The April 18 earthquake in Illinois, measuring a 5.2 magnitude and felt in parts of 16 states, has piqued the curiosity of seismologists because it did not originate on the notorious and much-feared New Madrid fault in Missouri Bootheel region but rather along the Wabash Valley fault — comparatively overlooked but potentially more dangerous.
It's just one of several "underlooked" faults in the United States that could pose greater risks than their better-known neighbors.
A series of infamous earthquakes occurred along the New Madrid fault at the end of 1811 and the beginning of 1812 — the last one, long estimated at a magnitude of 8.0 but now generally thought to be around 7.0, was felt as far away as Washington, D.C., and spurred what are believed to be the first calls for disaster relief from the United States government.
And while Midwesterners have remained vigilant about the threat posed by the New Madrid fault, seismologists now say the Wabash Valley zone — which covers the middle to middle-lower part of the Wabash River and has its epicenter in Terre Haute, Ind. — represents a far graver danger.
"New Madrid hasn't even had a magnitude 6 earthquake or above in the last century," said Michael Wysession, a seismologist at Washington University in St. Louis who has studied faults nationwide. "It hasn't had a magnitude 5 or above in the last 35 years. Wabash has had two magnitude 5's in the last six years.
"The other thing is we notice no strain accumulating around the New Madrid fault. There's no reason to suggest it will have another good-sized earthquake. Meanwhile, the Wabash Valley fault hasn't had a large earthquake in the past few centuries; it may be more likely to be due."
And if the "Big One" hits in the center of the country, its impact would be felt across a far greater distance than would a similar-magnitude earthquake in the western United States. That's because the earth's crust in the Midwest is very old, stiff and cold, allowing seismic waves to travel mile after mile; by contrast, the rock on the West Coast is much hotter, which dampens shock waves.
"Take a chocolate bar, leave one out in the sun, and put one in the freezer," Wysession said by way of analogy. "When you take the one out of the freezer and you snap it, that wave propagates through the whole rigid bar of chocolate. Take the soft bar and snap it. The wave doesn't propagate nearly as well; it just gets damped down to non-elastic behavior."
A recent re-survey of data by the United States Geological Survey showed that the New Madrid fault poses much less of a risk than was thought 30 years ago. And Wysession believes the Wabash Valley fault deserves the same level of study that seismologists have historically applied to New Madrid.
"Wabash is just like the New Madrid Fault except New Madrid has had its motion," he said. "Yes, we record lots of small earthquakes there, but that's because we have the seismometers there. If you look at earthquakes on a map, the spread is a function of both where the earthquakes are — and where the seismometers are. Now that there have been some earthquakes that have shaken people up a bit, a little more attention will be paid to Wabash."
And yet, neither Wabash nor New Madrid most concern Wysession; nor does the San Andreas fault in California, which caused the San Francisco of 1906.
"The San Andreas is a great example — a couple of seismometers at the time said it was a 7.9, which is a good-sized earthquake," he said. "But if the entire San Andreas ruptures from top to bottom, at that same level of four meters of split that happened in 1906, you'd get an 8.3. That's the largest earthquake you'd get. The Sumatra earthquake (which caused the 2004 tsunami) was a 9.3, the Alaska 1964 earthquake was 9.4 — these are more than 30 times larger."
Instead, Wysession cautions that the Cascadia Subduction Zone, which picks up in Northern California from the San Andreas fault and stretches to northern Vancouver Island, represents the most serious danger for the United States.
Paleoseismic evidence suggests that a huge earthquake strikes the region about every 500 years. The last came on Jan. 26, 1700, before Westerners settled in the area, when Japanese tidal records — accurate to the minute, according to modern scholars — registered the largest tsunami to slam the Pacific.
"If you look at the times that it hit different Japanese ports, you get a direction, and the direction points right back to Seattle and the Pacific Northwest," Wysession said. "It turns out all these trees and forested areas were dropped down and submerged under water. The Native Americans had these stories of gods battling, landslides shaking everything.
"It would take more than 30 quakes of the whole San Andreas rupturing to equal one big earthquake beneath Seattle and Portland. It's been over 300 years since the last one. We're starting to get into that realm where the likelihood is increasing of it happening again."
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