The probability of strong earthquakes striking eastern Japan within the next 30 years has markedly increased, according to experts.
The government's Headquarters for Earthquake Research Promotion released a series of upgraded "National Seismic Hazard Maps" on Dec. 21 that chart the probabilities of ground motion reaching lower 6 or higher on the Japanese seismic intensity scale of 7 over the period.
The probabilities rose remarkably from the previous release in 2010.
Ground motions were estimated from long-term earthquake occurrence probabilities along 110 major active geological fault lines across Japan and along ocean trenches.
Shizuoka, the capital of Shizuoka Prefecture, had a 30-year probability of 89.7 percent, the highest of all 47 prefectural capitals, followed by Tsu, the capital of Mie Prefecture, at 87.4 percent and Chiba, the capital of Chiba Prefecture, at 75.7 percent.
In the Kanto region, the 30-year risk of ground motions of at least lower 6 hitting Mito, the capital of Ibaraki Prefecture, jumped 31.0 points to 62.3 percent.
The number increased dramatically because the new hazard maps took into account the increased aftershock activity south of the hypocenter of the Great East Japan Earthquake.
The probabilities also increased along the Pacific coast from the Tokai region to Shikoku.
Tonankai and Nankai earthquakes have occurred along the Nankai Trough, south of Japan every 100 to 150 years. Their occurrence probabilities have risen with the passage of time since the last one struck in 1946, officials said.
Some nuclear power plant sites were rated as having a high risk of strong ground motion. The probability was calculated at 95.4 percent for the site of the Hamaoka nuclear plant in Shizuoka Prefecture.
The probabilities also rose 34.1 points to 67.5 percent for the Tokai No. 2 nuclear plant in Ibaraki Prefecture and gained 27.2 points to 40.6 percent for the Fukushima No. 2 nuclear power plant in Fukushima Prefecture, south of the stricken Fukushima No. 1 nuclear plant.
The researchers said lower probabilities do not indicate a lack of major seismic risk. For example, the city of Sendai experienced ground motions of lower 6 and higher during the magnitude-9.0 Great East Japan Earthquake, although the previous 2010 release of the hazard maps had assigned a scant probability of 4.0 percent to Sendai.
"Lower probabilities do not guarantee safety," said Yoshimori Honkura, chairman of the earthquake research committee.
The Headquarters for Earthquake Research Promotion chose to not upgrade the maps last year, as it concentrated on reviewing risk evaluation methods after the Great East Japan Earthquake. But the headquarters decided to upgrade this year because the maps can be used for determining earthquake insurance premium rates, and for efforts by local governments to draw up disaster management plans.
The hazard maps are accessible online on "Japan Seismic Hazard Information Station," a website operated by the National Research Institute for Earth Science and Disaster Prevention (www.j-shis.bosai.go.jp/en/).
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