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•Section 6.1 Objectives1. Discuss the elastic rebound theory.2. Explain why earthquakes generally occur at plate boundaries.
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Earthquakes & Plate Tectonics
•Earthquakes (EQs): vibrations of the Earth’s crust– Occur when rocks
under stress suddenly shift
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•Elastic Rebound Theory: the theory geologists use to explain many EQs– Rocks on each side of fault are moving– If fault is locked, stress increases– At certain point rocks fracture & rebound
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•Focus: the area along a fault where slippage 1st occurs.–Depths vary
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•Epicenter: the point on the Earth’s surface directly above the focus.
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•Aftershocks: series of smaller tremors.–Caused by released energy (seismic waves)•Increases stress in other rocks along the fault
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•Focus depths vary–Shallow focus - w/in 70 km of surface•Tend to cause the most damage
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•Focus depths vary– Intermediate focus - 70 to 300 km
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•Focus depths vary–Deep focus (occur in subduction zone)•300 to 650 km
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•Focus depths vary–About 90% of Continental EQs are shallow focus
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3 Major EQ Zones•Links btwn EQs & Plate Tectonics
– Plate movements = stress buildup
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•Pacific Ring of Fire: ring of tectonic/volcanic activity surrounding the pacific.
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•Mid-ocean ridges: spreading motion = stress
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•Eurasian-Melanesian mountain belt: Eurasian plate colliding w/African & Indian plates = stress
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•Fault Zones: are groups of interconnected faults.– Form at plate
boundaries b/c of intense stress•e.g. San Andreas
fault
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•Not all EQs occur at plate boundaries!– e.g. New Madrid, Missouri, 1812
•Ancient fault? (600 million yrs ago)