Kushiro, Japan Earthquake

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Posting Date: November 29, 2004, 9:00:00 AM

On Sunday, November 28, 2004 at 03:32 a.m. local time (18:32 UTC), a strong earthquake centered near the town of Kushiro shook Japan’s northernmost prefecture of Hokkaido. The U.S. Geological Survey (USGS) has estimated the magnitude of the quake at 7.0 and the focal depth at about 43 km. The epicenter was located about 55 km east of Kushiro, and about 930 km north-northeast of Tokyo. The quake was felt as far away as Yokohama, more than 950 km to the south of the epicenter. This event occurred in the Kuril (Chishima) subduction zone in northeastern Japan where the Pacific plate subducts under Hokkaido at a rate of about 8.5 cm/yr. The Chishima Trench is offshore to the southeast and has been the location of frequent large earthquakes. Large earthquakes occur both along the plate interface (the 1952 magnitude 8.2 and 1962 magnitude 7.9 earthquakes) and within the subducting Pacific plate (the 1993 magnitude 7.8 and the 1994 magnitude 8.1 events). Earthquakes occurring along the plate interface release seismic energy that has built up along the interface as the two plates slide past each other. In the Chishima trench off Hokkaido, three earthquakes of an approximate magnitude 8.0 ruptured the plate interface in each of the last two centuries. Earthquake mechanism studies of the current event indicate that it was likely an interface earthquake occurring along the lower portion of the plate interface. A similar earthquake occurred in 1961 in the same area. Seismologists in the Earthquake Information Center (EIC) of Japan inferred that Sunday’s event is basically a repeat of the 1961 event, releasing all the strain energy accumulated since 1961 in this part of the plate interface. Minimal damage has been reported, though as many as thirteen people were reported to be injured in the quake, according to local authorities. It cut electric power to about 480 households in the towns of Teshikaga and Shibecha in eastern Hokkaido. Rail service was temporarily suspended as officials inspected tracks for damage. In Nemuro, one of the three cities nearest the epicenter, the local government temporarily urged about 13,000 households to evacuate. Cracks were discovered in the city’s docks and at least two schools had broken windows and cracked water pipes. Using the available information about the seismological parameters of this event, the AIR earthquake team has run the Japan earthquake model. Although this was a relatively shallow event and one of significant magnitude, it occurred in an area of relatively sparse population and insured losses are therefore expected to be minimal.

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Posting Date: November 29, 2004, 9:00:00 AM