Deviations from plate rigidity, in areas of intraplate seismicity such as the Wabash Valley and New Madrid seismic zones (21), are being investigated. The results can be surprising. For example, GPS data in the New Madrid zone show very small deformation, suggesting that the recurrence time for large earthquakes may be much longer than previously assumed, and implying that the seismic hazard at New Madrid may be considerably less than often assumed. Such studies of intraplate seismic zones should shed light into the stresses within continental interiors and how they cause deformation and earthquakes.

Figure 8 - The expected recurrence interval for a great earthquake in the New Madrid seismic zone can be estimated from GPS observations showing at most a few mm/yr of fault-parallel motion, assuming the horizontal slip in the great 1811-1812 earthquakes was a few meters, as inferred from historical accounts.