What is a tsunami and what causes them? – The Sun

TSUNAMIS are incredibly powerful natural disasters leaving destruction in their wake.

The terrifying wave can decimate towns but what is a tsunami and how can you survive one?

What is a tsunami?

A tsunami, also called a seismic wave, is a series of waves caused by the movement of a large body of water.

Most of the time they are caused by earthquakes at the boundaries of tectonic plates, deep under water.

The movement of the plates cause a dramatic reaction in the water above which result in large waves.

Seemingly harmless waves can sometimes only be 30cm high in the open ocean, so go unnoticed by sailors.

But as it reaches shallower waters, the wave is slowed and the top of it moves faster than the bottom, causing the sea to dramatically rise.

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This wall of water can be strong enough to push boulders and collapse buildings, destroying entire areas on the coast.

Also called tidal waves, tsunami means "big wave in the port" in Japanese – coined by fishermen after they returned to shore to find their villages devastated by a giant wave they had not seen at sea.

Tsunamis can cause the sea levels to rise by as much as 30 metres, although they usually cause a rise averaging three metres.

The enormous energy of a tsunami can lift boulders, flip cars and plough down buildings.

Lituya Bay, Alaska was home to the largest ever tsunami in 1958, which rose to 1720ft (51metres) above sea level after an earthquake.

While this was a ginormous wave, it only claimed the lives on 2 people.

The deadliest tsunami was the 2004 Indian Ocean earthquake – this series of continuous tsunamis claimed 230,210 lives.

It caused devastation in ten countries and was initially 108ft (33metres) tall.

How is a tsunami formed?

A tsunami can be formed in a number of different ways but usually there are three things that have to happen.

An earthquake must measure at least 7.0 on the Richter scale, this moves the water with enough force to build the tsunami wave at sea.

Secondly the sea bed must be lifted or lowered by the earthquake, this is often where the earth's tectonic plates meet which allows the movement.

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Finally, the epicentre of the earthquake must be close to the Earth's surface, meaning the quake can impact things on the surface rather than deep in the earth's crust.

Earthquakes, volcanic eruptions, underwater explosions, landslides, meteorite impacts and other disturbances above or below water can potentially generate a tsunami.

While normal waves are caused by the winds as well as the Moon and the Sun, a tsunami is always caused by the displacement of a large body of water.

The term tidal wave is technically incorrect as tsunamis are not impacted by the tidal pull at all.

As the wave builds, travelling towards land, the height builds from the faster movement at the top of the wave.

This continues to pull in water until it crashes, unleashing destruction in its path.

Retreating sea water on the coast is one of the major warning signs that a tsunami is about to hit, although it only gives a warning of about five minutes.

How to survive a tsunami?

Be ready

If you are travelling to an area known as a tsunami hotspot be prepared.

Have an emergency kit packed and easily accessible in case of an occurrence.

In your emergency kit you should have food, water, climate-appropriate clothing, and if possible a small first aid kit.

However its essential that you pack light as you never know when you’ll need it and for how long you will need to carry it.

Know that it's coming 

There are three signs that you can use to know when a tsunami is coming

• shakes and tremors underfoot
• the water recedes
• if you hear a loud roar from the ocean

It is also vital that you are alert to any announcements and warnings made by local authorities.

Many tsunami prone areas will have an early warning system in place so may be able to warn you.

Evacuation

It is very important that you listen to any guidance that governments and local authorities give you.

If they say to evacuate – then evacuate.

Abandon any unnecessary belongings and get away from the danger zone.

Tsunami hazard zones usually have signs to direct you to safety.

These are not normal waves and even the strongest swimmer will not be able to hold their own in the water.

For those already in the water, grab on to something that floats like a tree trunk or a raft.

If you're in a boat, face the direction of the wave and head out to sea, get as far away from the coast as possible.

Get to high ground

If the tsunami has been brought on by an earthquake, make sure you protect yourself and hold on while the quake hits.

As soon as it is safe to move, go to higher ground. 

High ground is the safest place to be during a tsunami. 

Avoid downed power lines and weakened bridges and overpasses. 

If you are outside of a tsunami hazard zone, stay where you are.

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Authorities will let you know when it is safe to return.

Do not wade into the floodwater without professionals as it may have debris and could be deeper than it appears.

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