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(1) In an earthquake, huge masses of rock move beneath the earth’s surface and cause the ground to shake. Earthquakes occur constantly around the world. Often, they are too small for people to feel at all. Sometimes, however, earthquakes cause great loss of life and property. Most earthquakes are caused by changes in the earth’s outermost shell, or crust.

(2) The crust is made up of about a dozen rock masses called plates that are constantly moving. In different places, they move apart, collide, or slide past each other. Over time this movement causes great pressure to build up. When the pressure becomes too great, the rock masses suddenly shift along a crack in the crust, called a fault. The shifting rocks release energy in the form of shock waves. The waves spread through the rocks in all directions, causing an earthquake.

(3) A small number of earthquakes have other natural or human causes. The movement of molten rock beneath a volcano can trigger an earthquake. The pressure resulting from holding vast amounts of water behind large dams can cause minor quakes. So can underground nuclear explosions and the digging of mines.

(4) An earthquake may be powerful enough to change the surface of the Earth, thrusting up cliffs and opening huge cracks in the ground. Earthquakes can do great damage to buildings, bridges, railroads, and other structures. The violent shaking during earthquakes often causes other earth movements, such as avalanches. Some quakes that happen in or near oceans cause huge, destructive waves called tsunamis to sweep ashore.

(5) Scientists record the shock waves produced in an earthquake with an instrument called a seismograph. They measure the strength of a quake in different ways. The Richter scale measures earthquakes based on the amount of energy they release. The weakest earthquakes are close to zero on the scale, and the strongest measure about 9. The Mercalli scale measures the amount of destruction caused by an earthquake on a scale of 1 to 12.