Seismic Swarm S20101222.2 in the Bonin Islands Region

An earthquake swarm designated S20101222.2 occurred in the Bonin Islands region of Japan between 17:45 UTC on 21 December 2010 and 16:10 UTC on 26 December 2010. Over 118 hours and 24 minutes, the sequence produced 86 events, all recorded at shallow focal depths between 5 km and 13 km with magnitudes ranging from 3.9 to 5.0. No single mainshock dominated the activity; instead, events occurred in temporal clusters, characteristic of swarm behavior driven by fluid migration or aseismic slip along faults rather than a classic foreshock-aftershock sequence.

The Bonin Islands, also known as the Ogasawara Islands, lie approximately 1,000 km south of Tokyo on the Izu-Bonin-Mariana volcanic arc. This intra-oceanic arc formed through subduction of the Pacific plate beneath the Philippine Sea plate at rates of 4–6 cm per year. The tectonic setting produces frequent shallow seismicity and active submarine volcanism, including the nearby Nishinoshima and Fukutoku-Oka-no-Ba volcanoes. Historical records document recurrent earthquake swarms and moderate events linked to arc-normal extension and slab dehydration processes at depths less than 20 km.

Within the swarm, the first 24 hours on 21–22 December accounted for the highest event rate, with 38 earthquakes between magnitudes 4.0 and 4.9. Activity continued at a lower rate through 23–25 December before a final pulse on 26 December that included the two largest events, both of magnitude 5.0. Depths remained consistently shallow, with only three events deeper than 11 km. Such patterns align with known swarm dynamics in subduction-related extensional regimes, where pore-pressure changes facilitate distributed failure without a clear magnitude progression.

Since 1 January 2000, only one swarm of comparable character has been identified in the same region according to internal SeismoSight classification, underscoring the relative rarity of this style of clustered seismicity compared with isolated mainshock sequences elsewhere along the arc.

SeismoSight internal records provided swarm parameters and event list. Tectonic framework derived from standard geophysical literature on the Izu-Bonin arc.