An earthquake is the trembling or shaking of the ground. Earthquakes are caused by a sudden release of energy stored in rocks beneath the surface. The release of that energy may cause one large mass of rock to slide past another large mass of rock into a different position. The crack formed between these rocks is called a fault. this explanation of why earthquakes take place is called the Elastic Rebound Theory.
When Rocks break, waves of energy are released through the earth. The waves of energy produced by an earthquakes are called seismic waves. Two types of seismic waves are body waves and surface waves. Body waves travel through the Earth's interior, spreading outward from the focus, the place where the wave originated, in all directions. Surface waves travel on the Earth's surface away from the epicenter. The epicenter is the point on the earth's surface directly above the focus.
There are two types of body waves, a P wave and an S wave. A P wave is a compressional wave in which rock vibrates parallel to the direction the waves are moving. P waves are very fast and are the first waves to reach the seismograph at a recording station when an earthquake occurs. The second type of body wave is called the S wave. This type of wave is caused by a shearing motion and travels very slowly. The rocks vibrate perpendicular to the direction the wave is traveling. Both P waves and S waves can pass easily through solid rock, but S waves can not penetrate a solid.
The time interval between first arrivals of P and S waves at a seismic station is used to determine the distance between the seismograph and the epicenter. A station records the P and S waves from a quake, then matches the interval between the waves to a standard time travel curve. By reading directly from the graphs from at least three stations, a scientist can determine where earthquakes occurred.
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