On the relation between modified Mercalli intensity and body-wave magnitude

This paper is concerned with estimating body-wave magnitude, m_b, from the intensity distribution of an earthquake. Initially, it is assumed that modified Mercalli (MM) intensity values are directly related to the (A/T)_z values of 1-Hz, Lg-wave ground motion. By comparison with the intensity values of a reference earthquake, magnitudes are calculated for 41 western and central United States earthquakes. Magnitudes of these earthquakes also are determined independently, in the conventional manner, using teleseismic P-wave amplitudes. Comparison of the two sets of magnitude values indicates that the assumed relation between 1-Hz, Lg-wave (A/T)_z values and MM intensity does not hold exactly over the m_b range of 4.0 to 6.2. An empirical equation is derived to adjust the m_b values obtained from intensity data so that they agree with the teleseismic P-wave magnitudes.

The method then is applied to estimate m_b of some historical earthquakes which occurred prior to 1962. These include the set for which Kanamori and Jennings (1978) estimated M_L from strong-motion accelerograms. Some noteworthy United States earthquakes also are considered. These include: the 1811 New Madrid earthquake for which m_b is estimated to be 7.3; the 1886 Charleston, South Carolina earthquake, for which m_b is estimated to be 6.6 to 6.9; the 1897 Giles County, Virginia earthquake, for which m_b is estimated to be 5.8; the 1906 San Francisco, California earthquake, for which m_b is estimated to be 6.8 to 7.1.

The intensity-attenuation method cannot be used for estimating m_b of all historical earthquakes because the intensity data are not always adequate. In some cases, however, the total felt area or the area enclosed by the Modified Mercalli IV isoseism can be determined. It was found that empirical equations relating m_b to these areas, which were derived for central and northeastern United States earthquakes, also apply for events in the southeast. These empirical methods are used to estimate m_b values for a set of historical Virginia earthquakes.