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