Tsunamis

Afternoon, on the 11th of March, 2011, in the city of Sendai in Japan. It’s an ordinary day. Kei is at his office working. He has a few hours left of his workday. Suddenly, the building starts to rock.

The lamps swing back and forth, and a bookshelf falls over. Kei rushes out onto the street, just like many others. There is a major earthquake happening. The ground shakes for several minutes. As soon as it stops, Kei and some workmates run to his car and drive away.

They want to get as far from the coast as they can. They know the catastrophe might get worse in a short while. Out at sea is the centre of the earthquake, its epicentre. A long way below the surface, the seabed moves, and enormous masses of water are lifted several meters by the quake. When the water surface evens out again, waves occur.

The waves spread out in all directions. One wave now approaches the coast of Japan, minutes after the earthquake - which just shook Sendai and several other cities. So far, the wave is not visible with the naked eye. Although it is only one metre high, it is a hundred kilometres wide. The wave travels fast, at 800 kilometres an hour, toward the coast.

Closer to land, the ocean is shallower. The seabed and the shore slow the wave down. There isn't room for the water that the wave puts in motion. And when the water does not have room, it is pushed upwards. The wave becomes higher … and higher.

The wave builds to 10, 20, or even 30 metres high. It hits the coast with full power. Japan is struck by a giant wave, a tsunami. The tsunami grabs boats, cars, buildings… and people. No-one in the wave’s path stands a chance.

The power of the wave is massive. The tsunami sweeps along 600 kilometres of coastline, and floods large areas inland. Railways and roads, bridges, hospitals, and fields, electricity and water supply systems... most of what’s in the way of the wave is torn down. The water reaches as far as 1.5 kilometres inland in some places.

Then, gravity pulls it back to the sea again. The water that a short while ago was a wave moving inland, is now a wave moving toward the sea. It drags along the broken houses, cars, trains, debris, and people who have drowned in the wave. Thousands of bodies will never be found again. So, a tsunami can be created by an earthquake at the seabed, but also by volcanic eruptions or large landslides under water.

This is not something that humans can prevent, but we can do certain things to protect people, when the wave comes. One possibility is to build large walls to shelter inhabited areas. These can prevent water reaching people and buildings when the wave strikes. Another option is to place sensors on the seabed that sense when the ground is shifting -- when there is seismic activity. In this way, the sensors can sense volcanic eruptions and earthquakes.

They send a signal to a monitoring system. If the quake is a major one, and there is a risk of a large wave, a siren sounds, and messages are put out across all radio, TV and phones. A tsunami warning is issued. Now people know they need to get out of harm's way as quickly as possible, to evacuate. They move as fast as they can away from the coastline, or to elevated places the water can’t reach.

In Sendai, there were tsunami defense walls in place, but the wave was so powerful and high, it crashed over the walls and destroyed cities and villages. Sendai did have a warning system that activated. This allowed some people, like Kei and his workmates, to evacuate. But unfortunately many others did not have time to escape. About 18 000 people died in the catastrophe, and half a million homes were destroyed.

The 2011 tsunami was the worst Japan has ever experienced.