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Earthquake Hazards Program

Tectonic Summary

Saturday, October 23, 2004 at 08:56:00 UTC

Preliminary Earthquake Report
U.S. Geological Survey, National Earthquake Information Center
World Data Center for Seismology, Denver

The Islands of Japan lie on a ring of seismically active plate boundaries that surround the Pacific Ocean known as the Ring of Fire. Earthquakes throughout Japan are caused by the relative motion of several major and minor tectonic plates including the Pacific plate, the Philippine Sea plate, the Okhotsk plate, and the Amur plate. Earthquakes result from slippage on the interface between the plates and on faults some distance from the plate's boundaries. This thrust earthquake, of October 23rd 2004, occurred within the Okhotsk plate about 350 km west of the Japan Trench where the Pacific plate subducts beneath the overriding Okhotsk plate. Within the last 30 years, there have been several significant earthquakes in region nearby this earthquake. These include a June, 1964 magnitude 7.4 quake that occurred 125 km to the north and killed 36 people and an April, 1995 magnitude 5.4 quake that occurred 90 km to the north and injured at least 39 people. Since 1900, the country of Japan has suffered nine devastating earthquakes that have killed more than 1000 people each. These include the 1923 magnitude 7.9 earthquake that triggered the great Tokyo fire and killed 143,000 people and the more recent 1995 6.9 Kobe earthquake that killed 5,500 people.


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