71 Cities in Japan At ‘Very High’ Risk of 3-Meter-High Tsunami Within the Next 30 Years

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On January 24, the Earthquake Research Committee of the Japanese government issued a new report announcing the probability of a tsunami associated with an earthquake in the Nankai Trough (Southern Sea Trough) off Japan’s southeast coast in the near future.

The report states that the likelihood of a large tsunami of more than three meters high hitting areas along the Pacific coast of Japan within the next 30 years is “very high,” including impacts in the Tokai and Kyushu regions. 

Scientists hope that the report — which uses the newest tsunami probability data shared by the government for the first time — will contribute to better disaster prevention measures, such as maintaining and updating seawalls.

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Looking at More Likely Seismic Events to Calculate Risks

In 2012, the government announced the estimated height of a tsunami in the event of a giant 9.1 magnitude (M) megathrust earthquake. However, such large earthquakes are considered to be very rare, and the lack of past records made it impossible to calculate the probability of such an event.

The January 24 report sets aside the question of a rare megathrust earthquake and instead focuses on the probability of a tsunami caused by an earthquake in the range of magnitude 7.6 to 9.0, which is said to be likely to occur in the future. The probability was calculated for each of the 352 municipalities and 24 prefectures along the coast.

The studies show that there is a “very high” (more than 26%) probability of a major earthquake within the next 30 years followed by a tsunami of more than three meters high hitting the coast of 71 municipalities and 10 prefectures from the Tokyo, Tokai, Kinki, Shikoku, and Kyushu regions.

The Japan Meteorological Agency uses the three-meter mark as an indicator to announce major tsunami warnings. At this level, old wooden houses are likely to be washed away and the risk of human injury is higher. According to the committee, a probability of 26% within the next 30 years is equivalent to “once in 100 years.”

Naoshi Hirata, the chairman of the committee, emphasized the point, saying: “There is a very high chance that this (Nankai Trough earthquake and tsunami) will happen in our lifetime. Do not misunderstand this data as simple safety information, but take it as an opportunity to think about what you can do” to be prepared.

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Knowledge is Key to Survival

The report also revealed the “very high” (more than 26%) likelihood of an even more damaging tsunami of five meters or higher for 29 municipalities in seven prefectures, including Shizuoka, Mie, Wakayama, and Kochi.

The report did not find the same likelihood of 26% or more for a tsunami of 10 meters or higher for any region. Nevertheless, it assessed that there is still a “high” probability (6% to less than 26%) that such a 10-meter tsunami could occur, putting 21 municipalities in six prefectures at risk.

Researchers estimate the probability of an earthquake in the Nankai Trough within the next 30 years is as high as 70% to 80%. The probability of tsunami was calculated by analyzing past smaller-scale Nankai Trough earthquakes, together with data on the epicenters and areas where plates have collapsed over the years.

The committee hopes to further encourage countermeasures to strengthen community resilience against the risk of disaster through its research. To this end, within the next decade, it aims to sequentially calculate the probability of earthquake-caused tsunamis for each of the various regions throughout Japan.

(Click here to read the original report in Japanese.)

Author: The Sankei Shimbun