A notable seismic swarm designated PS20101221.1 occurred in the Bonin Islands, Japan region, from 17:19 on 21 December 2010 to 14:30 on 23 December 2010. Over this 45-hour period, 52 earthquakes were recorded, highlighting the dynamic tectonic environment of the area.
The swarm initiated with a magnitude 7.4 event at 17:19:40 on 21 December at a depth of 14 km. Subsequent activity included numerous events between magnitudes 5.0 and 6.4, with depths predominantly ranging from 2 km to 18 km. Notable later shocks comprised a magnitude 6.4 earthquake on 22 December at 21:49:40 (18 km depth) and several magnitude 5.5–5.6 events clustered in the initial hours. The sequence concluded with a magnitude 5.0 event at 14:30:40 on 23 December.
This activity aligns with the broader seismicity of the Bonin Islands, situated along the Izu-Bonin-Mariana arc system. Here, the Pacific Plate subducts westward beneath the Philippine Sea Plate at rates of approximately 4–6 cm per year, generating both shallow crustal events and deeper Wadati-Benioff zone earthquakes. The arc features volcanic islands and seamounts formed by this subduction, with the region experiencing recurrent moderate-to-large earthquakes due to plate interface coupling and intra-plate deformation.
Historically, the Bonin Islands have hosted significant seismic events, including deep-focus earthquakes exceeding magnitude 7. The 2010 swarm's proximity to a magnitude 7.4 mainshock underscores patterns of clustered aftershock-like activity in subduction forearc settings. Depths in the recorded data reflect a mix of shallow crustal and upper-plate seismicity typical of the overriding Philippine Sea Plate.
Such swarms provide insights into stress redistribution following large ruptures, often without a single dominant mainshock. Monitoring in this tectonically active zone supports improved understanding of subduction dynamics and regional hazard assessment.