Measuring Earthquakes

Geologists measure earthquakes in two general ways, quantitatively and qualitatively.

Quantitatively: measures the magnitude, or the energy released in an earthquake using a seismograph.

• Richter Scale*: (ML) developed in the 1930's to measure the magnitude of shallow earthquakes in California. This has been replaced by the moment magnitude scale.
• Moment Magnitude Scale*: (MW) developed in the 1970's, improving upon the Richter scale by considering the shaking measured by the seismograph and the area of the fault that slipped. This has replaced the Richter scale because it can easily by applied to earthquakes anywhere in the world.
  

*Note: these scales are logarithmic. That means every time you increase one scale degree, the energy released by the earthquake increases by a power of ten. For example, a MW 4.3 earthquake is ten times stronger than a MW 3.3 earthquake.

Qualitatively: measures the intensity of an earthquake by looking at the impact on people and infrastructure.

• Modified Mercalli Scale: (MMI) developed around 1900 to measure the intensity of earthquakes. The MMI degree for an earthquake will vary depending on the quality of infrastructure (i.e. roads, buildings, sewers, etc.). For example, a MW 4 earthquake may have a much greater MMI scale degree in a poor country where building standards are low than it would have in a wealthy country where building standards are high.

Follow Up:

• Visit this link to compare the Moment Magnitude scale with the Modified Mercalli scale: CLICK HERE!
• Compare the 2010 earthquakes in Haiti and Chile: CLICK HERE!
  1. What was the magnitude of each earthquake?
  2. How many people died in each earthquake?
  3. Look at the "shake maps" for each. How do the two earthquakes compare in intensity?
  4. What factors might explain the difference in mortality rates for the two earthquakes?