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.
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Add CommentThough I am not scientifically trained, I have lived in both California and Indiana and been present when a quakes occured. As a civilian, the difference that I've noticed is that the California quakes (ie 1971 quake, 6+) felt like one was sitting on a table and people were pulling/pushing the legs in several different directions as once...thus kind of a bucking effect. The quake we just had in Indiana early this spring was more like a rolling effect, like a billowing sheet. It was much more gentle (5.1).
Reply | Report Abuse | Link to thisThe article was interesting, thank you.
an apparent cycle of 500 years is sampled for 8 years and a conclusion is drawn? This is SCIENCE???
Reply | Report Abuse | Link to thisPlease!
I agree that 8 years of data may not be valid to draw conclusions about the status of phenomenon with a period of 500 years. We thought the N Atlantic meridional overturning circulation was shutting down, then new data suggested that it has a high degree of variability with a period of variation significant longer than the original data set. Also, the conclusion about the New Madrid system shutting down is based on the plate tectonics model, where slippage at subduction or transform boundaries provides the elastic energy released in large earthquakes. An alternative hypothesis to declaring a shutdown of a process that has repeated at least 3 times over millenia is that we don't understand the mechanisms that cause intraplate earthquakes. Classic plate tectonics may not fit every earthquake. Certainly there is no lack of seismic activity in the New Madrid zone. It has been suggested that seismic pumping of fluids out of the New Madrid zone was responsible for at least part of the Tri-State lead-zinc deposits. These deposits are late Permian to early Mezozoic, so, declaring a 'shutdown' may be a little premature. However, the result of no detectable surface deformation over 8 years is a very significant finding and a very interesting piece of data.
Reply | Report Abuse | Link to thisThe research data seems to show little LATERIAL movement.
Reply | Report Abuse | Link to thisIf i remember correctly the New Madrid fault movement is primarly vertical and that there is a sometimes mountain range just to the south sometimes it's up and presently it's down. Not to worry! it's at least 250 years to the next movement.
Something i've never heard mentioned but seems logical is the upheavals that would occur earthwide from a massive melting of glacial and polar ice- would seem to me that the natural path for much of that water would be the equator, seeing that it already bulges from centrifugal force of Earth's rotation. This would speed the spin of Earth as well. Whatever the case, there is no doubt that you could not shift that much weight around the Earth and not create mechanical imbalances in Earth's crust that are only corrected with an earthquake. I can foresee such a cascade of events helping to fulfill Christ' prophecy that there will be great earthquakes in one place after another before 'the end' comes.
Reply | Report Abuse | Link to thisHas anyone tried doing a stress analysis of the North American plate using something like NASTRAN? Is the New Madrid fault zone the result of a stress concentration in the middle of the plate?
Reply | Report Abuse | Link to this@PsySciGuy
Reply | Report Abuse | Link to thisThey measured ground deformation on sub-millimetre scales over 8 years; the 500 year cycle refers to major earthquakes. Although related, they are different phenomena. It is science and they do actually know what they're talking about.
I would love to know which is actually the larger fault, the SanAdreas or New Madrid fault, could someone please give me an answer
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