Story: Earthquakes

Part of page 1 – What causes earthquakes?

The earthquake magnitude scale (2nd of 2)

The magnitude scale measures the size of an earthquake at its source. This measurement is taken from a seismogram, using two variables. One is the time between the primary (P) and secondary (S) waves. This is known as S-P time (secondary minus primary) and gives an indication of the distance from the origin or focus of the earthquake, under the ground. The other variable is the height of the biggest wave (amplitude). A line can then be drawn between these measurements on a chart known as a nomogram. This has an S-P scale on one side and an amplitude scale on the other, and the intersection of a line joining the two values with the magnitude scale indicates the size of the earthquake. In the example shown here, the S-P time is 24 seconds, and the amplitude is 23 millimetres, giving a magnitude of 5. A magnitude 9 quake is extremely rare.