The disturbingly large variation in magnitude estimates observable in any earthquake catalogue has led to this analysis of one particular catalogue and definition of magnitude. The method employed by the United States Coast and Geodetic Survey is analyzed in the light of theoretical considerations concerning the probability distribution of the variables involved. In particular, the truncation procedure seems to lead to estimates which consistently underestimate the magnitude for large earthquakes, and perhaps overestimate the magnitude for small ones. An alternative method is suggested which leads to estimates which are in good agreement with those provided by Berkeley, Pasadena and Lamont.

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