Mid-Continent Earthquakes as a Complex System

Niels Bohr once observed: “How wonderful that we have met with a paradox. Now we have some hope of making progress.” This situation is happening in the long-frustrating effort to understand large earthquakes in continental interiors. The paradox arises from a series of GPS studies across the New Madrid seismic zone (NMSZ). Large (magnitude >7) earthquakes felt across the Midwest occurred here in 1811 and 1812, small earthquakes occur today, and paleoseismic records show evidence of large earthquakes about 500 years apart in the past 2,000 years. We thus expected to see strain building up for a future large earthquake, but found none. Successive studies confirm this surprising result with progressively higher precision. The most recent analysis shows that present-day motions within 200 km of the NMSZ are indistinguishable from zero and less than 0.2 mm/yr (0.2 mm is the thickness of a piece of fishing line). The NMSZ is thus deforming far more slowly—if at all—than expected if large earthquakes continue to occur as they have. Hence the high strain rates required by paleoearthquakes in the NMSZ must have been transient and have ended. This observation is consistent with the absence of fault-related topography, the small deformation that has accumulated over the fault system's long life, and the jagged nature of the faults thought to have broken in 1811 and 1812. All of these indicate that the cluster of large-magnitude events in the past few thousand years does not reflect the faults' long-term behavior. Such variable fault behavior is being widely recognized in continental interiors. In many places large earthquakes cluster on specific faults for some time and then migrate to others. Some faults that appear inactive today, such as the Meers fault in Oklahoma, appear to have been active within the past few thousand years. Thus mid-continental faults …