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Science Why a Sea of Japan quake jolts the Pacific coast


Unswerving cyclist
14 Mar 2002
Those of you living in Kanto might have felt yesterday's quake at 17:37. It registered a magnitude of 6.1, but the stronger jolts were all felt along the Pacific coast. Here's why:

According to the Japan Meteorological Agency (JMA), an earthquake hit the center of the Sea of Japan at around 5:37 p.m. on Sept. 29. The temblor, which struck at a depth of 400 kilometers, registered a 3 on the 7-point Japanese seismic intensity scale over a wide area on the Pacific Ocean side, from the northernmost prefecture of Hokkaido to the Kanto region in the east.
Although the quake was centered in the Sea of Japan, the areas that observed 2 and 3 on the seismic intensity scale were mostly on the country's Pacific side. Experts say this phenomenon is called "anomalous seismic intensity," where an earthquake's jolts are transmitted along tectonic plates and can be felt far from the epicenter.

According to Takashi Furumura, professor at the University of Tokyo's Earthquake Research Institute, a plate boundary exists on the eastern side of the Japanese archipelago, where the oceanic Pacific plate subducts under the continental plate. The Pacific plate stretches to areas below the Sea of Japan, and the Sept. 29 earthquake is believed to have occurred within this plate. The jolt was propagated through the plate, resulting in shakes across a wide area from Hokkaido to east Japan's Kanto region. The Pacific plate reportedly tends to transmit seismic waves without weakening them.


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