Location:

Magnitude:

8.1

Time:

25 Sep 2003 19:50:06

Depth:

27.0

M 7.0+:

M 7.4; 119 km ESE of Shizunai-furukawach?, Japan; (25 Sep 2003) (26km from the earthquake)

There are 3 swarms found nearby.

2003

PS20030925.1(56.8km)

25 Sep

1 day 0 hours

14 earthquakes

PS20030928.1(43.3km)

27 Sep

2 days 15 hours

13 earthquakes

2004

PS20041114.1(12.9km)

14 Nov

1 hours

5 earthquakes

AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

The 2003 Tokachi-Oki Earthquake and Regional Seismicity

The Tokachi region of southeastern Hokkaido, Japan, lies along the Kuril Trench subduction zone, where the Pacific Plate converges with and subducts beneath the Okhotsk Plate at rates of approximately 8–9 cm per year. This tectonic setting produces frequent megathrust earthquakes capable of generating strong ground shaking and tsunami across the Pacific margin. The area has a well-documented history of large events, including the 1952 Tokachi-Oki earthquake (M 8.2) and the 1968 event (M 8.3), both of which originated near the same segment of the plate interface.

On 25 September 2003 at 19:50 local time, an M 8.1 earthquake struck at a depth of 27 km. The mainshock epicenter was located directly offshore from the Tokachi coast. A closely associated M 7.4 aftershock occurred the same day, centered 119 km ESE of Shizunai-furukawachō and approximately 26 km from the mainshock. These events ruptured a portion of the plate boundary that had previously slipped in 1952, releasing accumulated strain in a classic example of subduction-zone seismicity.

Geological studies of the region highlight the presence of an accretionary prism and forearc basin sediments that amplify seismic waves in coastal lowlands. The 2003 sequence produced peak ground accelerations exceeding 1 g at some stations and triggered landslides in the Hidaka Mountains. Post-event analysis confirmed that the rupture propagated along the plate interface with a duration of roughly 40–50 seconds, consistent with the behavior of other great earthquakes in the Kuril-Kamchatka arc.

Seismic monitoring since 2000 has recorded these two events as the strongest in the immediate vicinity, underscoring the ongoing seismic hazard. The subduction zone remains capable of producing future M 8+ earthquakes, with recurrence intervals estimated in the range of 50–100 years for this particular segment.

References

United States Geological Survey Earthquake Catalog (event parameters for 25 September 2003)
Japan Meteorological Agency seismic records
Geological Survey of Japan, AIST: Hokkaido subduction zone reports