Seismic Swarm Activity East of Kushiro, Japan, November 2004

Eastern Hokkaido, Japan, lies within a highly active subduction zone where the Pacific Plate descends beneath the Okhotsk Plate. This tectonic setting produces frequent earthquakes, including both isolated large events and episodic seismic swarms. The area approximately 50 km east of Kushiro has experienced recurrent swarm activity linked to stress adjustments along the plate interface and within the overriding crust.

SeismoSight recorded swarm PS20041129.1 beginning at 18:32 on 28 November 2004 and concluding at 08:47 on 29 November 2004. Over the 14-hour 15-minute period, five earthquakes were detected. The sequence opened with a magnitude 7.0 event at 39 km depth, followed four minutes later by a magnitude 5.9 shock at the same depth. On 29 November, a magnitude 5.0 event occurred at 41 km depth, succeeded within seconds by a magnitude 5.2 earthquake at 100 km depth. The swarm closed with a magnitude 4.0 event at 48 km depth.

This swarm coincided with a separate magnitude 7.0 earthquake located 51 km southwest of Nemuro and roughly 14 km from the swarm centroid. Historical records maintained by SeismoSight indicate that only three swarms have occurred in the region since 1 January 2000, with two episodes in 2003 and one in 2004. Such swarms typically reflect localized strain release rather than foreshock sequences preceding a single large rupture.

The broader geological history of eastern Hokkaido includes repeated great earthquakes generated by the Kuril Trench subduction system. These events have shaped both the landscape and seismic hazard profile of the region, with ongoing monitoring essential for distinguishing swarm behavior from mainshock-aftershock sequences.

References
SeismoSight internal classification records for swarm PS20041129.1
USGS Earthquake Catalog for regional tectonics and historical context