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EARTHQUAKE MAGNITUDE SCALES

Uncertainty and confusion presently accompany the use and meaning of earthquake magnitude scales. An earthquake's magnitude will frequently be expressed without indicating whether it is local magnitude, surface wave magnitude, or body-wave magnitude (the spectral amplitudes of the ground motion at 1-5Hz, 0.05Hz, and 1Hz, respectively) being referred to. These are not the same since the shape of the near-field ground motion spectrum changes as the size of the earthquake changes. To add to the confusion, the term Richter magnitude is used for local magnitude when the local magnitude is no greater than 6.5 and for surface wave magnitude in the case of larger earthquakes. This practice arose because the seismographs measuring local magnitude are located nearby and become saturated or overdriven for the larger earthquake, leaving available only the surface wave magnitude values obtained from seismographic measurements of law frequency waves made at large distances. For purposes of earthquake engineering, the author recommends as magnitudes best describing the level of excitation of ground waves in the damaging frequency range local magnitude and body-wave magnitude, with the latter computed using the amplitude of one-second period higher-mode Rayleigh (Lg) waves instead of the one-second period compressional (P) waves which can be attenuated at depths between 75 and 200 km in some regions. Therefore, these magnitues should be used for equations or curves which specify the strong ground-motion acceleration, velocity, displacement, or response spectra as a function of distance from the earthquake source.

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  • Accession Number: 00361733
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Dec 30 1982 12:00AM