4 The size of earthquakes
4.2 Earthquake magnitude
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 Box 4, 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.
Box 4 Charles Richter and the Richter earthquake magnitude scale
Many scientists contributed to the evolution of the earthquake magnitude concept, but it was Charles Richter, a professor at the California Institute of Technology, who set up a scale on the basis of many years of observations and applied it to well-known earthquakes. He explained the scale in a now classic paper published in 1935. Professor Richter modestly never attached his own name to the scale. He even refused to call it the Richter scale in his papers, long after the press and public had made ‘Richter scale’ synonymous with ‘earthquake magnitude scale’.
Professor Richter often had trouble explaining to people that the Richter scale is a mathematical scale involving measurements and calculations on paper. ‘They seem to think it is some sort of instrument or apparatus. Every year they come by wanting to look at my scale’, he once said in an interview. Richter borrowed the term ‘magnitude’ from astronomy, in which he had an amateur interest. In astronomy the brightness of stars is measured on a magnitude scale.
Unlike earthquake intensity, any earthquake has only one Richter magnitude. The Richter scale is also quantitative, 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.
Activity 2 Investigating links between earthquake magnitude and location
In Activity 1 you established links between the depths of earthquake foci and the location of the earthquake epicentre. In this activity you will investigate links between magnitude and location.
You saw in Activity 1 that some of the Earth's surface features (the ocean trenches, mid-ocean ridges and certain mountain belts) have earthquakes associated with them, and that some of these features have both deep-focus and shallow-focus earthquakes. In this activity you will investigate whether there is also a relationship between the size of an earthquake and its location relative to the major surface features.
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(a) Study Figure 7, and for each of the surface features (i), (ii) and (iii) listed below, decide what is the highest earthquake magnitude (on the Richter scale) usually associated with the feature. Ignore any larger-magnitude earthquakes that occur very
infrequently.
- (i) Mountains and ocean trenches surrounding the Pacific Ocean.
- (ii) Mountain belts in Europe; mountain belts in Asia.
- (iii) Mid-ocean ridges.
- (b) Now fill in Table 1, using your answers from part (a) of this activity and the answer to Activity 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) | ||||
Note: With Internet access, you can examine other earthquake data in the British Geological Survey World Seismicity Database web site (http://www.gsrg.nmh.ac.uk).
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 7.)
- (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 |