Seismic Swarm PS20030928.1: Analysis of Activity South of Obihiro, Japan

Seismic swarm PS20030928.1 occurred in the Tokachi region of Hokkaido, Japan, approximately 74 km south of Obihiro. The sequence began at 08:06 on 27 September 2003 and concluded at 23:24 on 29 September 2003, spanning 63 hours and 18 minutes. During this period, 13 earthquakes were recorded, with magnitudes ranging from 5.0 to 6.5 and focal depths primarily between 25 km and 52 km.

The swarm initiated with a magnitude 5.3 event at 52 km depth on 27 September at 08:06:20 UTC. Subsequent events clustered at shallower depths around 33 km, including additional magnitude 5.3 and 5.0 shocks later that day. Activity intensified on 28 September with a magnitude 5.6 event at 04:17:54, followed by further magnitude 5+ tremors. The peak occurred on 29 September at 02:36:53 with a magnitude 6.5 earthquake at 25 km depth. The sequence tapered off with several magnitude 5.3–5.6 events through the evening of 29 September, ending with a magnitude 5.0 shock at 23:24:25.

This swarm unfolded in a tectonically active subduction zone where the Pacific Plate converges with the Okhotsk Plate. The region experiences frequent seismicity due to thrust faulting along the plate interface. Depths of 25–52 km align with typical intermediate-depth activity in this margin. Historical records indicate this was the sole swarm detected since 1 January 2000, with no prior comparable clusters in the immediate area.

The swarm followed closely after major regional events on 25 September 2003: a magnitude 7.4 earthquake 119 km east-southeast of Shizunai-furukawachō and the magnitude 8.1 Tokachi-Oki mainshock 43 km from the swarm center. These larger ruptures likely influenced stress conditions that triggered the subsequent swarm through afterslip or dynamic triggering mechanisms.

Geological context reveals Hokkaido’s position along the southwestern extension of the Kuril Trench system. Long-term strain accumulation from plate convergence produces both megathrust events and associated swarm activity. Depths and magnitudes observed here reflect brittle failure within the subducting slab and overlying crust.

References

SeismoSight internal classification data for swarm PS20030928.1.
Geological Survey of Japan, AIST – Regional tectonic framework of Hokkaido subduction zone.
Japan Meteorological Agency – Earthquake catalog and historical event summaries.