The Various Wave Types and Earthquake Magnitude

Earthquake magnitude is a measurement that reflects the energy radiated by an earthquake. Seismologists use the wave amplitudes of the various seismic waves emitted to calculate magnitude :
  • P-waves are longitudinal and compressive. They can reach propagation velocities of 3.5 to 14 km/s, depending on the nature of the rocks and the depth the waves travel at;
  • S-waves are shear, transverse vibrations running perpendicular to propagation direction. They are approximately 1.7 times slower than P-waves;
  • surface waves are superficial waves with large wavelengths and are even slower than S-waves.
Seismic waves are not emitted isotropically (ie identically in all directions) and they diminish according to the distance travelled. Seismic stations will therefore record different amplitudes. Even when the epicentral distance, ie between the quake and the seismic station, is factored in, the magnitudes calculated based on these amplitudes are slightly different. So the final magnitude will be the average of the magnitudes calculated at the various stations.

Depending on the waves examined, there are thus several magnitudes:
  • Mb magnitude calculated from body waves;
  • Ms magnitude calculated from surface waves; and
  • moment magnitude calculated from the signal as a whole.
They can differ by as much as a unit of magnitude.

Concepts of magnitude should not be confused with intensity. Intensity reflects the scale of damage caused by an earthquake. For example, intensity VI on the Mercali scale indicates that weaker walls have been cracked. A low-magnitude quake very near a town can cause major damage and therefore be classified as high-intensity (cf also the difference between a seismic risk and hazard).

Cf also Wikipedia article detailing the various seismic waves.