Plate Tectonic - Hazards (4)
Post Lab 

  • Comparing the Richter and Modified Mercalli scales.
  • Distinguishing the Richter and Modified Mercalli earthquake intensities.
  • Modified Mercalli Scale
  • Richter Scale
  • worksheet

Students evaluate the Modified Mercalli and Richter Scales.

Earthquake risk in United States. Reds and yellows are highest risk, blue and white are least. 


Earthquakes are caused when stress within the crust of the Earth builds up, causing an area of rock to "snap" along a fault. This breaking causes a release of energy. This energy is measured by how much "shaking" occurs. The shaking can vary in its intensity. Some earthquakes shake everything (high intensity), and others are not felt at all (low intensity). Seismologists use several methods to measure the intensity of an earthquake. This post lab focuses on two such scale.

First, the Richter scale is a mathematical measurement of the intensity of the ground shaking, as measured on a seismograph. It is actually a measurement of the height (amplitude) of the waves produced by the earthquake. The Richter Scale is an absolute scale; wherever an earthquake is recorded, it will measure the same on the Richter Scale. 

Second, the Modified Mercalli scales measures how people feel and react to the shaking of an earthquake. It is a relative scale, because people experience different amounts of shaking in different places. It is based on a series of key responses such as people awakening, the movement of furniture, and damage to structures. In general, the further one is from the epicenter of an earthquake, the less shaking is experienced. When an earthquake occurs, it is important for a student to have a way to interpret the size of the event. Learning the Modified Mercalli scale can give students this ability. It is sufficient to distinguish between small, moderate, or large earthquakes. This knowledge can help a student determine a course of action during and after the shaking.

  1. Using the worksheet, review the Modified Mercalli and Richter Scales with the students. Compare the two scales. If you live in an earthquake prone area, you may wish to have your students recall earthquakes that they have experienced. Ask them what Modified Mercalli scale rating they would give it.
  2. Have the students complete the worksheet. They are asked to predict the Modified Mercalli and Richter Scale magnitudes for a series of earthquake descriptions.
    1. IV - moderate: 4.3-4.8
    2. X - disastrous; 7-7.3
    3. II - feeble; 3.5-4.2
    4. VI - very strong; 5.5-6.1
    5. VIII - destructive; 6.2-6.9
    Mercalli scale = measures intensity compared to human terms
    Richter scale = strength of earthquake due to ground motion

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