Seismic 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 in the western Pacific Ocean. This archipelago forms part of the Izu-Bonin-Mariana volcanic arc, created by the subduction of the Pacific Plate beneath the Philippine Sea Plate at rates of 4–6 centimeters per year. The tectonic setting produces frequent seismicity, with earthquakes occurring along the trench, within the downgoing slab, and in the overlying crust. Depths typically range from shallow crustal events near 10 kilometers to deeper events exceeding 100 kilometers along the Wadati-Benioff zone.

The region has a long history of seismic activity linked to ongoing plate convergence. Major historical events include the 1896 Sanriku earthquake and numerous intermediate-depth shocks associated with slab dehydration. Volcanism in the arc, exemplified by islands such as Chichijima and Hahajima, adds to the geological complexity through magma-related fracturing that can influence local stress fields.

In December 2010, a notable earthquake swarm was recorded in this setting. The sequence, designated S20101222.4, began at 19:13 on 21 December and concluded at 06:38 on 24 December, spanning 59 hours and 24 minutes. During this period, 37 earthquakes were registered, with magnitudes between 4.1 and 4.9 and focal depths predominantly around 10 kilometers. The events clustered tightly in both space and time, characteristic of swarm behavior where no single mainshock dominates.

The sequence initiated with a 4.7 magnitude event and quickly intensified, featuring multiple 4.8 magnitude shocks within the first hours. Activity persisted through the night and into the following days, with additional peaks including two 4.9 magnitude events on 22 December. Magnitudes remained moderate throughout, and the swarm tapered off by the morning of 24 December with final events of 4.6 magnitude.

Such swarms in subduction zones often reflect fluid migration or aseismic slip that transiently alters fault stresses without producing a large rupture. In the Bonin region, similar patterns have been observed in association with slab-related processes. Historical statistics indicate that only two swarms have occurred in the area since 1 January 2000, underscoring the relative infrequency of this type of clustered activity.

Analysis of the 2010 swarm reveals a rapid onset followed by sustained elevated rates for roughly two days, after which seismicity returned to background levels. Depths remained consistently shallow, suggesting involvement of the upper plate or the shallowest portion of the slab interface. No damage or tsunami was reported from these moderate events, consistent with their magnitudes and offshore location.

Continued monitoring of the Izu-Bonin arc remains essential given its proximity to populated areas and potential for larger subduction earthquakes. The 2010 swarm provides a useful case for understanding stress transfer mechanisms within this tectonically active margin.

References

USGS Earthquake Catalog
Japan Meteorological Agency Seismic Data
Global CMT Project
Geological Survey of Japan Reports