Earthquake Swarm in the Bonin Islands Region, December 2010

The Bonin Islands, also known as the Ogasawara Islands, lie approximately 1,000 kilometers south of Tokyo within the Izu-Bonin-Mariana arc system. This intra-oceanic subduction zone forms where the Pacific Plate descends beneath the Philippine Sea Plate at rates of 4–6 cm per year. The region experiences frequent seismicity due to plate convergence, including both shallow crustal events and deep-focus earthquakes extending to depths exceeding 500 km. Volcanic activity associated with the arc has shaped the islands’ geology over millions of years, producing andesitic stratovolcanoes and submarine ridges.

Between 17:44 UTC on 21 December 2010 and 04:28 UTC on 26 December 2010, a swarm comprising 92 earthquakes was recorded in the Bonin Islands region. The sequence lasted 106 hours and 43 minutes and was centered near the location of a magnitude 7.4 mainshock that occurred on 21 December 2010 at a depth of 11 km. All events in the swarm were shallow, with focal depths predominantly at or near 10 km; only a few events registered depths between 2 km and 13 km. Magnitudes ranged from 4.0 to 4.9, with the largest events (M 4.9) occurring on 22, 23, and 25 December.

Activity began immediately after the magnitude 7.4 shock, with 16 events recorded within the first six hours on 21 December. The following day saw the highest daily count, including multiple M 4.7–4.9 earthquakes clustered between 00:00 and 23:00 UTC. Seismicity remained elevated through 23 December before gradually declining, with the final event (M 4.5) marking the swarm’s termination on 26 December. The temporal distribution showed no clear migration of hypocenters, consistent with a localized stress adjustment following the mainshock.

Since 1 January 2000, only one swarm has been identified in this segment of the Bonin Islands region, making the 2010 sequence the sole documented example in the instrumental record. The proximity of the swarm to the M 7.4 mainshock suggests it represents aftershock activity modulated by post-seismic stress redistribution along the subduction interface and overlying crust.

This swarm illustrates how moderate-magnitude sequences can cluster tightly in both space and time within subduction-zone forearcs. The consistent shallow depths and narrow magnitude range indicate failure on small fault patches under uniform stress conditions rather than widespread rupture propagation. Continued monitoring of such episodes provides valuable constraints on the mechanics of plate-boundary deformation in the Izu-Bonin arc.

References

SeismoSight internal swarm catalogue S20101222.1
USGS Earthquake Catalog (events since 2000)
Global CMT Project focal-mechanism solutions for 21 December 2010 mainshock