Body wave magnitude scales are of the form  
where B and C are constants, D is the epicentral distance, and A is some measure of maximum ground motion amplitude at the period T. By accepted definition, mb applies to waves with periods of 1 second. However, the Lg wave amplitude data in eastern United states, typically within the period range of 0.6 to 1.5 seconds, are currently being used in determining mb magnitudes of eastern earthquakes. Implicit in the use of periods other than 1 second is the assumption that the ground velocity (A/T), is constant over that period range.

This paper points out the errors introduced into the mb estimate of an earthquake when T is not restricted to lie relatively close to 1 second. These errors arise because of the shape of the seismic source spectrum and the relative scaling of 1 second period amplitudes with respect to other periods. As an example, it will be shown that the use of amplitudes with a period of 0.3 seconds can lead to error in the estimated mb magnitude by as much as 0.5 magnitude units. However, by using the known source spectrum scaling in a region, it will be shown how to obtain an equivalent mb magnitude from waves with periods other than 1 second.

First Page Preview

First page PDF preview
You do not currently have access to this article.