What is Shown on a Seismogram?

Seismograms are used to determine the location and magnitude of earthquakes.

An earthquake's magnitude may be considered to vary as a function of the amount of energy released at the rupture point. When an earthquake occurs, two main types of vibratory waves move through the body of the earth from the point of fracture. The primary, or P, waves travel most quickly and are the first to be registered by the seismograph. Secondary, or S, waves travel more slowly.

As S waves have a greater amplitude than P waves the two groups are easily distinguishable on the seismogram. By measuring the time interval between the arrivals of the P and S wave groups seismologists are able to calculate the distance between the seismograph and the origin of the earthquake. Magnitude is then derived from the amplitude of the waves on the seismogram and the distance of the earthquake from the seismograph.

When P and S waves strike the surface of the earth they initiate a third kind of wave, called surface waves, which travel over the earth's surface. These are the slowest waves. On recordings of local earthquakes the surface waves are small and can seldom be distinguished from the S waves that preceded them. However, since surface waves attenuate much more slowly than do P or S waves they are generally the largest waves to appear on long period seismograms of distant earthquakes.

If an earthquake is recorded by three or more seismograph stations its precise location can be determined from the set of distances. In seismically active areas, a network of sensitive seismographs may be installed to locate even very minor tremors.

What do seismic waves look like?

Suggested Reading

• "The Amateur Scientist", Scientific American, July 1957 and July 1979: Basic principles and how to build a simple seismograph.
• Hodgson, John. Earthquakes and Earth Structure. New Jersey, Prentice Hall, 1964, p. 60-69: How seismographs work and interpretation of seismograms.