Tsunamis

After the devastating Tsunami that has hit many parts of Asia and east Africa on Boxing Day we thought we would give you an in depth look at Tsunami’s and just how destructive they can be.

What is a Tsunami?

A tsunami is a natural phenomemon that generates a series of waves when the water in a lake or the sea is rapidly displaced on a massive scale.

All sorts of other natural disasters can trigger a tsunami such as earthquakes, landslides, volcanic eruptions and even meteorites can generate tsunamis which can devastate coastlines.

The term tsunami comes from the Japanese language and means Harbour Wave.

Causes

A tsunami can be generated by any disturbance that displaces a large amount of water such as the natural disasters listed above. But the most common cause is an undersea earthquake.

The earths crust sits on a number of moving plates that are constantly moving together and pulling apart. When these plates make a big move everything sitting on top of them is moved too, whether it is water or land.

Small undersea landslides can also cause tsunamis but often these are on a much smaller scale and rarely affect coastlines. But large displacements caused by exceptionally large landlides or impacts can cause mega tsunamis that are far larger and potentially much more devastating than an ordinary tsunami. The largest known in recent times occurred in Lituya Bay, Alaska in 1958 after a massive landslide and the waves were estimated at 100 feet high.

Characteristics of a Tsunami

Tsunamis move at high speeds and act quite differently from typical surf swells. They can travel great distances with little energy loss. They can cause damage thousands of kilometres away from their origin so there could be a delay of several hours between when they originate and when they impact with the coastline.

Tsunamis way out to sea are often not noticed. They can be like a small swell of only a metre high so ships may not even notice them. The energy of the tsunmai passes through the entire water column to depths of 4000m or more, unlike surface waves which typically reach only 10m or so.

The waves can travel across the sea at speeds from 500 to 1000kmh. The second or third waves may not arrive for up to an hour after the first one.

As the wave approaches land, the sea shallows and the wave no longer travels quickly so it begins to pile-up. The wave becomes steeper and taller. So while it only reaches 1m high out to sea it can build to over 30m as it nears shallow water.

As it gets closer to shore the sea will recede from the beaches. If the slope of the beach is shallow the sea can recede up to 800m. This is the first warning sign and it can be from 5-10 minutes before the tsunami hits the beach.

As it approaches the shore it doesn’t look like a normal wave you would see on a beach but looks more like an on-rushing tide – hence why they are sometimes called tidal waves. Most of the damage is caused by the huge amount of water behind the first wave.

The sheer weight of water is enough to devastate everything in it’s path, often reducing buildings to their foundations and to carry large objects like ships, several miles inland.

Prevention and Warnings

Some countries and coastlines are more prone to tsunamis than others. In Japan they have built tsunami walls up to 4.5m high in front of populate coastal areas. Other places have built floodgates and channels to direct the water away when a tsunami hits.

These measures are questioned though as to their effectiveness as waves can be three times the height of the barriers. But they may succeed in slowering down and moderating the height of a tsunami.

Other prevention systems include the use of tsunami warning systems which forecast tsunamis and warn the general population. On the west coast of the United States, which is prone to Pacific Ocean tsunamis, warning signs advise people where to run in the event of an on coming tsunami.

Donations and Help for the Asian Tsunami Relief effort

The Asian Tsunami has had absolutely devastating affects throughout the Indian Ocean. Aid is needed in all these countries. Below are a few sites that are set up to provide information and handle donations for victims throughout the region or you can contact your local aid agencies. Our thoughts are with everyone who has been affected.

 

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