Frequency-magnitude-time relationships in the NGSDC earthquake data file

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 M 3 = [ M S , if it is present Max ( m b , M L , M u ) , m b < 6 , M S is not present Max [ ( 2 . 0 m b − 5.4 ) , M L , M u ] , m b ≧ 6 , M S is not present . 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.