Continental North America's greatest earthquake sequence struck on the western frontier of the United States. The frontier was not then California but the valley of the continent's greatest river, the Mississippi, and the sequence was the New Madrid earthquakes of the winter of 1811-1812. Their described impacts on the land and the river were so dramatic as to produce widespread modern disbelief. However, geological, geophysical, and historical research, carried out mostly in the past two decades, has verified much in the historical accounts. The sequence included at least six (possibly nine) events of estimated moment magnitude M ??? 7 and two of M ??? 8. The faulting was in the intruded crust of a failed intracontinental rift, beneath the saturated alluvium of the river valley, and its violent shaking resulted in massive and extensive liquefaction. The largest earthquakes ruptured at least six (and possibly more than seven) intersecting fault segments, one of which broke the surface as a thrust fault that disrupted the bed of the Mississippi River in at least 2 (and possibly four) places.
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The enigma of the New Madrid earthquakes of 1811-1812