The NASA SCIence Files™
Earthquake, Earthquake, Read All About It!

Purpose: To understand the various types of scales used to measure earthquakes

Background
Magnitude is a measure of the strength of an earthquake or strain energy released by it, as determined by seismographic observations. This is a logarithmic value originally defined by Charles Richter (1935). An increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released. In other words, a magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy of a 4.7. There is no beginning or end to this scale. However, most earthquakes are not smaller than about -1 nor larger than about 9.5. There are three versions of the Richter scale in use around the world. They are the magnitude local (ML), body-wave magnitude (mb), and surface-wave magnitude (MS). Due to limitations of all three scales, a new scale called the moment magnitude (MW) was developed and gives the most reliable estimate of earthquake size.

Intensity is a measure of the effects of an earthquake at a particular place on humans, structures and (or) the land itself. The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to the point and the local geology at that point. The current scale used in the United States to measure intensity is the Modified Mercalli (MM) Intensity Scale. Unlike the Richter scale, it does not have a mathematical basis, instead it is an arbitrary ranking based on observations.

Materials

Procedure

  1. Open the web page listed above.
  2. Read the abbreviated descriptions of the 12 levels of the Modified Mercalli Intensity Scale.
  3. Using the scale, classify the following earthquakes.
    1. I was taking a nap upstairs when I thought I felt the bed shake.
    2. I was in the kitchen cooking when the whole house moved and trembled. I was frightened when the plaster started falling off the ceiling. I thought I was a goner.
    3. It was late at night and I thought I felt a tremble, and then I noticed creaking or cracking sound. I thought maybe a truck had run into the house!
    4. During the earthquake, almost all of the buildings in my downtown area were destroyed. Even the bridge collapsed and the train tracks were bent.
  4. Write a story as if you are a newspaper reporter describing a town that has just experienced an earthquake.
  5. Exchange or share stories with your partner, group, or class. Determine what number on the intensity scale each story best fits.