How Old is the New Madrid Seismic Zone?

Current seismicity levels on the New Madrid seismic zone should produce about 0.11 cm/year of horizontal slip which, when compared with uplift of 42 m in the subsurface strata below the Lake County uplift and assuming a 31° reverse fault model, indicates that the present seismicity levels could not have been present for more than about 64,000 years. If seismicity in the region has persisted for a much longer period of time, then (1) the seismicity has moved spatially between several deformed zones (Crowley's Ridge and the Crittenden County fault zone); (2) the seismicity is episodic in nature, and active periods similar to the present occur between long quiescent times; or (3) there have been far fewer large earthquakes than predicted by extrapolation of the Gutenberg-Richter relation to higher magnitudes. Any of these scenarios indicates that assessing the hazard from large earthquakes is more complicated than conventional analyses have assumed because either the seismicity locations or rates change or analysis techniques relying on the Gutenberg-Richter relation are invalid for estimating the recurrence times of large earthquakes in the New Madrid area.