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Earthquakes and Seismic Waves Key Terms: Earthquake: Is the shaking that results from the sudden movement of rock along a fault. Focus: Is the area beneath Earth's surface where rock that is under stress breaks, triggering an earthquake. Epicenter: The point on the surface directly above the focus. Introduction: Earth is never still.Everyday, worldwide, there are several thousand earthquakes.Most earthquakes are too small to notice.But a large earthquake can change Earth's surface and cause great damage.The forces of plate movement cause earthquakes.Plate movements produce stress in Earth's crust, adding energy to rock breaks.An earthquake begins.In seconds, the earthquake releases a large amount of stored energy.Most earthquakes begin in the lithosphere within 100 kilometers of Earth's surface.

Types of Seismic Waves Like a pebble thrown into a pond, an earthquake produces vibrations called waves.These waves carry energy as they travel outward.During an earthquake,seismic waves race out from the focus in all directions.Seismic waves are vibrations that travel through Earth carrying energy released during an earthquake. P waves- are seismic waves that compress and expand the ground like an accordion. S waves- are seismic waves that vibrate from side to side as well. Surface waves- move more slowly than P and S waves,but they can produce servers ground movements.

Measuring Earthquakes There are three comonly used methods of measuring earthquakes: the Mercalli scale,the Richter scale,the Moment Magnitude scale. The Mercalli Scale- a scale that rates earthquakes according to their intensity and how much damage they cause at a particular place. The Richter Scale- a scale that rates an eathquake's magnitude based on the size of its seismic waves. The Moment Magnitude Scale- a scale that rates an earthquake by estimating the total energy released.

Locatin the Epicenter Seismic waves travel at different speeds.P waves arrive at a seismograph first, with S waves following close behind.To tell how far the epicenter is from the seismograph,scientists measure the difference between the arrival times of the S an P waves.The father away an earthquake is, the greater the time between the arraival of P and S waves.