M6.6 Honshu, Japan Earthquake of 23 October 2004

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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 at some distance from the plates' boundaries. The 23 October 2004 earthquake had a thrust fault mechanism and occurred within the Okhotsk plate, about 350 km west of the Japan Trench, where the Pacific plate subducts beneath the over-riding Okhotsk plate. Within the past 30 years, there have been several significant earthquakes in the vicinity of this earthquake, including a June 1964 magnitude 7.4 earthquake (about 125 km to the north) that killed 36 people and an April 1995 magnitude 5.4 shock (about 90 km to the north) that injured at at least 39 people. During the 20th century, Japan suffered nine devastating earthquakes that 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 magnitude 6.9 Kobe earthquake that killed 5,500 people.