seismic Swarm PS20150522.1: Analysis of Activity Near the Solomon Islands

An earthquake swarm designated PS20150522.1 occurred in the Solomon Islands region, centered 206 km east-southeast of Kirakira. The sequence began at 21:45 on 22 May 2015 and concluded at 00:39 on 23 May 2015, spanning 2 hours and 54 minutes. During this interval, seven earthquakes were recorded, with two events exceeding magnitude 6.0. The initial shock reached magnitude 6.9 at a depth of 11 km, followed minutes later by a magnitude 5.3 event at 10 km depth. Subsequent activity included magnitudes of 5.1, 4.3, 6.8, 5.1, and 5.3, all at depths of approximately 10 km.

This swarm reflects clustered seismicity typical of the tectonically active southwest Pacific. The Solomon Islands sit at the convergent margin where the Pacific Plate subducts beneath the Australian Plate along the Solomon Trench. Such settings produce frequent moderate-to-large earthquakes due to ongoing plate convergence at rates of 7–10 cm per year. Shallow focal depths around 10–11 km indicate rupture within the upper crust or near the plate interface, consistent with interplate thrust faulting.

Historical records since 2000 show six prior swarms in the same general area, occurring in 2004, 2005, 2008, 2013 (two events), and 2014. These episodes underscore episodic seismic clustering rather than isolated mainshock-aftershock sequences. A notable nearby event was the magnitude 7.0 earthquake of 9 November 2004, located 61 km from the 2015 swarm centroid. That quake highlighted the region’s capacity for strong shaking, though the 2015 swarm remained below that threshold.

Swarm behavior in subduction zones often arises from fluid migration, stress transfer along faults, or slow slip events that load adjacent segments without producing a single dominant rupture. Depths consistently near 10 km across the 2015 sequence suggest activity along a relatively uniform structural horizon. No surface rupture or tsunami generation was associated with these events, aligning with their moderate magnitudes and offshore location.

The Solomon Islands experience some of the world’s highest rates of seismicity. Regional monitoring networks have documented thousands of events annually, many linked to the same plate boundary responsible for the 2015 swarm. Updated catalogs confirm ongoing activity through the present decade, reinforcing the area’s persistent seismic hazard.

• USGS Earthquake Catalog (earthquake.usgs.gov)
• Incorporated Research Institutions for Seismology (IRIS) event database
• Pacific Tsunami Warning Center historical bulletins