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, have clearly been active within the past few thousand years. Thus mid-continental faults “turn on” and “turn off” on timescales of hundreds or thousands of years, causing large earthquakes that are episodic, clustered, and migrating. We hypothesize that this spatio-temporal variability results from interactions among the faults in a region. The faults form a complex system, in the sense that the system’s evolution cannot be understood by considering an individual fault. Complex systems arise in many branches of the physical, biological, and social sciences, as reviewed in the April 2, 1999 issue of Science. In complex systems, the whole behaves in a fashion more complicated than can be understood from analysis of its component parts. As a result, such systems can evolve in many different ways. Studying them requires moving beyond the traditional reductionist approach, in which we focus on the system’s simplest component, understand it in detail, and generalize it for the entire system. Instead, the system should be viewed as a totality, such that local effects in space and time result from the system as a whole. Thinking of mid-continent faults as a complex system may explain much of why we have been unsuccessful in understanding them. To date we have focused on individual faults in isolation, whereas in a complexsystem approach, the key is the interactions among faults. An ideal isolated fault behaves relatively simply. Earthquakes occur when stress on the fault exceeds its frictional strength. When stress reaches the critical value, the fault slips in an earthquake, releasing the accumulated elastic strain, and the cycle repeats. An isolated fault acted on by constant or slowly varying forces thus produces quasi-periodic earthquakes because the forces steadily build up stress. This process becomes more complicated due to interactions among faults. A large earthquake not only releases stress on the hosting fault, but also changes the stress on other segments of that fault or nearby faults by instantaneous (elastic) stress transmission and transient stress migration related to viscous relaxation in the lower crust. Furthermore, long periods of mechanical locking (absence of earthquakes) or clusters of repeated earthquakes on one fault can affect the loading rate on neighboring faults. In such a system the rate of strain accumulation on a given fault is not constant. Instead, it varies depending on the forces acting within the plate, the geometry of the fault system, and the rheology of both the faults and the material between them. As a result, the locations of large earthquakes vary in space and time. This view explains why mid-continent earthquakes and ones at plate boundaries behave differently. Because the physics of fault rupture in these two environments is essentially the same, we have been puzzled about why the spatio-temporal patOpinion