Most earthquakes occur at plate boundaries earthquake

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– most earthquakes occur at plate boundaries Earthquake focal depths are – Deep – Intermediate – Shallow The deep-focus earthquakes are concentrated in subduction zones (thick lithosphere)
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World Seismicity (1979-1995)
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Divergence = normal faults
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Convergence = reverse faults
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Transform = Strike-slip faults
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Seismic Waves Energy released by an earthquake will send seismic waves out from the focus Body Wave – travels through the interior of the earth – P waves are compression waves (fast) – S waves are shear waves (slower) Surface wave – travels along the surface – Larger ground displacement than body waves – Results in most earthquake damage
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Early Seismograph
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Measuring Earthquakes Seismograph Seismogram
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Fig. 4.06 P Waves S Waves
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Rayleigh and Love Waves (surface waves)
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Wave arrival times
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Locating the earthquake
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Triangulation Epicenter is 1350 km from SF, 1900 km from Dallas 2400 km from Chicago Epicenter must be near Salt Lake City
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Earthquake Energy Energy is released during an earthquake As the waves of energy are transmitted through the rock, this energy will be felt by people at the surface Magnitude – related to the amount of ground motion associated with an earthquake Intensity – effect on humans, and their structures, caused by the energy released by an earthquake
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Measuring Earthquakes Magnitude - Richter magnitude amount of ground motion (log scale of 10 times more each 1.0 step) - Moment magnitude or relative energy released (30 times more energy each 1.0 step) - Intensity (Mercalli, qualitative response of people and structures to an earthquake). Hard to compare as damage relates to many parameters
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