Comparing Earthquake Scales

An earthquake is caused by movements of the earth's crust that occur when plates shift and release potential energy. Most earthquakes result from movements of the earth's crust along faults, as we looked at in Part two of this series. It is along the fault where the rocks first break and move; this point is called the focus and this is where the earthquake begins. The epicenter is the point on the earth's surface directly above the focus. The strongest shaking in an earthquake will be felt at the epicenter. The seismic waves (energy) moves in all directions from the focus.

The energy from an earthquake travels quickly travels outward in waves from the point of breakage. The energy of an earthquake can break and move rock and soil. This energy is measured and scientists use two types of scales to measure earthquakes. The Richter scale measures the magnitude of an earthquake based on the amount of shaking caused by the quake. It is based on energy released by the earthquake. On this scale, each increase of 1 magnitude number equals an increase of 10 in ground motion caused by seismic waves. So, a earthquake with a magnitude of 6 would cause 10 times more ground motion than an earthquake with a magnitude of 5. The Mercalli scale measures the intensity of an earthquake. It is based on personal observations. The intensity of an earthquake includes ground motion and resulting damage.

This table shows a basic relationship between the Richter scale and the Mercalli scale. Download and give each student a copy of this table, comparing the each earthquake scale. Then ask students to answer the following questions:

1. How are the two scales similar? How are they different? 
2. What occurs during a Richter magnitude 8.0 earthquake? What is the measurement for the same quake on the Mercalli scale?
3. At which measurement on each scale does damage to buildings begin?
4. Why are two different scales used for measuring earthquakes?

For a bonus activity, have students research recent earthquakes and list how they measured on each of the scales. Have them explain any damage that was reported. They can also look at how the earthquakes of different sizes compared in the resulting damage. They may take another copy of the above table and mark off where each of their researched earthquakes fell on the scales.