The lack of knowledge of the cause of earthquakes in the Eastern United States and the short length of seismic history lead to decisional and statistical uncertainties concerning the seismicity of the East. The effect of these uncertainties on the calculated seismic hazard for the east coast is assessed by deriving probability distributions on the important parameters.
The available earthquake catalog for the Eastern United States is adequate to establish activity rates for seismic sources but not maximum possible sizes of events. Thus it is not justifiable on a statistical basis to assume that events larger than those observed historically in areas of the East will not occur in the future. The Modified Mercalli intensities associated with an annual probability of 10−4 of being equaled or exceeded range from VIII to IX for selected sites from Florida to Maine; these intensities are generally insensitive to the manner in which seismic sources are drawn to represent seismicity, except for those sites which lie within an important seismic source under one hypothesis and outside all seismic sources under a different hypothesis.
Several disadvantages of using a strictly deterministic procedure to determine design intensities are disclosed by this study. Specifically, design intensities established deterministically are sensitive to seismic source geometry and to the largest event observed historically in each source. The risks associated with these design intensities vary by more than a factor of ten for the sites on the east coast examined here.