Seismic Swarm PS20081220.1: Analysis of Activity Offshore Iwaki, Japan

Seismic swarm PS20081220.1 occurred approximately 138 km east-southeast of Iwaki in Fukushima Prefecture, Japan. The sequence began at 10:29 on 20 December 2008 and concluded at 09:16 on 21 December 2008, spanning 22 hours and 47 minutes. During this interval, six earthquakes were recorded, with magnitudes ranging from 4.2 to 6.3.

The sequence opened with a magnitude 6.3 event at a depth of 19 km. Subsequent shocks included a magnitude 5.2 at 10 km depth, followed by a magnitude 5.1 also at 10 km. Two closely spaced events occurred around 19:11–19:13, registering magnitudes 4.2 and 5.4 at 10 km depth. The swarm closed with a magnitude 5.9 shock at 13 km depth.

This activity represents the sole swarm recorded in the region since 1 January 2000. No additional swarms have been identified in the internal classification up to the present.

The location lies along the Japan Trench subduction zone, where the Pacific Plate descends beneath the Okhotsk Plate at rates of approximately 8–9 cm per year. This tectonic setting produces frequent intermediate-depth and shallow seismicity. The offshore Fukushima area has a documented history of clustered events, often linked to stress transfer along the plate interface and within the overriding crust.

Regional geology features a well-developed accretionary prism and forearc basin sediments overlying the subducting slab. Historical records show elevated seismic rates east of Iwaki, with notable activity preceding larger regional ruptures. The 2008 swarm occurred three years prior to the 2011 Tohoku-Oki earthquake, whose epicenter lay farther north yet within the same subduction system.

Swarm sequences in subduction forearcs commonly reflect fluid migration or aseismic slip episodes rather than classic mainshock-aftershock patterns. The tight temporal clustering and limited magnitude range observed here align with such characteristics, though detailed source modeling remains outside the scope of this summary.

• 20 Dec 2008 10:29:23, magnitude 6.3, 19 km depth
• 20 Dec 2008 10:48:23, magnitude 5.2, 10 km depth
• 20 Dec 2008 17:37:16, magnitude 5.1, 10 km depth
• 20 Dec 2008 19:11:30, magnitude 4.2, 10 km depth
• 20 Dec 2008 19:13:38, magnitude 5.4, 10 km depth
• 21 Dec 2008 09:16:44, magnitude 5.9, 13 km depth

Continued monitoring of the Japan Trench remains essential given the persistent convergence and potential for both swarm-type and great earthquake activity.

References

SeismoSight internal swarm classification database.
Japan Meteorological Agency earthquake catalog.
USGS Earthquake Hazards Program regional tectonics summary.