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21 Japan municipalities at risk of Nankai Trough tsunami of 10m or more within 30 yrs: data

In this Sept. 2, 2019 file photo, the coastline in the town of Kuroshio, Kochi Prefecture in western Japan, one of the areas said to be at risk of being hit by a tsunami triggered by a Nankai Trough earthquake, is seen from a Mainichi Shimbun aircraft. (Mainichi)

TOKYO -- Tsunamis at a height of at least 10 meters could hit 21 Japanese municipalities along the Pacific coast at the probability of 6-26% in the event of a Nankai Trough temblor within 30 years, according to estimates released by the government's Earthquake Research Committee on Jan. 24.

In the latest estimates, 26% is equivalent to "once in 100 years," whereas 6% is equivalent to "once in 500 years," according to the committee. It warns that although the chances are very high for municipalities in the category "26% or more," those in the "less than 6%" category are by no means safe, and people need to be on their guard.

The research committee estimated that there is a 26% or higher chance of waves measuring at least 5 meters high hitting the coast of 29 municipalities in prefectures near the predicted epicentral area, including Shizuoka, Mie, Wakayama and Kochi prefectures on Japan's southeast coast.

Furthermore, 71 municipalities located over a wide area between Shizuoka and Miyazaki prefectures face a 26% or higher chance of being struck by waves measuring at least 3 meters high. The probability of waves measuring 10 meters or higher striking the coast of 21 municipalities in Shizuoka, Mie, Wakayama, Kochi and other prefectures ranges from 6-26%.

The latest government estimates on the anticipated arrival of tsunamis are the first to be accompanied by probabilities. Using the same method, the committee plans to estimate tsunami damage from earthquakes that occur around the Japan Trench, the Chishima Trench and the Sea of Japan.

The committee focused mainly on big quakes that are likely to occur repeatedly every 100-200 years, rather than tremors in the very largest class.


A megaquake registering magnitude 8-9 is said to have a 70-80% chance of happening along the Nankai Trough, a submarine trench off Japan's Pacific coast, within 30 years. The government's Central Disaster Management Council and other institutions announced the estimated damage of a top-class Nankai Trough earthquake registering magnitude 9.1 in 2012. According to their estimation, the town of Kuroshio and the city of Tosashimizu in Kochi Prefecture will be slammed with waves measuring up to 34 meters in height, while waves measuring between 20-30 meters will strike along the Pacific coast from the Tokai to the Shikoku region. Researchers say a massive Nankai Trough quake would leave 231,000 people dead in a worst-case scenario.

The research committee, however, did not take such megaquakes into consideration in the latest research, on the assumption that such quakes "are rare" as they haven't occurred over the past 2,000 years.

Instead, the likeliness of tsunamis reaching municipalities was based on records of temblors with 8-9 magnitudes that have occurred since 1361, including the 1707 Hoei Earthquake, which had an estimate magnitude of 8.6. The committee calculated the probability of waves measuring at least 3 meters, 5 meters or higher or at least 10 meters reaching the coast of 352 municipalities that are said to be at risk of being hit by a Nankai Trough tsunami within 30 years. The probability rate was separated into three categories: "26% or more," "from 6% to under 26%," and "less than 6%."

As wave heights vary depending on factors including geographical features, the committee divided some municipalities by districts, and provided multiple figures for a single municipality where necessary.

According to the Japan Meteorological Agency, 3-meter waves can sweep away wooden homes, and the number of completely destroyed buildings rises sharply when waves measure 5-6 meters.

Committee head and University of Tokyo professor Naoshi Hirata commented, "We would like people to know that (earthquakes), even if not the largest class, have a high chance of triggering a tsunami that can cause great damage."

(Japanese original by Mayumi Nobuta, Science & Environment News Department)

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