Earthquakes

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Earthquakes • Visit our website: http://www.sliderbase.com/ • Free PowerPoint Presentations for teaching and learning • What are Earthquakes? • The shaking or trembling caused by the sudden release of energy • Usually associated with faulting or breaking of rocks • Continuing adjustment of position results in aftershocks • What is the Elastic Rebound Theory? • Explains how energy is stored in rocks • Rocks bend until the strength of the rock is exceeded • Rupture occurs and the rocks quickly rebound to an undeformed shape • Energy is released in waves that radiate outward from the fault • The Focus and Epicenter of an Earthquake • The point within Earth where faulting begins is the focus, or hypocenter • The point directly above the focus on the surface is the epicenter • Where Do Earthquakes Occur and How Often? • ~80% of all earthquakes occur in the circum-Pacific belt • most of these result from convergent margin activity • ~15% occur in the Mediterranean-Asiatic belt • remaining 5% occur in the interiors of plates and on spreading ridge centers • more than 150,000 quakes strong enough to be felt are recorded each year • How is an Earthquake’s Epicenter Located? • Seismic wave behavior • P waves arrive first, then S waves, then L and R • Average speeds for all these waves is known • After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter. • How is an Earthquake’s Epicenter Located? • Time-distance graph showing the average travel times for P- and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves • How are the Size and Strength of an Earthquake Measured? • Modified Mercalli Intensity Map • 1994 Northridge, CA earthquake, magnitude 6.7 • Intensity • subjective measure of the kind of damage done and people’s reactions to it • isoseismal lines identify areas of equal intensity • What are the Destructive Effects of Earthquakes? • Ground Shaking • amplitude, duration, and damage increases in poorly consolidated rocks • Can Earthquakes be Predicted? • Earthquake Prediction Programs • include laboratory and field studies of rocks before, during, and after earthquakes • monitor activity along major faults • produce risk assessments

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