Ch11_Outline - Earth: An Introduction to Physical Geology,...

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Earth: An Introduction to Physical Geology, 9e Tarbuck & Lutgens
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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 11.2
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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 ) Earthquakes most often occur along existing faults whenever the frictional forces on the fault surfaces are overcome
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Offset produced by the 1906 San Francisco earthquake
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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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San Andreas: An active earthquake zone San Andreas is the most studied fault system in the world Displacement occurs along discrete segments 100 to 200 kilometers long Some portions exhibit slow, gradual displacement known as fault creep Other segments regularly slip producing small earthquakes
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San Andreas: An active earthquake zone Displacements along the San Andreas fault Still other segments store elastic energy for hundreds of years before rupturing in great earthquakes Process described as stick-slip motion Great earthquakes should occur about every 50 to 200 years along these sections
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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 Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape
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A vertical ground motion seismograph Figure 11.8
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Ch11_Outline - Earth: An Introduction to Physical Geology,...

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