CORES OFFER EVIDENCE OF FAULT IN NEW MADRID SEISMIC ZONE

FAYETTEVILLE, Ark. - A University of Arkansas researcher has found evidence that verifies the presence of a suspected fault in the New Madrid seismic zone. This may help scientists better understand the distribution of earthquakes in the seismic zone, which includes the large cities of Memphis and St. Louis.

Margaret Guccione, professor of geosciences, will present her findings today at the American Geophysical Union in San Francisco.

Researchers have speculated that one or more faults might occur within the New Madrid seismic zone, the site of earthquakes in 1811 and 1812 that rang bells in Boston and temporarily blocked the flow of the Mississippi River. However, faults that extend to the surface and have mostly horizontal movement have been obscured by liquefaction of sand in the New Madrid seismic zone. Liquefaction is caused by shaking saturated sand during an earthquake. The liquefied sand may build up pressure and "explode" toward the surface, causing the soil and clay to collapse into the resulting hole. Collapse of the soil can look similar to a fault.

Guccione studied the Bootheel lineament, a 135-kilometer-long linear feature that can be seen by satellite. She took 100 cores up to eight meters deep from a 300 by 300-meter area to reconstruct the geologic history of the lineament for the past 12,000 years. She examined their composition and used radiocarbon dating to determine the age of the materials in the cores.

The cores show that sand deposited by the Mississippi River has subsided a total of 10 feet on the west side of the fault during at least two earthquakes, creating a depression in the ground. During the first earthquake, which occurred between 12,000 and 10,500 years ago, there is evidence for faulting but no liquefaction at the study site. Flooding of the Mississippi River deposited a thick clay at the bottom of the depression but on the other side of the lineament only a thin layer of clay was deposited on the uplifted sand.

During the second earthquake, one that is widely recognized throughout the New Madrid seismic zone and dated about A.D. 1470, both faulting and liquefaction caused additional subsidence west of the fault. This time, liquefied sand partially filled the depression along the fault. Clay subsequently buried the sand to fill the remaining depression and in areas far beyond the liquefied sand.

A channel of a small stream that eroded across the lineament between the first and second earthquakes was displaced by lateral movement along the fault during the at least A.D. 1470 earthquake.

"To me, this movement is a very strong argument that there is a surface fault," Guccione said. "We can demonstrate three meters of vertical offset in the last 10,000 years and more than 10 meters of lateral offset in the last 2,500 years."

Graduate student Curtis Nunn has contributed to this research, which is funded by the U.S. Geological Survey.