Chpt11_pptteacher - Earthquakes Chapter 11...

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Earthquakes Chapter 11
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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 Movements that produce earthquakes are usually associated with faults Most of the motion along faults can be explained by plate tectonic theory
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New Zealand Earthquake
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Earthquake focus  and epicenter Figure 11.2
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   What is the cause of earthquakes? 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 Slippage at the weakest point (the focus ) occurs. Displacement will exert stress farther along the fault where slip will release the built up stress as strain (brittle deformation) Vibrations (earthquakes) occur as the deformed rock “springs back” to its original shape ( elastic rebound )
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What is an earthquake? Summarizing: earthquakes are produced by the rapid release of stored energy. Once the elastic limit (strength) of the rock is exceeded, it ruptures, causing vibrations. Earthquakes most often occur along existing faults whenever the frictional forces on the fault surfaces are overcome.
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Figure 11.3 (left)
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Offset produced by the 1906 San  Francisco earthquake
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Figure 11.4
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Foreshocks and aftershocks Adjustments, along the same fault plane, that follow a major earthquake often generate smaller earthquakes called aftershocks Small earthquakes, called foreshocks , often precede a major earthquake by days or maybe several years
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Earthquake Rupture and propagation Most of the time faults are locked in position, and no movement is occurring, due to confining pressure. But strain is building up. Eventually the fault ruptures at the focus, overcoming the frictional forces holding the sides together. The slippage propagates away from the focus Slippage stops when the rupture reaches a fault section with less built up strain.
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Figure 11.5
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Figure 11.6
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San Andreas:  An active earthquake zone San Andreas is the world’s most studied fault system Displacement occurs along segments 100 to 200 km long Some portions exhibit slow, gradual displacement known as fault creep , with little seismic activity. Other segments regularly slip producing small earthquakes 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
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