In performing a statistical analysis of the NGSDC data base, we have found completeness problems in time and magnitude which can affect the accurate determination of derivative frequency-magnitude relations. Techniques are suggested which may circumvent most inaccuracies. In particular, the choice of event magnitude is important in minimizing distortion in frequency-magnitude relations. A new working magnitude, M3, is suggested as useful for the NGSDC data base, where


After correcting for time bias the use of M3 reduces distortion of frequency-magnitude distributions derived from the NGSDC data base as compared to previous magnitude definitions used.

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