What are tsunamis?

Tsunamis are powerful, enormous waves caused by earthquakes under the sea. Tsunamis can race across the ocean at up to 500mph (800km/h) and may be up to 30m tall when they break. They can destroy whole towns and cities by carrying bits of cars, boats and even buildings, and slamming them into whatever gets in their way.

Tsunamis often occur in the Pacific Ocean because it is surrounded by The Ring of Fire, a collection of fault lines that run around its edge. Two of the biggest ever recorded were in Indonesia in 2004 (sometimes called the Boxing Day Tsunami) and in Japan in 2011.

external image tsunami-formation.gif external image japan-tsunami.jpg

Early warning systems

Some places in the world have systems to warn people when there is an earthquake that could cause a tsunami. They are usually places that are both near the sea and near fault lines. There are two types of early warning system: International systems monitor earthquakes around the world while regional systems just watch out for earthquakes nearby.

When there is an earthquake, there are two types of shockwave (these are like ripples in the earth's surface, not waves on the water): P-waves travel very quickly through the ground but don't do much damage, while S-waves travel more slowly but do more damage. Early warning systems measure the P-waves so they can give people warning that an S-wave (and possibly a tsunami) is coming. This will give them time to get into a safe place.

In Japan, people are sent text messages on their phones and newsflashes appear on the TV.