Ch06_Outline - Chapter 6 Restless Earth Earthquakes...

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Chapter 6 Restless Earth: Earthquakes, Geologic Structures, and Mountain Building
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What Is an Earthquake? An earthquake is the vibration of Earth produced by the rapid release of energy Energy released radiates in all directions from its source, the focus Energy is in the form of waves Sensitive instruments around the world record the event
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Earthquake Focus and Epicenter Figure 6.2
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What Is an Earthquake? Earthquakes and faults Movements that produce earthquakes are usually associated with large fractures in Earth’s crust called faults Most of the motion along faults can be explained by the plate tectonics theory
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What Is an Earthquake? Elastic rebound Mechanism for earthquakes was first explained by H. F. Reid Rocks on both sides of an existing fault are deformed by tectonic forces Rocks bend and store elastic energy Frictional resistance holding the rocks together is overcome
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What Is an Earthquake? Elastic rebound Earthquake mechanism Slippage at the weakest point (the focus) occurs Vibrations (earthquakes) occur as the deformed rock “springs back” to its original shape (elastic rebound)
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What Is an Earthquake? Foreshocks and aftershocks Adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks Small earthquakes, called foreshocks, often precede a major earthquake by days or, in some cases, by as much as several years
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Seismology The study of earthquake waves, seismology, dates back almost 2000 years to the Chinese Seismographs, instruments that record seismic waves Record the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape
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Seismology Seismographs More than one type of seismograph is needed to record both vertical and horizontal ground motion Records obtained are called seismograms Types of seismic waves Surface waves Travel along outer part of Earth
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Seismology Types of seismic waves Surface waves Complex motion Cause greatest destruction Exhibit greatest amplitude and slowest velocity
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Seismology Types of seismic waves Body waves Travel through Earth’s interior Two types based on mode of travel Primary (P) waves Push-pull (compress and expand) motion, changing the volume of the intervening material Travel through solids, liquids, and gases
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Seismology Types of seismic waves Body waves Secondary (S) waves “Shake” motion at right angles to their direction of travel Travel only through solids Slower velocity than P waves
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Locating an Earthquake Terms Focus The place within Earth where earthquake waves originate Epicenter Location on the surface directly above the focus Epicenter is located using the difference in velocities of P and S waves
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Locating an Earthquake Locating the epicenter of an earthquake Three station recordings are needed to locate an epicenter
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