In 1811-1812 a series of large intra-plate earthquakes struck near New Madrid in the Mississippi valley, the largest were estimated to be between M7 and M8 (no instruments were around at the time so magnitude estimates are based on contemporary felt effect reports). The largest of these were probably bigger than the famous 1906 San Francisco earthquake. Since this time the New Madrid Seismic Zone has been quietening down but still has a much higher level of seismicity than most places in the USA (until 2009 the New Madrid seismic zone was the most seismically active area of the USA east of the Rockies, since then it has been overtaken by Oklahoma) Scientists estimate that such large earthquakes probably occur in this region every 500 years or so.
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