Using the maximum likelihood method and a simple least-squares fit relating magnitude and the number of earthquakes per year in the central Mississippi Valley, Nuttli obtained equations which predict that the average recurrence period for a New Madrid-size earthquake is 810 and 980 years, respectively. Using different variants of Gumbel’s theory of extreme events, the average recurrence period is found to be 1100 years (Type I) to 215,000 years (Type III based on recent data only). Conventional methods, including Gumbel’s Type I theory, which make no allowance for an upper limit on the size of expectable earthquakes may overpredict the hazard from earthquakes which are larger than any which have occurred in the historic past.

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