Tsunami: why Japan studies them

In short

How a tsunami starts

When a large earthquake happens under the seabed, the fault moves and the seabed rises or sinks. The sea surface moves with it, and big waves spread out in every direction. This is a tsunami.

There is an old saying that the sea always pulls back before a tsunami. This is not always true. Sometimes a big wave comes first.

Fast in deep water, tall near land

A tsunami travels faster where the sea is deeper. Far out at sea, it is as fast as a jet plane.

As the water gets shallower, it slows down. The back of the wave catches up with the front, and the wave gets taller.

Even when it slows down, people cannot outrun it. If you feel an earthquake near the coast, or a tsunami warning is issued, get away quickly, even if you cannot see a tsunami.

The largest tsunami in Japan

The tsunami from the earthquake of March 11, 2011 ran up to a height of 40.1 m at Ryori Bay in Ofunato, Iwate Prefecture. This is the largest tsunami recorded in Japan.

Before that, the largest was the Meiji Sanriku tsunami of 1896, about 38.2 m, also in Ofunato.

A tsunami can also come from far away. The tsunami from an earthquake in Peru in 2007 crossed the Pacific Ocean and reached Japan more than 20 hours later.

Think about it

• Q1Why does a tsunami grow taller as it comes near the land?
• Q2Why should people leave even when they cannot see the tsunami?
• Q3Find the nearest high place to your home or school.

There is no single right answer. Talking it over with someone at home is a good way to learn.