Section 6.1 Objectives

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• Section 6.1 Objectives
• 1. 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 Earths
  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 Earths 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)