Geophysical Institute, Faculty of Science, Tohoku University, Sendai 980, Japan
Abstract:
A relation between surface-wave magnitude Ms and fault area S for great earthquakes has been proposed
log S 2Ms ? 11.5 (Ms > 7.5)
A similar formula has been also derived for body-wave magnitude mb* redetermined from maximum amplitudes of short-period P-wavetrains
log S ∝ 1.7 mb*
These are quite different from a theoretical relation expected on the basis of long-wave approximation. Because wavelengths of seismic waves used for the above magnitudes are very short compared to the size of earthquake sources, these relations represent the short-period nature of the earthquake process. The statistical theory of extreme values has been applied to understand the relations considering that the component waves which constitute the wavetrains for mb* and Ms determinations originate from the random fracture of fault heterogeneities.