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  • Created by: Jelena
  • Created on: 15-05-13 15:09

General Terminology

  • Earthquake occurs when rocks being stressed suddenly break along a new or pre-existing fault
  • The two blocks of rocks on either side of the fault slip suddenly, setting off ground vibrations or seismic waves
  • Focus = the point at which the earthquake begins
  • Epicenter = point on Earth's surface directly above the focus
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Stress and Strain

  • Fractures form when stress (the forces per unit area acting on a material) exceed the strength of the rocks involved
  • 3 types of stress:
  • 1. Compression: decreases the volume of a material
  • 2. Tension: pulls a material apart
  • 3. Shear: causes a material to twist
  • Strain = the deformation of materials in response to a stress
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  • Fault = a fracture (crack) in the Earth's crust along which movement occurs
  • Key terms:
  • 1. Fault Plane = surface along whcih movement takes place
  • 2. Hanging Wall = occurs above the fault plane
  • 3. Foot Wall = occurs below the fault plane
  • (
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3 Types of Faults

  • 1. Normal fault = the crust is being pulled apart (tensional force); hanging wall drops down
  • 2. Reverse fault = form as a result of horizontal compression; hanging wall mvoes up
  • 3. Strike-slip fault = fractures caused by horizontal shear; walls move sideways, rather then up or down. Fault plane is vertical, so there is no hanging wall or foot fall.
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Seismic Waves

  • The vibrations of the ground during an earthquake are called seismic waves
  • Every earthquake generates 3 types of seismic waves
  • 1. Primary waves (P-waves) = compressional waves --> they travel through solid, liquid, or gaseous materials as a succession of compressions and expansion
  • 2. Secondary waves (S-waves) = shear waves --> they displace materials at right angles to their path of travel and do not travel through liquids or gases
  • 3. Surface waves = one type of surface material sets up a rolling motion in the ground; another type shakes the ground sideways
  • P-waves and S-waves pass through the Earth's interior and are called BODY WAVES
  • P-waves are the fastest to travel through solid rock of Earth's crust at 6 km/sec, followed by S-waves (about 3 km/sec), followed by surface waves
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Seismic Waves Picture


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Earthquakes and Plate Tectonics

  • Earthquakes are concentrated along the major plate boundaries; these areas are called earthquake belts
  • Most earthquakes occur at divergent boundaries, tranform-fault boundaries and convergent boundaries
  • A small percentage of earthquakes occur far from plate boundaries --> called intraplate earthquakes
  • Many intraplate earthquakes occur on old faults that were once part of ancient plate boundaries. The faults no longer form plate voundaries but remain zones of crustal weakness
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