FAYETTEVILLE, Ark. - A University of Arkansas researcher and her colleagues have used surface uplift along the Mississippi River to more precisely date a 15th century earthquake and estimate its magnitude to be similar to the one that shook the New Madrid Seismic Zone (NMSZ) in 1811 and 1812. Their techniques for determining uplift and reconstruction of the river's response to the quakes may help locate past epicenters and estimate the relative magnitude of earthquakes in this and other parts of the NMSZ.

Margaret Guccione, professor of geosciences, Karl Mueller of the University of Colorado, Boulder, and their colleagues report their findings from several years of study in the current issue of Geomorphology.

The researchers examined an area called the Tiptonville Dome and Reelfoot Lake, nestled in western Tennessee and Kentucky. They focused on the Reelfoot Scarp, a geologic fold that crosses an abandoned Mississippi River channel near Tiptonville, Tenn. Scientists study the dome because past earthquakes in the NMSZ shaped its formation. Guccione and Mueller determined that the movement along the fault beneath the Reelfoot Scarp averaged about six millimeters per year over the past 2,300 years but that most of this movement—about 13 meters—has occurred in the past 500 years.

Historic reports from the 1811-1812 earthquakes indicate that the lake and at least part of the dome formed during that seismic event, but until now researchers had not determined how much of the dome might have been present before 1811 or how long it has existed.

"The uplift of the Tiptonville Dome, and therefore the much of the movement on the fault beneath the dome, probably happened in two earthquakes in the past 500 years," Guccione said. Previous estimates have incorporated three historic earthquakes into the equation. "This means that the same amount of energy was released in two earthquakes rather than the three or more earthquakes other researchers have estimated. Thus, the two earthquakes were probably stronger than previously estimated," she said.

The Mississippi River provides opportunities for geologists to learn more about earthquakes because it cuts through an active seismic zone. The Mississippi, like all rivers, is sensitive to changes in elevation associated with earthquakes. Researchers date the earthquakes by radiocarbon dating river sediment.

When Guccione and Mueller looked at the Mississippi River's historic response to geologic events, they discovered that the Tiptonville Dome rose, and the basin dropped, causing a difference in elevation of about four meters in approximately A.D. 1470 to 1480. Guccione also found evidence that small streams in the area had dammed up and filled around this time, the water in them unable to surmount the uplift caused by quakes. A similar amount of uplift occurred during the 1812 earthquake.

The Reelfoot data show that, in some cases, researchers can determine the amount of surface deformation for individual events, instead of averaging the deformation over time.

"This has been done along faults that have cut bedrock, but we have shown that it can also be done for folding of unconsolidated sediment," she said.

The researchers also discovered that the mighty Mississippi River is still adjusting to the earthquake that rocked its banks 190 years ago. The team estimates that the channel at Tiptonville experienced about 4 meters of uplift in the 1812 earthquake, reducing the river's depth by 15-20 percent

"Apparently it was easier for the river to widen its channel rather than scour the base of the channel back to its normal depth, and it has continued in this pattern for the past 190 years," Guccione said. "This suggests that the river is not able to adjust to large vertical topographic changes very rapidly, even though it is eroding into unconsolidated sediment." # # #