The simple model of an earthquake used in Evernden, et al. (1973) was extended to the conterminous United States and observed patterns of isoseismals for major earthquakes studied in relation to the model. Regional attenuation, a known major factor controlling isoseismal patterns, was quantitatively evaluated. Incorporating this regional variation into the model results in the prediction that in terms of energy released, the San Francisco earthquake of 1906 was 50 times as large as Owens Valley 1872 and more than 100 times larger than the Charleston 1886 and New Madrid 1811 earthquakes. All of these were probably of comparable “magnitude”. Other relevant parameters are investigated. Analysis of probability of occurrence of major earthquakes in the Eastern United States suggests average annual return times of intensity X and IX of thousands of years at least. Presently unknown local conditions may lead to much greater probabilities at some localities, and the importance of developing procedures for determining regions of abnormally high risk is stressed.

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