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earthquakes - A Violent Pulse Earthquakes San Francisco...

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A Violent Pulse: Earthquakes San Francisco, 1906
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Seismic Risk
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W. W. Norton Rock under stress Stress = applied push pull or shear Stress > rock strength + friction = movement Fault movement releases stress as vibrations Faulting is a process of breaking, therefore is BRITTLE What is an earthquake? “Episode of ground shaking”
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National Geophysical Data Center/NOAA W. W. Norton What does a geologist see? Slip and Displacement = Distance of offset
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How much displacement occurs? - Can be small (mm to cm) during smaller quakes - Can be massive - 1906 San Francisco Earthquake ruptured 430- km long by 15-km high segment of the San Andreas Fault - Entire length was displaced between 2-7m - 1964 Alaska Quake raised ground level 2 meters
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Important terms Focus or hypocenter Epicenter Fault scarp Hanging wall Footwall Hypocenter HW FW Fault scarp
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W. W. Norton shear) n) Fault types (low angle)
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Elastic Rebound Theory Tectonic forces slowly cause rock to strain (change shape) Rocks store elastic energy Stress too great and …..pow, energy is released! Movement on fault = release of stress Energy released as spherical vibrations No more stress - rock “snaps back” to original shape Snap back (elastic rebound) causes the vibrations Build up of stress can cause small quakes - foreshocks before the big one Also can have aftershocks after main quake as rocks readjust
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W. W. Norton
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W. W. Norton Stick-slip
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Friction - resists sliding Force of friction overcome and movement on fault
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WHEN THE FAULT MOVES, SHOCKS OCCUR The more rock that moves and the greater amount of slip Results in BIGGER EARTHQUAKES Foreshocks - movement along small cracks, precursors to main event Aftershocks - Follows main quake, 10x smaller, fault readjusting to new Stresses caused by major earthquake, can exist days to weeks after
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