KYTL+2.2

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Vocabulary

 * Focus**-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
 * Seismic** **wave**-vibration that travels through Earth carrying the energy released during an earthquake
 * P** **wave**-type of seismic wave that compresses and expands the ground
 * S** **wave**-type of seismic wave that moves the ground up and down or side to side
 * Surface** **wave**-when P and S waves reach the surface some are transformed into these
 * Seismograph**-device that records ground movements caused by seismic waves as they move through Earth
 * Magnitude**-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 provides accurate measurements for small nearby earthquakes
 * Moment** **magnitude** **scale**- scale that rates earthquakes by estimating the total energy released by an earthquake

Summary

 * earthquakes always begin in rock below surface

Seismic Waves

 * carry the energy of an earthquake away from the focus through Earth's interior, and across the surface
 * three categories:
 * Primary Waves
 * a.k.a. P Waves
 * causes buildings to contract and expand
 * travels through both solid and liquid
 * Secondary Waves
 * a.k.a. S waves
 * when reached surface, it shakes structures violently
 * cannot travel through liquid but can travel through solid
 * Surface Waves
 * when P and S waves both reach the surface, some of them transforms into Surface Waves

Detecting Seismic Waves

 * a mechanical seismograph includes a heavy weight attached to a frame by a spring or wire
 * the pen (connected to the weight) rests its point on a rotating drum
 * the electronic seismograph converts ground movements into a signal that can be recorded and printed

Measuring Earthquakes

 * at least 20 different measures for rating an earthquake
 * the Mercalli Scale
 * rate earthquakes according to its intensity
 * earthquake's intensity is the strength of its ground motion in a given place
 * same earthquakes can have different ratings because it causes different amounts of damage at different places
 * the Mercalli Scale is developed early in the twentieth century
 * Richter Scale
 * developed in 1930s
 * provides accurate measurements for small, nearby earthquakes
 * doesn't work very well if the earthquakes are large and/or far away
 * Moment Magnitude Scale
 * can be used to rate earthquakes of all sizes near or far

Locating the Epicenter

 * use seismic waves to locate/find the epicenter
 * seismic waves travel at different speeds
 * P waves arrives before the S waves
 * scientists measure the difference between the arrival time of P waves and S waves to tell how far away the epicenter is from the seismograph
 * the farther away an earthquake is, the greater the time between the arrival of P and S waves
 * geologists draw __at least__ 3 circles
 * center of each circle is the seismograph's location
 * radius of each circle is the distance from epicenter
 * point where each circle intersect is epicenter

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