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4.4.2: Earthquake Magnitude

  • Page ID
    2109
  • The magnitude of an earthquake is a measure of the amount of seismic energy released by it, so it is a quantitative scale. The scale of earthquake magnitude is called the Richter scale. Its development is described in below, Charles Richter and the Richter earthquake magnitude scale. The Richter magnitude is calculated by first measuring the size of the largest ground motion recorded by a seismometer, a sensitive instrument that detects the ground movements produced by earthquakes. This is then corrected for the distance from the earthquake, since the closer the seismometer is to the earthquake, the larger the ground motion will be.

    , being based on numerical measurement. The Richter scale has no upper limit, but in reality the Earth itself provides an upper limit due to the strength of rocks. The largest earthquakes ever recorded had Richter magnitudes of 8.9.

    The sizes of earthquakes vary enormously, so the size of the ground motion produced can differ by thousands or even millions from earthquake to earthquake. In order to deal with such enormous variation, the Richter scale is based on powers of ten, which means that an increase of one unit on the scale implies a tenfold increase in the amount of ground motion. For example, a magnitude 2 earthquake produces 10 times more maximum ground motion than a magnitude 1 earthquake. A magnitude 3 earthquake produces 10 times more again, which is 10 × 10 = 100 times greater maximum ground motion than a magnitude 1 earthquake.

    • What is the difference in maximum ground motion between a magnitude 3 earthquake and a magnitude 6 earthquake?

    • Magnitude 6 is 3 points more on the Richter scale than magnitude 3, so a magnitude 6 earthquake has 10 × 10 × 10 = 1 000 (or 103) times greater maximum ground motion than a magnitude 3 earthquake.

    Similarly, the difference between earthquakes of magnitude 3 and 7 (4 points on the Richter scale) will be 104 in maximum ground motion. What appears at first to be a small change in Richter magnitude of an earthquake (say from 3 to 7, 4 points) really represents a very large change in earthquake size.

    (b) Now fill in Table 1, using your answers from part (a) of this activity and the answer to Exercise 1. The completed table will provide a summary of the relationship between earthquake depth, size and location.

    Table 1 The depths and sizes of earthquakes at different locations

    Mountains and ocean trenches surrounding the Pacific Mountain belts Mid-ocean ridges
    Europe Asia
    depth (shallow-focus, intermediate-focus or deep-focus)
    largest magnitude (up to magnitude 7.9, or over magnitude 8)
    With Internet access, you can examine other earthquake data in the British Geological Survey World Seismicity Database web site

    Answer

    • (a) (i) Mountains and ocean trenches surrounding the Pacific Ocean: magnitude 8.0-8.9

      • (ii) Mountain belts in Europe: magnitude 7.0-7.9; mountain belts in Asia: magnitude 8.0-8.9

      • (iii) Mid-ocean ridges: magnitude 7.0-7.9. (In fact the maximum magnitude at mid-ocean ridges is 7.5, but this is not shown on Figure 3.2.1.)

    • (b) See Table 2 below.

    Table 2 The depths and sizes of earthquakes at different locations

    Mountains and ocean trenches surrounding the Pacific Mountain belts Mid-ocean ridges
    Europe Asia
    depth (shallow-focus, intermediate-focus or deep-focus) shallow-, intermediate and deep-focus mainly shallow-focus, a few intermediate mainly shallow-focus, a few intermediate shallow-focus
    largest magnitude (up to magnitude 7.9, or over magnitude 8) over 8.0 up to 7.9 over 8.0 up to 7.9

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