Earthquakes usually happen on the edges of large sections of the Earth's plates. These plates slowly move over a long period of time. Sometimes the edges, which are called fault lines, can get stuck, but the plates keep moving. Pressure slowly starts to build up where the edges are stuck and, once the pressure gets strong enough, the plates will suddenly move causing an earthquake.
Generally, before and after a large earthquake there will be smaller earthquakes. The ones that happen before are called foreshocks. The ones that happen after are called aftershocks. Scientists don't really know if an earthquake is a foreshock until the bigger earthquake occurs.
Shock waves from an earthquake that travel through the ground are called seismic waves (地震波). They are most powerful at the center of the earthquake, but they travel through much of the earth and back to the surface. They move quickly at 20 times the speed of sound.
Scientists use seismic waves to measure how big an earthquake is. They use a device called a seismograph to measure the size of the waves. The size of the waves is called the magnitude.
To tell the strength of an earthquake scientists use a scale called the Moment Magnitude Scale or MMS (it used to be called the Richter scale). The larger the number on the MMS scale, the larger the earthquake. You usually won't even notice an earthquake unless it measures at least a 3 on the MMS scale. Here are some examples of what may happen depending on the scale:
4. 0— Could shake your house as if a large truck were passing close by. Some people may not notice.
5. 0—If you are in a car, it may shake. Glasses and dishes may rattle. Windows may break.
6. 0— Items will fall off shelves. Walls in some houses may crack and windows break. Pretty much everyone near the center will feel this one.
7. 0— Weaker buildings will collapse and cracks will occur in bridges and on the street.
8. 0— Many buildings and bridges fall down. Large cracks in the earth.
9. 0 and up— Whole cities flattened and large-scale damage.
