Seismic Swarm PS20030925.1: Analysis of the 2003 Hokkaido Sequence

The seismic swarm designated PS20030925.1 occurred 78 km southeast of Obihiro in Hokkaido, Japan, spanning from 19:50 on 25 September 2003 to 20:38 on 26 September 2003. Over this 24-hour and 48-minute period, 14 earthquakes were recorded, forming part of the aftershock sequence following the great 2003 Tokachi-Oki mainshock.

The sequence began with an 8.1 magnitude event at 27 km depth. Subsequent events clustered predominantly at 33 km depth, including magnitudes of 5.5, 5.3, 5.6, 5.7, 5.5, 7.4, 5.4, 5.0, 5.6, 5.2, 5.3, 5.9, and a final 6.0 magnitude shock. This pattern reflects typical aftershock decay following a megathrust rupture, with the largest aftershock reaching 7.4 magnitude approximately 82 km from the swarm centroid.

Hokkaido lies at the convergent margin where the Pacific plate subducts beneath the Okhotsk plate at rates of 8–9 cm per year. The 2003 Tokachi-Oki earthquake (M 8.1) ruptured a 200 km segment of the plate interface, releasing strain accumulated since the previous great event in 1952. The swarm’s location aligns with the southeastern edge of this rupture zone, where afterslip and triggered slip on adjacent fault patches produced the observed cluster.

The region has a well-documented history of great earthquakes, with recurrence intervals of 100–150 years along the Kuril Trench. Paleoseismic records and geodetic data confirm that the 2003 event filled a seismic gap identified after the 1952 Tokachi-Oki earthquake. Post-2003 monitoring shows ongoing viscoelastic relaxation and episodic tremor, consistent with the subduction zone’s long-term behavior.

The provided swarm parameters indicate a compact spatial distribution at intermediate depth, typical of intraslab and interface aftershocks in this tectonic setting. No surface rupture was associated with the swarm itself, though the mainshock generated a modest tsunami that affected the Pacific coast of Hokkaido.

References

USGS Earthquake Catalog
Japan Meteorological Agency Strong Motion Database
Geological Survey of Japan, AIST – Active Fault and Seismic Hazard Maps