Earthquake Magnitude and Intensity

Key Concepts

• Magnitude

  • Represents the energy released at the source of an earthquake.
  • Calculated using measurements from seismic instruments.
  • Expressed as a single value.
  • Consistent across all locations.

• Intensity

  • Measures the strength of shaking at a specific location.
  • Determined from effects on people, human structures, and the natural environment.
  • Varies from place to place.
  • Produces a range of shaking intensities in different locations.

Factors Affecting Intensity

1. Magnitude

   • The larger the magnitude, the stronger the potential intensity.

2. Distance from Hypocenter

   • Intensity decreases with increasing distance from the earthquake's source.

3. Local Rock and Soil Conditions

   • Geological conditions can amplify or dampen shaking.

Analogy

• Light Bulb Analogy:
  • Hypocenter: Location within the Earth where the earthquake begins.
  • Magnitude (Wattage): Represents the total power/energy of the earthquake, similar to the power of a light bulb.
  • Intensity (Light Amount): Similar to how light varies in intensity throughout a room, earthquake intensity varies by location.

Visual Representation

• USGS "Did You Feel It?" Intensity Maps
  • Show varying intensities of ground shaking as experienced by different locations during the same earthquake.

Summary

• A given earthquake has only one magnitude but produces different intensities of ground shaking at different locations.