RLNI+2.2

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Vocabulary

 * Focus**: point beneath earth's surface where rock that's under stress breaks, causing an earthquake
 * Epicenter**: point on the surface directly above the focus
 * Seismic Waves**: vibrations that travel through earth carrying the energy released during an earthquake
 * P Waves**: earthquake waves that compress and expand the ground like an accordian
 * S Waves**: earthquake waves that vibrate from side to side and up and down
 * Surface Waves** : move slower than P and S waves, but they make the most severe ground movements
 * Seismograph**: records ground movements caused by seismic waves as they move through earth
 * Mercalli Scale**: developed to rate earthquakes according to their intensity
 * Richter Scale**: rates the size of seismic waves as measured by a particular type of mechanical seismograph
 * Moment Magnitude Scale**: rating system that estimates the total energy released by an earthquake

Seismic Waves

 * three types of seismic waves
 * Primary waves (P waves)
 * earthquake waves that compress and expand ground
 * Secondary waves (S waves)
 * earthquake waves that vibrate from side to side and up and down
 * surface waves
 * move slower that P and S waves but produces the most severe ground movements

**Detecting Seismic Waves**

 * seismograph records ground movements caused by seismic waves
 * electronic seismograph converts ground movements into a signal that can be recorded or printed

**Measuring Earthquakes**

 * magnitude measures earthquake strength based on seismic waves and movement along faults
 * mercalli scale
 * rates earthquakes according to their intensity
 * developed in twentieth century
 * earthquakes's intensity is the strength of its ground motion in a chosen place
 * richter scale
 * rates the size of seismic waves measured by a particular type of mechanical seismograph
 * developed in 1930s
 * accurate for nearby or small earthquakes
 * isn't very accurate if earthquake is far or large
 * moment magnitude scale
 * used to rate earthquakes of all sizes

**Locating the Epicenter**

 * geologists locate an earthquake's epicenter using seismic waves
 * seismic waves travel at different speeds
 * P waves arrive before S waves
 * scientists measure the difference between the arrival time of P and S waves to find out how far the epicenter is from the seismograph
 * the greater the time difference is with P and S waves, the farther away an earthquake is
 * geologists draw at least three circles using data from seismographs
 * center of each circle is a particular seismograph's location
 * radius of each circle is the distance from the seismograph to the epicenter
 * point where the three circles intersect is the location of the epicenter