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Approximately 90% of the seismic activity in the contiguous United States occurs in California and western Nevada (Figure 8--2). Although the risk of catastrophic earthquakes in the western part of the United States is widely recognized, few people realize the high probability that a major earthquake will hit the eastern United States in the next several decades. For example, a series of three great earthquakes (estimated magnitudes 8.6, 8.4, and 8.7) all of intensity XII occurred during a 3-month period in the winter of 1811-1812 near the town of New Madrid, Missouri. Although little loss of life occurred in the then sparsely populated area, the earthquakes were felt over most of the United States east of the Rocky Mountains and caused destruction for hundreds of miles. The New Madrid fault system is less well studied than the San Andreas system, but New Madrid-sized earthquakes may recur at 600- to 700-year intervals. Seismologists now feel that enough strain may have developed in the New Madrid seismic zone to produce a magnitude 7.6 earthquake, which would be damaging over 200,000 square miles.