The first of a series of Wabash Valley quakes, before 5 am April 18th, 2008, 128 miles east of St. Louis, 38 miles NNW of Evansville, Ind., shook the Midwest from Canada to Georgia, but caused very little damage.
Wabash Valley Indiana/Illinois Shakes - "New Kid on the Block"
The concern of Douglas Wiens, Ph.D., and Michael Wysession, Ph.D., seismologists at Washington University in St. Louis, is that the New Madrid Fault may have seen its day and the Wabash Fault is the new kid on the block.
"There hasn't been a magnitude 6 earthquake on the New Madrid zone in more than 100 years, yet in 20 years there have been three magnitude 5 or better earthquakes on the Wabash Valley Fault," said Wyssession, associate professor of earth and planetary sciences. "There is evidence that sometime in the past the Wabash Valley Fault has produced as strong as magnitude 7 earthquakes. On the other hand, the New Madrid Fault has been very quiet for a long time now. Clearly, the Wabash Valley Fault has gotten our deserved attention."
Wysession said that the North American Earth's crust is filled with cracks and faults, and that an earthquake can occur anywhere on the continent. Many of the faults are undetected.
"As the continents bang into each other, sometimes they pull apart, and the crust cracks and ruptures, causing earthquakes," he explained. "This whole region of New Madrid and the Wabash Valley seismic zone became a rift zone about 750 million years ago when the continent almost broke apart. There was a lot of volcanic activity, a lot of seismic activity. The crust got stretched and thinned. By looking at seismometers, we can actually see many of these faults in the thinning of crusts underground."
Wysession said that an earthquake in the Midwest will be felt ten times farther away than one occurring in the western United States because the crust beneath the Midwest is very old, stiff and cold. The rock is about 1.7 billion years old and the seismic waves can travel very long distances through this type of crust. It can be felt hundreds of miles away, even if it was a smaller earthquake. In the western United States, the rock is hotter, and thus it dampens the shock waves and they are not felt as far away.
The quake was not part of the New Madrid fault, but is in the Wabash Valley seismic zone. The USGS says quakes from this area are generally not major, but are felt in a wide area, as was this one.
The Caborn Fault (named after a tiny town just west of Evansville IN), is a steep, nearly vertical, strike-slip fault, probably dating back to the Precambrian Era. It may have "reawakened" earlier this decade.
The latest quake would be a red dot just below the northernmost red dot on this map from a 1987 quake. The second red dot, just west of Evansville shows a quake from 1968. All three quakes are in the 5.1 - 5.3 magnitude range.
Click the map above to download a poster of Wabash Valley fault.
Many earthquakes are too small to make news or even be noticed. In the 3 hours immediately after the quake, 20 more happened in the US, probably unrelated to this one. See USGS current list of all quakes
The USGS estimates that there are 500,000 detectable earthquakes in the world each year. Of those 500,000 earthquakes, only 100,000 can be felt and 100 of those earthquakes cause damage.
Map of the New Madrid and Wabash Valley seismic zones. Red circles indicate earthquakes that occurred from 1974 to 2002 with magnitudes larger than 2.5 (University of Memphis).
Green circles denote earthquakes that occurred prior to 1974. Larger earthquakes are represented by larger circles.
From USGS Fact Sheet 131-02, " Earthquake Hazard in the Heart of the Homeland"
ST. LOUIS (AP) -- Eastern Missouri residents are scrambling to get earthquake insurance after last week's 5.2 magnitude trembler rattled windows and shook houses throughout the area. But they are finding out there's a waiting period for coverage, even as aftershocks continue to shake the ground.
The state's biggest earthquake insurance providers have imposed a 30-day moratorium on the policies after last week's quake. State Farm Insurance, the largest provider, and American Family Insurance, the second largest, both say the moratorium is standard policy.
"You wouldn't want to be in a situation where there was an event, and people rushed out to buy earthquake coverage for the possibility of an aftershock," said American Family Insurance spokesman Steve Witmer. "It's an industry principle that people spread the risk on assumptions of risk, and not wait until the event happens and then buy the insurance."
U.S. Geological Survey Earthquake Hazards Program
Missouri is the third-largest market for earthquake insurance coverage in the United States, according to the Missouri Earthquake Insurance Task Force, created by Gov. Matt Blunt last year.
In 2006, Missouri homeowners and businesses spent about $75.9 million on earthquake coverage, according to the task force. Only California and Washington spent more. Most of the policies were held in eastern Missouri, where residents are worried about a cataclysmic quake on the New Madrid fault.
The insurance task force estimates that a 7.2 magnitude earthquake today would cause $80 billion in damages.
Both American Family and Shelter Insurance said last week's quake created minimal damage. American Family inspected a number of homes, but didn't find any damage that resulted from the tremors, Witmer said.
Shelter spokeswoman Alicia Robinson said the company is investigating 63 damage claims in Missouri.
About 38 percent of all homes in Missouri are covered for earthquakes. In communities around St. Louis, between 58 percent and 81 percent of homes are covered.
The Illinois State Geological Survey in 1996 said Wabash Valley has produced a dozen moderate quakes in the century, after the last 1895 major New Madrid region quake. It says Wabash Valley likely had several shake of magnitude 6.5 or larger, 4000 to 6000 years ago. But those are not what's shaking lately, so it's difficult to say where and how many shakes are ahead for this region.