Megaquake risk along Pacific coast higher than anticipated

December 20, 2014

Megaquake risk higher for areas along Japan's Pacific coast

http://ajw.asahi.com/article/0311disaster/quake_tsunami/AJ201412200028

THE ASAHI SHIMBUN

The likelihood of a catastrophic quake striking within 30 years was higher in areas along the Pacific coast, including densely populated parts of the Kanto region, according to a national earthquake research body.

The probabilities were included in a revised ground motion probability map released by the Headquarters for Earthquake Research Promotion on Dec. 19.

The new map is based on the latest earthquake projections and results of underground geological studies nationwide.

Several prefectural capitals in the region showed high probabilities of being hit by an earthquake with a seismic intensity of at least lower 6 on the Japanese scale of 7. The area around Yokohama showed a 78 percent probability--the highest--with 73 percent for Chiba, and 70 percent for Mito and Kochi. Tokyo's rating was 46 percent.

Percentages were higher for the Kanto region and southern Hokkaido compared with methods used in the past, while many other locations showed only differences of a few percentage points from the tentative survey in 2013.

The figures were calculated using 250-meter square units of land.

Many areas along the Pacific coast of the main Honshu island, such as Shizuoka, showed a high quake risk as they are situated along the Nankai Trough, the subterranean source of an expected megaquake. The 4-kilometer depression stretches 700 kilometers from Suruga Bay off Shizuoka Prefecture to areas southeast of Kyushu in southern Japan.

After the Great East Japan Earthquake in March 2011, changes were made to the way estimates are calculated in order to account for earthquakes that tend to be excluded from the realm of possibility.

Yoshimori Honkura, the chairman of the earthquake research committee under the Headquarters for Earthquake Research Promotion, said: "The probability map contains some uncertainties. Low probabilities do not mean the area is stable."

The map can be accessed at "Japan Seismic Hazard Information Station" (J-SHIS) website operated by the National Research Institute for Earth Science and Disaster Prevention (http://www.j-shis.bosai.go.jp/en/).