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An underwater earthquake is the most

common cause of a tsunami, but not

every earthquake causes a tsunami.
The earthquake must measure at least

7.0 on the Richter scale to cause a
tsunami.

But why do these underwater
earthquakes happen? They happen

because some areas of the earth’s crust

push into each other and build up pressure. The pressure builds and builds until the
areas under the earth’s crust releases all the energy in the form
How does an underwater
earthquake cause a tsunami? of an earthquake. When the underwater earthquake is large

enough, the floor of the ocean makes movements, therefore
the water above the sea floor also moves. The water is then pushed upwards and this

becomes the ‘tsunami wave’. It travels at very high speeds of upto 600 miles per hour.
The wave has plenty of room to move but when it gets near the shore the wave starts to

build in height because the water has nowhere to travel. The first wave that can grow

upto 30 metres high at the beach will usually be followed by more waves that are even
more dangerous. Eventually, the wave hits the shore and then travels inland. However,

the wave settles down and all the water that came onto land from the tsunami moves
back into the ocean. Unfortunately, such large amounts of water can cause loss of lives

and property, if people are not warned of the danger.

But then how do we identify a tsunami?

The difference between a tsunami wave and a normal wave is the extreme distance
between one wave crest to the next wave crest. When a tsunami wave approaches the

land, the water will be pulled back into the sea by enormous currents. If the people on

the beach notice the water on the sea-bed getting drained, they will have a few minutes
to escape to higher ground.

And do you know that a young girl noticed the early signs of a tsunami and warned all

the people on the beach? But how did she know of it? She had learnt
What is one way to
all about it in her geography class! identify a tsunami?

104 Arrow-English Plus-5