Tokyo’s exposure to Tsunami, explained

Following the events of 3/11, those living in Tokyo should be asking, “How vulnerable is Tokyo to flooding by tsunami?” — should a seismic event the same size as 3/11, or larger, occur near Tokyo Bay. To answer this question, Beacon Reports spoke to coastal engineer Hiroshi Takagi, Associate Professor at the Tokyo Institute of Technology.

3/11’s magnitude Mw 9 earthquake occurred 400 km northeast off Miyagi Prefecture. The resultant tsunami devastated the Japanese coast from Tōhoku to Kanto, with water levels rising as high as 40 meters. Lesser known, is that the tsunami entered Tokyo Bay, where water levels rose on average by .9 meters. The Sumida river swelled by 1.5 meters. Overflow and run-up heights of 2.91 meters and 2.84 meters were recorded, respectively, in the Shin-Futtsu Fishery Port and the Funabashi Fishery Port.

According to Takagi, Tokyo Bay is vulnerable to a tsunami originating from the Tōkai region, which is located 100 – 150 km southwest of the Bōsō Peninsula.

Major fault lines run along the stretch of the Nankai, Tōnankai, and Tōkai regions (known as the Nankai trough). Tōkai earthquakes have occurred, on average, every 100 – 150 years. The last Tōkai earthquake struck in 1854, 158 years ago. According to Takagi, scientists believe another Tōkai earthquake could happen at any time; the level of risk is high. There are, respectively, 50% and 40% chances that a Tōnankai and Nankai earthquake will occur within the next 30 years. But, says Takagi, “scientists now believe that all three earthquakes could occur simultaneously,” as happened in the Hōei earthquake of 1707, where 5,000 people died. How would Tokyo Bay be affected should this occur?

Earthquakes are caused when subducted plates slip, causing one of the plates to spring violently upwards.

That lifts the column of water above the slippage. The deeper the ocean, the more energy that is released.

The depth of the ocean along the Nankai trough averages 3,500 meters, which is very deep. The energy released by lifting a 3,500 meter column of water is enormous. That energy then ripples outward from the epicenter.

Now, here’s the physics: As a tsunami moves towards an ever-shallowing shoreline, the seismic energy has nowhere to go, but up. Hence, although its speed decreases, a tsunami will increase in height as it approaches the shoreline. In the below video demonstration of wave shoaling, the tsunami travels from right to left. The green line represents an ever-shallowing shoreline. The blue line represents a tsunami.

 

Note how high the tsunami (blue line) reaches, as it approaches the shore (green line).

The depth of the water outside the southwestern facing mouth of Tokyo Bay averages 500 meters. On the Tokyo Port side of the bay, the average depth is only 20 – 30 meters. Seismic energy, originating in a 3,500 km column of water — the result of a future Nankai trough seismic event — would be subject to concentration by wave shoaling and wave refraction as it moves towards Tokyo Bay, through the mouth, and finally into the bay itself.

Further, the southwestward facing mouth of Tokyo bay is V-shaped. V-shaped bays concentrate seismic energy as would a funnel. At its widest, this funnel measures 19 km in width, between the southernmost tip of the Bōsō Peninsula and the Miura Peninsula. Seismic energy entering that funnel would be concentrated into an opening measuring 9 km at its narrowest point, before being released into Tokyo Bay.

Takagi is concerned about what will happen to the seismic energy once it enters the bay. This energy is effectively trapped. Depending upon the topography of the bay and coastal structures, the energy can be reflected, refracted, deflected, or concentrated.  According to Takagi, most of the energy will follow the line of least resistance by heading straight towards Chiba. Some of that energy, however, would be deflected. The deflection is caused by the differential rates of shallowing along the shoreline. Each concentric ring on this overlaid bathymetric map of Tokyo Bay marks a one-meter decrease in water depth. This topographic feature would cause some seismic energy to bend towards Tokyo Port. Both Tokyo Port and Chiba are therefore at increased risk of flooding.

Tokyo-Bay-Diversion-Effects2

 

Vertical coastal structures such as flood barriers are also of concern to Takagi. Vertical walls reflect seismic energy. According to Takagi, tsunami wave heights are amplified by a factor of two, where it hits head-on a vertical wall or floodgate (excluding the splash). This video shows how seismic energy is amplified when hitting a vertical wall head-on:

 

Reflected energy, depending on where it is focused, can concentrate and put other areas at increased risk. For instance, a tsunami traveling along rivers and canals can concentrate at the end of waterways, where docked boats are then easily washed away. Canal endpoints are particularly at risk, as there is nowhere for the water to go, other than up.

Takagi pointed out that low-lying delta areas, such as in Koto Ward, are already below sea level. Many areas around Tokyo Bay are vulnerable to subsidence due to liquefaction as well. Subsidence of up to 3 meters was recorded in the Great Hanshin-Awaji Earthquake of 1995. All of the above puts Tokyo Bay, and those residing nearby, at risk.

How high does Takagi think that waters could reach within Tokyo Bay? Takagi says scientists believe that earthquakes are getting larger. Researchers are only beginning to take these factors into consideration in their modeling. Should a large earthquake occur off Tokyo Bay, Takagi says the consensus amongst researchers is that average tsunami heights of 2 – 3 meters might be reached. This translates into a height of 4 – 6 meters, should a tsunami hit a vertically walled structure at right angles. But then, he adds, “these are very rough predictions.” Nobody knows for certain.


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