MAGNITUDE (Ms) AND BODY-WAVE MAGNITUDE (mb)
The successful development
of the local-magnitude scale encouraged Gutenberg and Richter to develop magnitude
scales based on teleseismic observations of earthquakes. Two scales were developed,
one based on surface waves, Ms, and one on body waves, mb.
Surface waves with a period near 20 s generally produce the largest amplitudes on a standard long-period seismograph, and so the amplitude of these waves is used to determine Ms, using an equation similar to that used for ML.
The body-wave magnitude, mb, which was developed specifically to treat deep-focus earthquakes, presents yet another alternative scale for magnitude determination. Although it presently is the most commonly reported teleseismic magnitude, current practice in its determination differs from that employed by Gutenberg, and so it is omitted from Table 6.1. As a short-period magnitude, modern mb values measure the same part of the earthquake energy spectrum as ML.
The magnitudes listed by Gutenberg and Richter (1954) that appear in Table 6.1 as Miff are essentially Ms according to Geller and Kanamori (1977); magnitudes attributed to Richter (1958) are based on ML or Ms.
Useful estimates of Ms can be obtained from many different types of long-period seismographs, including the undamped instruments deployed by Milne beginning in 1897. Abe and Nognchi (1983) constructed estimates of Ms from Milne seismograms to resolve a longstanding controversy concerning an apparent peak in global seismicity between 1904 and 1912. Abe (1988) later used the Milne data to determine magnitudes for smaller earthquakes in California between 1898 and 1912. His procedures have been used to compute Ms for additional California events occurring between 1898 and 1934, which are listed in Table 6.1.
SEISMIC INTENSITY AND EARTHQUAKE MAGNITUDE (MI)
Before the development of seismographs in the late 19th century, descriptions of the effects of earthquakes provided the only means for assessing earthquake size in all but the rare cases where surface faulting was well described. A robust method for relating the area undergoing shaking of a given intensity or greater to ML was developed by Toppozada (1975) for California and western Nevada. Using these relations, Toppozada and others (1981, 1982) successfully assigned an intensity magnitude, MI, to many earthquakes. The isoseismal maps developed in the course of their research also generally provide our best estimates of epicentral locations. New MI values have been determined for several events, using the same procedures as part of this study.