Earthquakes

Earthquakes:  Terrible Trembling
 
Earthquakes are caused by plate movement underground.  The plates under the earth usually move slowly, but sometimes large pieces can get caught.  The plates keep trying to move, but large blocks of old rock hold them back.  The pressure and energy builds up.  Sudddenly, the rocks give way, releasing all the pressure and energy.  The plates jerk forward and the ground shakes.  Far above, people feel an earthquake.
 
Some earthquakes are small and harmless, or so small they are not even noticed.  Others can be so strong that buildings crumble, bridges collapse, and large cracks open up in the ground.  The strenght of an earthquake is measured with a special instrument called a seismograph (first invented by the ancient Chinese).  Each earthquake is given a
number on a special scale depending on its strength.  This number scale is called the Richter Scale.  There is no limit to the Richter Scale, but the highest ever recorded earthquake was a 9.5.  For each number on the Richter Scale, an earthquake is ten times as strong as the previous number.  A 6.0 earthquake is ten times stronger than a 5.0 and 100 times stronger than a 4.0.
 
Earthquakes have other effects on the Earth besides property damage.  Animal habitats can be destroyed.  Fires can be started with downed electrical lines or gas leaks.  Vegetation is then ruined. People can also be affected by earthquakes.  Obviously, it can ruin personal property like their house or car.  It can also cause anxiety or stress and even death. 
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