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Deadly tsunami could hit Tokyo 2020 sailing venue in 5 min. after massive quake


The Mainichi Shimbun has obtained a prefectural government tsunami simulation showing that most of the sailing venue for the Tokyo 2020 Olympic Games could be flooded to a depth of 1 meter -- a level said to produce a 100 percent death rate -- about five minutes after a massive temblor.

Sailing at the 2020 Games will be held at the Port of Shonan in the Enoshima district of Fujisawa, Kanagawa Prefecture, south of Tokyo.

Data earlier made available had indicated that a less powerful quake could produce tsunami waves to that depth at the port in roughly 6 to 8 minutes. Experts say measures are needed to counter the threat in response to the most recently uncovered simulation.

The simulation was for a temblor resembling the Genroku earthquake that struck near Edo (present-day Tokyo) in 1703. Among nine quake scenarios produced by the Kanagawa Prefectural Government, this type of quake would produce the biggest tsunami, arriving at the port in the shortest amount of time.

Under this simulation, it was envisaged that tsunami waves up to 7.3 meters in height could arrive in the Enoshima district seven minutes after a magnitude-8.5 quake occurred. The Mainichi Shimbun obtained the simulation information through a freedom-of-information request.

It is said that deaths start to occur when tsunami waves reach a depth of 30 centimeters, and that the death rate rises to 100 percent at 1 meter. Data shows that about 90 seconds after the quake -- the rough standard for the Japan Meteorological Agency to announce the intensity of a temblor -- much of the area from which boats leave and depart would be hit by tsunami waves reaching at least 1 meter in depth. Five minutes after the temblor, parts of an area where some spectators may be (area B in the diagram) would be hit by tsunami waves reaching 1 meter or higher.

A separate simulation based on a report of the 1923 Great Kanto Earthquake with a magnitude of 6.4 predicted that tsunami waves would take a few minutes longer to reach the area.

A temblor on the scale of the Genroku quake is said to occur once every 2,000 to 3,000 years, while a Great Kanto-type quake occurs once every 200 to 400 years. The Tokyo Organising Committee of the Olympic and Paralympic Games explained that based on the time between earthquakes, it made a realistic decision to formulate evacuation plans based on the Great Kanto quake scenario.

Organizers do not envisage a Genroku-type quake occurring. However, the nine earthquake scenarios, including the Genroku-type earthquake, were drawn up by the Kanagawa Prefectural Government in 2014 following the Great East Japan Earthquake and tsunami of 2011. They are requirements for formulating local disaster prevention plans and tsunami evacuation plans.

Fumihiko Imamura, a professor of tsunami engineering and director of the International Research Institute of Disaster Science at Tohoku University, commented, "If a whole country is coming together to hold the games under the banner of 'reconstruction ' (following the March 2011 earthquake and tsunami), then the organizing committee should properly formulate countermeasures."

Nobuaki Shimizu, a professor at Aichi Prefectural University who is versed in crisis management including evacuation from tsunamis, commented, "There was a problem from the outset with the organizing committee's evacuation plans based on the possibility of a Great Kanto-type quake. If a Genroku-type quake were to occur, evacuation would become even more difficult."

Besides sailing, there are five other sports that will be held in coastal areas during the games. Four of them -- triathlon, beach volleyball, canoeing and rowing -- will be held around Tokyo Bay, while the surfing competition will be held in Chiba Prefecture east of Tokyo. For the sports around Tokyo Bay, the fastest a tsunami would arrive would be roughly 86 minutes later, while it would take a tsunami around 33 minutes to reach the Chiba Prefecture town of Ichinomiya, where the surfing will be staged. This sets these venues apart from the sailing venue in the Enoshima district, where there would be much less time to evacuate.

For the triathlon event, efforts are underway in Japan to offer protection from a tsunami -- and these efforts have not been limited to the Olympic Games. Plans have been made to evacuate athletes and spectators at the ITU World Triathlon Yokohama in the city of Yokohama in May in the event of a tsunami, through such efforts as requesting assistance from the operators of buildings that stand at least three stories tall.

Tadahiko Kaneko, head of the ITU World Triathlon Yokohama secretariat, commented, "We formulated measures following the Great East Japan Earthquake disaster. If something happened, what would we do? It's the responsibility of the organizer to prepare and answer that question."

(Japanese original by Itsuro Goto, Special Reports Group)

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