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**Vocabulary**
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 * Focus** - The point beneath Earth's surface where rock breaks under stress and causes an earthquake.
 * Epicenter** - The point on Earth's surface directly above an earthquake's focus.
 * P waves** - A type of seismic wave that compresses and expands the ground.
 * S waves** - A type of seismic wave that moves the ground up and down or side to side.
 * Surface waves** - A type of seismic wave that forms when P waves and S waves reach Earth's surface.
 * Seismograph** - A device that records ground movements caused by seismic waves as they move through Earth.
 * Magnitude** - The measurement of an earthquake's strength based on seismic waves and movement along faults.
 * Mercalli scale** - A scale that rates earthquakes according to their intensity and how much damage they cause.
 * Richter scale** - A scale that rates seismic waves as measured by a particular type of mechanical seismograph.
 * Moment magnitude scale** - A scale that rates earthquakes by estimating the total energy released by an earthquake.

**Outline**
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Seismic Waves

 * During an earthquake, seismic waves race out from the focus in all directions.
 * Seismic Waves are vibrations that travel through Earth carrying the energy released during an earthquake.
 * Seismic waves carries the energy of an earthquake away from the focus, through the Earth's interior, and across the surface.
 * There are three categories of seismic waves: P waves, S waves, and Surface waves.
 * Primary Waves (P waves)
 * P waves are the first wave to arrive.
 * P waves are earthquake waves that compress and expand the ground.
 * P waves cause buildings to contract and expand.
 * Secondary Waves(S waves)
 * After P waves comes S waves.
 * S waves are earthquake waves that vibrate from side to side as well up and down
 * They shake the ground back and forth.
 * When S waves reach the surface, the shake structures and buildings violently.
 * S waves cannot move through liquids like P waves can.
 * Surface Waves
 * When S and P waves reach the surface, some of the waves are transformed into a seismic wave called surface waves.
 * Surface Waves move more slowly than P and S waves, but the produce more severe ground movements.

Detecting Seismic Waves

 * To recurd and measure the vibrations of seismic waves, geologists use instruments called seismographs.
 * Untill recently, scientists use mechanical seismographs.
 * Today, geologists use electronic seismographs.

Measuring Earthquakes

 * There are three types of ways to measure earthquakes: The Mercalli Scale, the Richter Scale, and the Moment Magnitude Scale.
 * The Mercalli Scale
 * Early in the 20th century, the Mercalli scale was developed th rate earthquakes according to their intensity.
 * The Mercalli Scale is not a precise measurement.
 * The Richter Scale
 * The Richter Scale is a rating of the size of a seismic wave mueasured by a particular type of mechanical seismograph.
 * The Richter scale was developed in the 1930's.
 * Geologists use this scale all over the world for about 50 years.
 * The Richter Scale provides accurate measurements for small, nearby earthquakes.
 * The scale does not work well for large or distant earthquakes.
 * The Moment Magnitude Scale
 * Today, geologists still use this scale.
 * This scale can be used to measure /rate earthquakes of all sizes, near or far.
 * Earthquakes with a magnitude below 5.0 on the scale can cause small and little damage.

Locating the Epicenter

 * Geologists use seismic waves to locate an earthquake's epicenter.


 * Seismic waves travel at different speeds.
 * P waves comes first
 * S waves comes second.(close behind)
 * The farther away an earthquake is, the greater the time between the arrival of the P and S waves.
 * Geologists then draw 3 circles using data from different seismographs set up at stations all over the world.





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