abstract

This paper presents a methodology for correcting body-wave magnitudes for the effect of focal mechanism in a routine manner. The method requires a knowledge of the prevailing or dominant mechanism for a geographic region, from which tables are constructed which enable one to make the necessary correction. Included in the paper are tables for Aleutian Island, Kamchatka and mid-Atlantic Ocean earthquakes.

From a study of seven earthquakes, it is concluded that the present method gives essentially the same average magnitude with the same standard deviation as a more exact method of correcting for the focal mechanism. The latter method uses the focal-mechanism parameters of the earthquake, which must be determined independently for each earthquake.

The existing distribution of seismograph stations is such that transform-fault earthquakes of the mid-Atlantic Ocean will consistently have their P-wave magnitudes underestimated by about 0.2 magnitude units, if no correction is made for the focal mechanism. On the other hand, P-wave magnitudes of earthquakes in Kamchatka and south of the axis of the Aleutian Trench will be overestimated by about 0.2 units.

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