The geology of sand blows (sand ejected by water squirting out of the ground due to strong earthquake shaking) created by large earthquakes in the New Madrid seismic zone shows that previous sequences of 1811-1812 size earthquakes occurred around 1450 AD, 900 AD and maybe 300 AD. This is an astonishingly high rate of activity: one series of large earthquakes every 500-1000 years on average.