Magnitudes of great shallow earthquakes from 1904 to 1952

Abstract

The “revised magnitudes”, M, converted from Gutenberg's unified magnitude, m, and listed by Richter (1958) and Duda (1965) are systematically higher than the magnitudes listed by Gutenberg and Richter (1954) in Seismicity of the Earth. This difference is examined on the basis of Gutenberg and Richter's unpublished original worksheets for Seismicity of the Earth. It is concluded that (1) the magnitudes of most shallow “class a” earthquakes in Seismicity of the Earth are essentially equivalent to the 20-sec surface-wave magnitude, M_s; (2) the revised magnitudes, M, of most great shallow (less than 40 km) earthquakes listed in Richter (1958) (also used in Duda, 1965) heavily emphasize body-wave magnitudes, m_b, and are given by M = 1/4 M_s + 3/4 (1.59 m_b - 3.97). For earthquakes at depths of 40 to 60 km, M is given by M = (1.59 m_b − 3.97). M and M_s are thus distinct and should not be confused. Because of the saturation of the surface-wave magnitude scale at M_s ≃ 8.0, use of empirical moment versus magnitude relations for estimating the seismic moment results in large errors. Use of the fault area, S, is suggested for estimating the moment.