Earthquakes  

The Earth has a crust under the oceans and the land that we live on.  This crust is made of massive areas of flat rock called tectonic plates, which float on the Earth’s mantle, the inside layer of the Earth. 

Earthquakes

Where these plates meet is called a fault. The picture above shows the Earth’s tectonic plates and how they fit together like a jigsaw. 

When the plates move slowly together, this movement forces energy through the crust to the earth’s surface.  

The energy causes the earth to tremble and shake. These tremors are called an earthquake. 

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Any shaking of the Earth’s crust is an earthquake.  Some earthquakes are very small and can only be detected by sensitive instruments.  Other earthquakes are huge and cause massive destruction and loss of life. 

The strength of earthquakes are measured by an instrument called a seismograph.  This word is taken from the Ancient Greek words, seismos, which means earthquake, and graphein, which means writing.  The study of earthquakes is called seismography. 

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Instruments for investigating earthquakes have existed from very early times.  A Chinese scientist called Choko invented an instrument in AD 136 which recorded earthquakes and the direction from which the shock came. 

Modern seismographs can record the strength of an earthquake very accurately.  Seismographs are also known as seismometers (meaning earthquake-measurer). 

The way in which an earthquake develops is rather like what you see if you throw a stone into water.  When the stone hits the water, a series of waves move out in circles.  We call these concentric waves, meaning they are moving out from the centre in circles. 

An earthquake works in the same way.  There is a sudden movement between tectonic plates and concentric shock waves move outwards from that movement.  The centre point of the earthquake is called the epicentre. 

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Earthquakes are measured on a scale called the Richter scale.  The numbers on the Richter scale measure factors of 10, which means that an earthquake which measures 5 on the Richter scale is 10 times bigger than one that measures 4. 

·        Charles Richter developed the Richter scale for measuring earthquakes in 1935. 

·        Earthquakes measuring 2 and under on the Richter scale can only be detected by seismometers.  We cannot feel these small tremors at all.  These very small earthquakes are called microquakes and are happening all the time. 

·        Earthquakes measuring between 2 and 6 on the Richter scale are classed as moderate.  They may cause minor damage. 

·        Earthquakes measuring 7 and above on the Richter scale can cause terrible damage and be disastrous.   

·        The largest recorded earthquake was in Chile in 1960 and measured 9.5 on the Richter scale. 

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The Earth is shaken by at least one powerful earthquake every day.  Fortunately most of these take place in areas far from towns and cities.  Many of them take place under the sea. 

Undersea earthquakes are not always harmless, however.  An earthquake under the sea can cause a terrible tidal wave called a tsunami, which can sweep onto the land destroying everything in its path. 

Indian Ocean tsunami

 

·        Indian Ocean tsunami, 2004. In December 2004 an undersea earthquake with an epicentre off the west coast of Indonesia caused a tsunami with waves of up to 95 feet. Over 230,000 people were killed in 14 different countries. 

Japanese tsunami

 

·        Japanese tsunami, 2011. In March 2011 there was an earthquake under the sea just off the Japanese coast.  The earthquake measured 8.9 on the Richter scale and created a giant tsunami.  Both the shock waves of the earthquake and the tsunami caused terrible damage in Japan and killed more than 20,000 people.