The potential for a large earthquake along the New Madrid seismic zone in the central Mississippi Valley should be considered a serious threat, according to a new study spearheaded by a University of Colorado at Boulder professor.
The new analysis is one of many studies that have been conducted in the New Madrid fault region, extending through Arkansas, Missouri, Kentucky, Illinois and Tennessee, said CU-Boulder geological sciences Assistant Professor Karl Mueller. Between 1811 and 1812, three catastrophic earthquakes with magnitudes greater than 7.0 occurred during a three-month period, followed by hundreds of aftershocks.
The study by Mueller and his colleagues focused on the Lake County region of Tennessee, where they gathered various types of new evidence on a pivotal "blind-thrust" fault. These faults, lurking deep beneath Earth聮s surface and which can raise or lower the landscape during earthquakes without breaking the surface -- were largely unknown until 1994, when the Northridge Earthquake demolished parts of the Los Angeles area.
"Our evidence shows the New Madrid seismic zone is indeed a threat, which contradicts a recent study of the seismic hazard of the region taken with satellite data," Mueller said. "For the first time we can see how fast the earthquake engine is running and how long it takes to build up energy for a quake."
A paper on the subject by CU-Boulder聮s Mueller and graduate student Jocasta Champion, Margaret Guccione of the University of Arkansas, Fayetteville, and Keith Kelson from William Lettis and Associates of Walnut Creek, Calif., appears in the Nov. 5 issue of Science, the nation聮s leading weekly science magazine.
As part of the study, the research team hired backhoe operators to dig long, deep, trenches in the floodplain of the Mississippi River in the Reelfoot Lake region, exposing recent deformation in the sediments. A hallmark of blind-thrust faults is the folding of rocks or sediments beneath the surface as they absorb the upward motion of the energy released by ruptured faults.
The team measured the widths of "kink bands" in exposed trenches 聳 sharp bends in rocks and sediment formed as material is pushed upward 聳 and the age of the folded sediment. They concluded the slip rate of the Reelfoot thrust fault was about six millimeters per year over the past 2,300 years, totaling nearly 45 feet.
"The kink bands allow us to infer where the bends are in the fault and how it has changed direction," said Mueller. "If you know the shape of the fault, then you know where the seismic gun is pointed."
They also used an imaging technique adapted by former CU-Boulder graduate student Adam Bielecki, now with Research Systems Inc. in Boulder, to produce 3-D topographic relief maps of the area that enhance the vertical hills, bumps and wrinkles. "It聮s like taking a map of an area and turning it into a landscape, allowing you to see features that you cannot see from the ground," Mueller said.
The team also calculated the annual slip rate of other large faults in the area by combining the slip rate on the blind-thrust fault with the shape of other large, nearby faults. Although the Reelfoot fault is one of many faults underlying the New Madrid seismic region, the faults are linked together into a multi-forked, lightning- bolt shape, causing them to slip in a consistent manner during earthquakes, he said.
Researchers have been able to estimate the length, depth and location of the New Madrid seismic zone by measuring scores of aftershocks that still occur annually as the crust adjusts to the 1811-1812 quakes. "The locations of these aftershocks illuminate the fault as if we were shining a light on it," he said.
Although no one knows the exact magnitude of the 1811-1812 earthquakes, Mueller and his team estimate they were about 7.5, based on the speed of the past fault slip. Relatively few people inhabited the region during the great 1811-1812 quakes, which reportedly caused church bells to ring as far away as Boston.
"If a similar event occurrs in the future, Memphis and St. Louis would be the hardest hit," said Mueller. "And if the Mississipi River was running high at the time, there is the possibility of broken levees along some 150 miles of the river."