What is the magnitude of an earthquake?

What is the "magnitude" of an earthquake?



Magnitude is a measure of the amount of energy released during an earthquake. It may be expressed using several magnitude scales. One of these, Used in Southern California, is called the Richter scale. To calculate magnitude, the amplitude of waves on a seismogram is measured, correcting for the distance between the recording instrument and the earthquake epicentre. Since magnitude is representative of the earthquake itself, there is thus only one magnitude per earthquake.

Taking the Lawton earthquake of 1998 APR 28, as an example, one could not therefore speak of magnitude 4 at Lawton and magnitude 3 at Tulsa. The effects ( or intensities) at the two places were different, but the magnitude of an earthquake is unique; in this example, it was 4.2 on the Richter scale.

The magnitude scale is logarithmic. This means that, at the same distance, an earthquake of magnitude 6 produces vibrations with amplitudes 10 times greater than those from a magnitude 5 earthquake and 100 times greater than those from a magnitude 4 earthquake.

In terms of energy, an earthquake of magnitude 6 releases about 30 times more energy than an earthquake of magnitude 5 and about 1000 times more energy than an earthquake of magnitude 4.

It is very unlikely that an earthquake of magnitude less than 5 could cause any damage.


Copyright Geological Survey of Canada, modified and used with permission.