The center of the U.S. saw earthquakes two centuries ago that were powerful enough to briefly reverse the flow of the Mississippi River. But unlike Californians, who must live with the specter of “the big one,” Midwesterners may have already seen the last of them. New research suggests the crack in the earth behind the Mississippi Valley events may actually be shutting down. If so, geoscientists will need to rethink how earthquakes work.

Three of some of the largest earthquakes recorded in U.S. history struck over the course of two months from 1811 to 1812, the last of which destroyed the small town of New Madrid, Mo. Geologic evidence indicates that major earthquakes also shook this region around A.D. 1450, 900 and 490, implying a cycle of about every 500 years. This pattern fit in with the conventional view of faults—they build up stress and eventually relieve it violently, only to start anew.

Now millions of people dwell in the area of the New Madrid fault zone, which includes parts of Illinois, Indiana, Missouri, Tennessee, Arkansas, Kentucky and Mississippi. Conflicting evidence has emerged, however, regarding what risk they face. To assess how much energy a fault is building up, investigators look at the amount of motion in the ground there. Estimates in the past 20 years have varied from seven millimeters of movement a year to none whatsoever. As a consequence, the risk calculations have varied wildly, from a 10 percent chance of a disastrous earthquake in the next 50 years to a chance of just 1 percent or less.

To settle the matter, geophysicist Eric Calais of Purdue University and seismologist Seth Stein of Northwestern University monitored the heart of the fault in southern Missouri and Tennessee with nine GPS antennas mounted on this ground for eight years. The devices can detect as little as 0.2 millimeter of movement—roughly the thickness of a fishing line—but they recorded no movement at all. Even if the earth moved at just below the detection limit of 0.2 millimeter, it would take a minimum of 10,000 years to accumulate enough energy for a magnitude 7 or greater quake to occur, the researchers conclude in the March 13 Science.

Hence, instead of recharging as the San Andreas fault is, New Madrid may be deactivating. The difference might be placement—whereas the San Andreas fault is found near the edge of tectonic plates, where a great deal of seismic violence occurs, the New Madrid fault is located in the heart of a continent, smack in the middle of a plate. “There’s a whole new worldview that’s emerging about how faults turn on and off within continents,” Stein says.

Investigations of past records of big midcontinent earthquakes now hint they occur in clusters at one fault for a while and then migrate somewhere else, Stein explains, with the New Madrid fault “just being the one that’s been active the most recently.” How this migration occurs and where the energy comes from in midcontinental earthquakes still remains a big mystery. “Instead of focusing on one major, long-lived fault like the San Andreas, we need to think of how different faults interact,” Stein says. Calais suggests that major earthquakes at one fault increase stress at others, making some of them more likely to “unclamp.” Geophysicists Mian Liu of the University of Missouri–Columbia and Qingsong Li of the Lunar and Planetary Institute in Houston are currently modeling how such energy might transfer within plates from one fault to another.

Seismologist Susan Hough of the U.S. Geological Survey office in Pasadena, Calif., thought that Calais and Stein’s findings were strong. But she points to research in 2000 that speculates that major earthquakes could still occur with little evidence of warping at the surface, “just like hidden time bombs.” And, Stein observes, “research might look for other faults that are turning on.” So even if New Madrid is shutting down, the Midwest might not be safe from earthquakes yet.

Where the Fault Lies within a Plate
Major faults occur at the point where two tectonic plates collide. But they can also exist within plates, as the New Madrid fault does in the middle of North America. Scientists are still unsure what powers these intraplate earthquakes; they may result from some combination of forces, such as the removal of glacial sheets after the last ice age or the drag from the fluid mantle underneath the plates. Earthquakes from such faults can be quite powerful and deadly. “Perhaps the most damaging earthquake in Australia’s recent history was in Newcastle in 1989, when a quake measuring 5.6 killed 13 people and left a damage bill of billions of dollars. It was in the middle of the continent,” recounts Seth Stein of Northwestern University. China and northern Europe may also be good places to investigate such activity.

Note: This article was originally printed with the title, "Rumble Off".