Tokyo Bay at risk to tsunami

Sagami TroughOn December 31st, 1703 an 8.2 magnitude earthquake along the Sagami Trough triggered a tsunami that rippled towards Japan’s then capital, Edo. Six thousand five hundred people are thought to have drowned, with another 4,000 lives lost to fire and shaking. How at risk is Tokyo Bay from future tsunami? The issue is of importance if not only because active development on reclaimed land in Tokyo Bay, where the new Tsukiji market is located and the Olympic Village will be built, puts structures and lives at possible risk.

Japan’s Earthquake Research Center Committee estimates there is between a zero to 5% chance that a similar sized earthquake will occur within the next 30 years along the Sagami Trough. It also predicts a 70% chance of a smaller magnitude 7 earthquake happening by then.

Rest assured, the Tokyo 2020 Press Office told Beacon Reports that the topography of Tokyo Bay, with its narrow and curved entrance, naturally protects Tokyo from tsunami. They also assured that Tokyo Metropolitan Government’s up-to-date ‘Plan for the development of protective coastal infrastructure in the Tokyo Bay’ includes comprehensive anti-tsunami measures, such as seawalls, floodgates and breakwaters.

Box2Tokyo Bay is S-shaped. The mouth separating the inner from the outer bay is only nine kilometers apart at its narrowest point. These should offer some protection.

Yet other people believe that the bay’s V-shaped mouth, known as the Uraga Channel, concentrates tsunami energy. As it moves into the funnel towards an ever-shallowing shoreline, the energy has nowhere to go but up. Rather than offering natural protection, they believe the V-shaped topography puts structures and lives at increased risk.

We turned to coastal engineer Hiroshi Takagi, Associate Professor at Tokyo Institute of Technology, to determine which of these two natural topographic factors has the greater effect over the other. Kindly, he modeled one of four seismic event scenarios considered by the Tokyo Metropolitan Government in their most recent assessment dated April 2012. He chose to simulate the 8.2 magnitude Genroku-type Kanto Earthquake of 1703 which created the largest tsunami, reaching 2.51 meters at Chuo-ku where the Olympics Village is planned.

Takagi found that inner bay water heights rose to levels consistent with those estimated by the Tokyo Metropolitan Government. More intriguingly, he found that Tokyo Bay’s S-shaped curve and narrow mouth substantially mitigates funneling effects. The shallow waters of the inner bay also dissipate tsunami energy through increased friction with the sea floor. The bay’s topography therefore offers inner bay protection from tsunami, as claimed.

But Professor Robert J. Geller, a seismologist at The University of Tokyo, is not so reassured. Scientists, he notes, have only 2,000 years of tsunami data to base predictions on. Yet the Japanese islands separated from the Eurasian continent about 15 million years ago. The earth itself is 4.6 billion years old. Because of what little data we have, “I’m not sure what the 2.51 meter figure proves,” says Geller.

Takagi admits that by changing the epicenter of the assumed earthquake, even by a little, tsunami heights almost doubled at some locations in the inner most part of the bay.

What scientists do know is that Japan sits at the boundary of four tectonic plates. About 20% of the world’s earthquakes happen here. We also know that bigger quakes happen less often than smaller ones. There is no maximum seismic event — the sky is the limit.

Accurately forecasting earthquakes with today’s knowledge is probably impossible. “Some scientists and government officials were claiming the so called ‘Tokai’ quake was imminent in 1978 but it never happened, whereas other large quakes like Tohoku in 2011 have,” notes Geller.

He compares earthquake hazard assessment with papal indulgences of the Middle Ages: The client pays an expert, who in turn creates specifications for building structures capable of withstanding seismic events the client finds tolerable. “Then, if a bigger quake or tsunami hits — it’s just, “Sorry, sotei gai (想定外),” he says.

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