Earthquakes: Magnitude and Intensity

Magnitude

  • magnitude: a measure of energy released by an earthquake
    • quantitative
  • as magnitude increases, the earthquake tends to:
    • affect a broader region
    • shake for longer (damage more buildings, cause more harm)

Richter Magnitude Scale

  • measures amplitude
    • amplitude: size of the waves on seismograph
  • logarithmic scale: each degree of magnitude is 10x bigger waves
    • equates to 50x more energy per degree of magnitude
  • not accurate for large, deep, or distant earthquakes
  • no longer used

Moment Magnitude Scale

  • the scale that’s now used
  • measures strain energy along rupture surface (energy released)
  • logarithmic scale: each degree of magnitude is 10x more energy released
  • more accurate than richter
  • most widely accepted and used by scientists

Intensity

  • qualitative
  • what we feel in an earthquake
  • modified mercalli scale
  • “I'“ is felt by very few people or not at all to “X” which is total destruction

Earthquake Intensity

  • relates to ground motion’s effects on population and structures
  • rely on “felt” reports
    • people provide location and their own intensity rating

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