Lost in Japan: How to defend against tsunamisDr. Jörn Lauterjung, who develops tsunami early warning systems at the German Research Centre for Geosciences, tells us more about these monster waves.
Jörn Lauterjung: This tsunami was triggered by an 8.9-magnitude earthquake around 250 miles (400 kilometers) from Tokyo, off the coastal city of Sendai.
The tremor itself was a result of the Pacific tectonic plate submerging beneath the North American and Eurasian tectonic plates.
I always compare this with a flat spring as they are used for utility vehicles. What basically happened was that the submerging plate, our flat spring, couldn’t stand the stress anymore.
When it cracked, the upper part of the Pacific plate rebounded, thereby rising the sea level by up to 4 meters along a 100 to 150 kilometer-long fracture zone. This resulted in a wave that travelled at high speed through the ocean towards the shore.
How big was this tsunami compared to the Indian Ocean tsunami in 2004?
Even if we talk of the Japan earthquake as one of the four or five strongest quakes ever recorded, the energy set free by the Sumatra tremor was even higher in terms of sea level displacement. The Japan tsunami reached a wave height of about 10 meters. The one which hit Indonesia surged by up to 25 meters.
But it’s not only the height of the wave that is so dangerous. It’s the wave length of 200 meters and more that unfolds the deadly impact as this huge volume of water enriched with debris, cars, ships and even parts of houses crashes into shore. The chances of surviving in this water are close to zero.
What the 2004 tsunami exposed was that there was no warning system in place. Unlike in the Pacific Ocean, there was no real precedent for a tsunami like this in the Indian Ocean.
Of course that changed immediately afterwards. Structures were developed to deal with this kind of natural force. National Tsunami Warning Focal Points (TWFPs) were established, local authorities were trained in disaster management, which led to a higher awareness on the part of the people.
Escape routes from beaches now point out where to evacuate in case of emergency: parking garages, hotels and other public houses can be used as shelter.
In some areas with no natural hills and higher buildings even Tsunami Evacuation Raised Earth Parks (TEREPs) have been established. I would say that if another tsunami similar to the one of 2004 happened far less people would perish.
Speaking of tsunami warning systems, did the Japanese one fail?
Not at all. The Japanese warning system is known as one of the best in the world. They have a very dense net of seismometers. They know that they face one of the highest tsunami risks in the world.
Three minutes after the first shock a major tsunami warning was issued. The only problem was that the wave hit the coastline just 15 minutes later. It was just not enough time for evacuation.
There is very little you can do against a 10 or 20-meter tsunami. The best protection is nature itself. This was the lesson to be learned from the Indian Ocean tsunami. Coral reefs provide a good defence. They weaken the tsunami on their way ashore. If the coral reefs off the coast of Sri Lanka had not been removed by humans, the impact would have been less disastrous.
Of course you can think of artificial control structures, like breakwaters made out of concrete. But these would be enormous building works raising the question of cost and benefit.
Statistically speaking, a tsunami of this size happens once every 100 to 150 years. For the Indian Ocean this natural disaster only occurs once every 400 to 600 years.