2003 Tokachi earthquake

Tectonic setting

The location and moment tensor solution of this earthquake are consistent with it being a result of thrust faulting between the North American plate and the subducting Pacific plate. In addition to experiencing large thrust earthquakes that originate on the interface between the plates, eastern Hokkaidō experiences great earthquakes that originate from the interior of the subducted Pacific plate.

The region experienced a catastrophic earthquake and tsunami with an estimated magnitude of 9.0 a few decades before 1667, a magnitude 8.2 event in 1952, a 1968 quake measuring 8.3 , and one in 2008 measuring 7.1, all bearing the name Tokachi-Oki, and a 1973 event to the immediate north along the Kuril Trench plate boundary.

Earthquake

With a magnitude of 8.0, the earthquake occurred as the result of shallow megathrust faulting on or near the plate interface between the overriding North American Plate and the subducting Pacific Plate. According to a finite fault model provided by the United States Geological Survey (USGS), earthquake rupture was recorded on an area of around 240 km × 210 km with a majority of the slip being concentrated on a 120 km × 115 km patch of the torn subduction interface and a maximum slip of 7.746 m.

The earthquake had a maximum intensity of Shindo 6- on the Japan Meteorological Agency seismic intensity scale, which was observed in Kushiro, Tokachi and Hidaka Subprefectures. It also measured IX (Violent) on the Modified Mercalli intensity scale. By 31 December 2003, there were 181 aftershocks measuring 4.0 or higher. The largest of them measured 7.4 and struck at 21:08 UTC. At 09:06 UTC on 8 October, a 6.7 aftershock occurred near Kushiro.

Damage and casualties

Despite the earthquake's great intensity, structural damage to the region was comparatively light; the epicenter was located nearly 100 km offshore, with most structures in its vicinity reported to be resistant to earthquake shaking. The majority of the destruction was confined to coastal areas, such as sea and fishing ports, mostly inflicted by subsequent tsunami waves. Although soil liquefaction was observed over a broad geological area, it occurred in localized areas almost exclusively limited to man-made embankments. The earthquake affected a total of 36 local rivers, including the major Abashiri and Ishikari Rivers. Monetary losses in Hokkaido amounted to at least ¥213 billion (2003 JPY), or $1.9 billion (2003 USD). One person died after being hit by a car while cleaning up earthquake damage. The tsunami swept away two fishermen in the Tokachi River near Toyokoro, killing one and leaving the other missing.

The earthquake and tsunami destroyed several oceanside home communities and damaged many others. Over 1,500 houses or buildings, most of them in Kushiro city, suffered considerable damage, with of a total of 141 reported to be partially or completely destroyed. Strong shaking affected many bridges in the region, some sustaining severe damage due to relative motion between spans in excess of design standards. The center of the Rekifune Bridge, located in Taiki, Hiroo, was reported to have sunk about 0.12 m (0.39 ft) at the joint section following significant ground deformation. Some local schools were also damaged, ranging from shattered windows to severed expansion joints and columns. Two town halls in Kushiro and Taiku suffered partial collapses.

At Kushiro Airport, the tremor caused the control tower ceiling to collapse, prompting officials to halt control work for several days. Small cracks were reported in the gates of the Takami Dam, though no threat of dam failure existed.

Several sea ports in the area sustained moderate damage, such as cracks and collapsing walls, due to lateral ground spreading caused by liquefaction. Some 123 coastal fishing ports and facilities in eastern Hokkaido reported significant damage, with an additional 25 ports damaged in Iwate. Tsunami waves stranded several small boats onshore; various ship containers and oil tanks along coastlines sustained damage. The earthquake left marine oil spills in its wake, though the conditions were quickly normalized.

References

References

1. {{cite anss. M 8.2 - 134 km SSW of Kushiro, Japan. 2003. usp000c8kv
2. S. F. A. Rafeeqi and S. H. Lodi. (April 2004). "Earthquakes Cause Damages in Pakistan, Iran, Japan and Morocco". Cowasjee Earthquake Study Centre.
3. AIST. (December 10, 2004). "Transient Uplift After a 17th-Century Giant Earthquake in Hokkaido".
4. "かがくナビ > 読む > 科学よみもの > 地球科学現象の解明に役立つ地震研究".
5. ISC. (2022). "ISC-GEM Global Instrumental Earthquake Catalogue (1904–2018)". [[International Seismological Centre]].
6. (26 September 2003). "Earthquake information". [[Japan Meteorological Agency]].
7. (2004). "The tsunami source area of the 2003 Tokachi-oki earthquake estimated from tsunami travel times and its relationship to the 1952 Tokachi-oki earthquake". Earth, Planets and Space.
8. "USGS earthquake catalog". [[United States Geological Survey]].
9. {{cite anss. M 7.4 - 119 km ESE of Shizunai-furukawachō, Japan. 2003. usp000c8mb
10. {{cite anss. M 6.7 - 39 km SSE of Kushiro, Japan. 2003. usp000c9x7
11. EQE Japan Division. (2003). "The 2003 Tokachi-oki Earthquake of September 26". [[American Bureau of Shipping]].
12. (November 2003). "EERI Newsletter Volume 37, Number 11". [[Earthquake Engineering Research Institute.
13. National Geophysical Data Center / World Data Service (NGDC/WDS). (1972). "Significant Earthquake Database". [[National Geophysical Data Center]], [[National Oceanic and Atmospheric Administration.
14. link. Kiso-Joban Company. (2003)
15. (26 September 2003). "Strong quake hits north Japan". BBC News.
16. "平成15年（2003年）十勝沖地震". [[Japan Meteorological Agency]].
17. link. [[Ministry of Land, Infrastructure, Transport and Tourism (Japan). MLIT]]. (2003-10-03)
18. link. (2003-09-26). [[Cabinet of Japan. Cabinet Office, Government of Japan]]