Ky. geologists drilling quake observatory
Friday, October 20, 2006
SASSAFRAS RIDGE, Ky. -- In the ancient sediment deposited by the Mississippi River, scientists are looking for answers to modern problems.
Their findings could mean buildings that better resist earthquakes or roads and bridges that survive severe shaking so relief efforts don't falter at a critical time.
To get the answers, geologists from the University of Kentucky are drilling the deepest earthquake observatory in the eastern United States, going down 2,000 feet or more to the bedrock under far western Kentucky. Drilling crews have been working since September, logging changing layers of sediment so scientists know where to place motion sensors and other instruments.
"A lot of what is in building codes is based on information from California," said John Kiefer, assistant state geologist for the Kentucky Geological Survey. "We need similar information for the central United States."
Crews from Layne Christensen Inc., a Memphis, Tenn., firm that specializes in water wells, are working at a 60-foot tall derrick behind an abandoned grocery at the edge of a soybean field. Late last week they had drilled a 9-7/8-inch hole 1,530 feet into the earth, with a projected completion date by the end of the month.
When finished, the hole near the northern end of the New Madrid Fault will be fitted with a 4-inch pipe that will be cemented into place. Seismic monitors will be placed in the hole based on where the sediment layers change.
So far, driller H.C. Lowery said, the crew hasn't hit anything unexpected and is making progress at the rate of 75 to 100 feet per day.
The New Madrid Fault, a break in the rocks five to eight miles underground, is the most active seismic zone in the central United States. A series of catastrophic quakes in late 1811 and early 1812 rang church bells in Philadelphia.
The fault remains active, with 14 minor quakes in the past seven days, including a 3.4-magnitude temblor Wednesday about two miles northwest of Marston, Mo.
The new seismic observatory will be the "flagship observatory" in a network of motion sensors in Kentucky. There are hundreds of seismic monitors throughout the central United States, including Southeast Missouri, but this is the first attempt to put monitors at every significant level of the sediment layers of the Mississippi embayment, which is what geologists call the delta region.
The embayment stretches from just south of Cape Girardeau, from Crowley's Ridge in the west to the limestone hills of Kentucky and Tennessee in the east and south to the Gulf Coast. Millions of people live above the soil deposited over the eons by the great river.
"The whole embayment area has similar concentrations of material," Kiefer said. "There are silts, sand and gravel layers."
Information gathered by the seismic station will be generally transferrable to the rest of the region, he said.
"What we are really trying to find are the measurements, or estimated measurements, of the seismic waves as they come out of the bedrock," Kiefer said. "No one has actually measured that, put a hole all the way down to bedrock and measure them at levels all the way to the surface."
Depending on the density and composition of the deposits they pass through, seismic waves can be amplified or dampened, he said.
Scientists have documented the waves' motion and understand how unconsolidated sediment like that in the Mississippi delta can slow them down, making the ground motion more violent.
"Earthquakes move in deep rocks at 6,000 to 7,000 miles per hour," said Gary Patterson, information services director at the Center for Earthquake Research and Information at the University of Memphis. "Under the borehole there are 2,000 feet of sediments, with different layers within those sediments, and each layer will modify the wave."
The most immediate impact of the seismic station at Sassafras Ridge, aside from pure scientific value, will be potential modifications of building codes. A new international standard was introduced in 2003, and it is a conservative standard for the central United States, Kiefer said.
Slower shaking in the ground does the most damage to tall buildings, while faster shaking will do greater damage to shorter structures. The results at Sassafras Ridge will allow a fuller understanding of what to expect in any particular location, he said.
"Some people are preventing damage by not building at all," he said. "Right now, you can't build a two-story brick building in Paducah."
Current hazard maps detail only the likelihood of an earthquake and the kind of ground shaking that could be expected. The borehole could result in "different types of hazard maps for engineers to develop different height buildings," Patterson said.
The borehole is being funded in large part by the U.S. Department of Energy, which wants to know if it is possible to construct a safe landfill for low-level radioactive waste at the Paducah Gaseous Diffusion Plant.
"There are a lot of people saying 'not in my back yard,'" Kiefer said. "I think a secure landfill can be built."
The Center for Earthquake Research and Information has an even more ambitious drilling project in mind, Patterson said. It wants to drill a hole right into the heart of the fault, 10,000 to 15,000 feet below the surface. Such a project has already been done in California, he said. "We could study rupture mechanics, how do the earthquakes begin."
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