The Cabinet Office released on March 31 slightly lower death and disaster projections from an expected Nankai Trough megaquake than calculated a decade ago, but the numbers remain horrific and far short of the government's targets. 

The estimates indicate the potentially devastating earthquake, with an 80 percent chance of occurring within 30 years, would result in approximately 298,000 fatalities and 2.35 million buildings collapsing in a worst-case scenario. 

The expected damage and death toll have decreased only slightly compared to estimates from 10 years ago and remain far from meeting the government's disaster mitigation efforts. 

The expert panel assumed a magnitude-9 level earthquake originating in a sea area from Suruga Bay in Shizuoka Prefecture to the Hyuganada coast off Miyazaki Prefecture in the Kyushu region.

The panel then recalculated the seismic intensity, tsunami height and potential flood area using the latest soil and topographical data for the potentially affected regions.

The estimates have been updated for the first time since 2012 and 2013 when the government last released projections. The previous estimates indicated that the death toll would be 323,000 and the number of completely destroyed buildings would reach 2.386 million.

The government had designated 707 cities, towns and villages in Tokyo and 28 prefectures across the nation, including inland areas, as areas that must take countermeasures against the megaquake when the government issued an Nankai Trough megaquake advisory for the first time in August last year.

In the updated estimates that were released this time, 764 municipalities in Tokyo and 30 prefectures from Fukushima to Okinawa prefectures are said to be at risk of a seismic intensity of a lower 6 earthquake or stronger, or three-meter high or higher tsunami. The number of designated municipalities is likely to increase.

A tsunami reaching a maximum 34 meters in height would engulf Kuroshio and Tosashimizu in Kochi Prefecture.

Flooding of 30 centimeters or more, which will put lives at risk, will occur over a total area of approximately 1,151.5 square kilometers from Fukushima Prefecture to Okinawa Prefecture.

A tsunami of one meter or higher is expected to reach the cities of Shizuoka and Yaezu in Shizuoka Prefecture, and the towns of Taiji and Kushimoto in Wakayama Prefecture, within two minutes of the earthquake at the quickest.

The number of municipalities where a seismic intensity of 7 is expected to strike increased mainly in coastal areas from 143 to 149.

In addition, over these 10 years, the country’s situation has changed including a shrinking and aging population, the increasing number of high-rise condominiums and vacant houses as well as inflation.

These changes have led to additional risks, including the need for more assistance for those needing to evacuate and affected areas being cut off.

On the other hand, the rate of houses that have been earthquake reinforced increased from about 79 percent in 2008 to approximately 90 percent in 2023. The rate of seawalls constructed rose from about 39 percent in 2014 to about 65 percent in 2021.

The rate of municipalities where disaster prevention drills have been conducted increased from about 79 percent in 2018 to around 86 percent in 2024. These factors reduced the estimated damages that would result. 

DISASTER-RELATED DEATHS COULD HIT 52,000

Disaster-related deaths were also estimated for the first time, with a maximum of about 26,000 to about 52,000 based on the experiences of the 2011 Great East Japan Earthquake and the 2024 Noto Peninsula earthquake.

The government announced the basic plan for promoting disaster prevention measures for the Nankai Trough megaquake in 2014.

Using a different methodology from this time, it announced damage estimates of 332,000 deaths and 2.504 million completely destroyed buildings. Then, the government set a goal of reducing deaths by 80 percent and completely destroyed buildings by 50 percent over 10 years.

If the current estimates are calculated based on the same conditions as those used in 2014, the death toll would be 264,000 and the number of buildings completely destroyed would be 2.084 million, a reduction of only 20 percent, respectively.

After receiving the report, Manabu Sakai, the disaster management minister, said he would "take the results seriously," and told reporters that he would review disaster mitigation measures by around this summer. 

(This article was written by Shoko Rikimaru and Wataru Netsu.)