Alana Burke can recall the summer morning long ago when the windows in her Lincoln County, Ga., home began to rattle.
It was subtle at first, just a vibration.
"Then the glass in the windows started to buzz, and it almost sounded like a train coming through," she recalled.
Seconds later, it was over.
"It took us a few minutes to figure out what happened," she said. "We thought maybe it was a jet, but it was an earthquake."
The Aug. 2, 1974, quake was the Thurmond Lake area's largest -- registering 4.2 on the Richter scale. But it was by no means the only temblor to jar the earth beneath the 70,000-acre reservoir.
For decades, scientists have monitored the region around the U.S. Army Corps of Engineers' earth-and-concrete dam for seismic activity that has included dozens of small earthquakes.
According to the U.S. Geologic Survey's National Earthquake Information Center in Golden, Colo., Thurmond Lake is one of the major earthquake centers in Georgia and South Carolina.
Other primary areas include the Monticello reservoir near Columbia, the Summerville area near Charleston and Lake Jocassee in upstate South Carolina.
Although east coast quakes are nothing like their deadly counterparts in California, they're still worth studying.
"We monitor them, but by and large, there isn't much damage," said Dr. Pradeep Talwani, a University of South Carolina geophysics professor and director of the South Carolina Seismic Network.
Portions of Thurmond Lake, he said, are atop a series of prehistoric faults -- or ruptures deep within the Earth -- that can spawn seismic activity.
"The Eastern Piedmont Fault System is a long system from the Alabama-Georgia border all the way through South Carolina," Dr. Talwani said.
THE FAULT BISECTSThurmond Lake near Modoc, S.C., and intersects with a smaller crack -- known as the Belair fault -- that runs from the Augusta area to connect with the Eastern Piedmont system, he said.
Such faults, estimated to be about 250 million years old, typically don't yield serious earthquakes. But scientists say the influence of reservoirs like Thurmond Lake can cause mild quakes.
"There was the 4.2 earthquake near Lincolnton, which was between McCormick (S.C.) and Lincolnton right on the river itself," Dr. Talwani said. "Those earthquakes, we think, were induced by the reservoir itself."
As water levels change, pressure in subterranean rock changes also.
"Think of it like air in a balloon," he said. "If you increase air in a balloon, there's greater force in the walls."
Earthquake activity near Augusta is unpredictable, despite ongoing studies -- both by scientists and by the Corps of Engineers, which maintains three major dams along the Savannah River above Augusta.
"It may be very quiet for 10 years," Dr. Talwani said. "Then in '93 or '94, for example, in the month of March, we had more than 200 small shocks, all in McCormick-Abbeville."
IN SEPTEMBER 1996,earthquake activity in the region increased again. The three biggest quakes in the state that year were all on the Edgefield County, S.C., side of the reservoir, measuring 2.5, 2.3 and 2.2 in magnitude.
The Corps is well aware of the earthquake-prone nature of the Thurmond Dam area and monitors the site constantly, said Jim Parker, a Corps spokesman.
When Thurmond Dam was built almost half a century ago, engineers pumped grout into cracks in the bedrock found below the dam site, he said.
"But that's a standard practice for a project like that," Mr. Parker said.
In the 1970s, during construction of Russell Dam above Thurmond Lake, scientists again raised concerns about problems with earthquakes and the structure was redesigned to be stronger and more resistant to quakes.
"There were some seismic concerns about Russell," Mr. Parker said. "Extra funding was used, and it was designed to withstand a 5.5 magnitude earthquake occurring directly beneath the dam with a very shallow epicenter, a very unlikely earthquake in this part of the country."
THURMOND DAM WASdesigned with similar precautions, including a mix of various materials within the earthen portion of the dam on each side.
"The concrete is not a problem," Mr. Parker said. "During an earthquake, the earth embankments are the problem because of liquefaction."
Liquefaction is the process of the earth turning liquid during a violent quake. The use of different materials within the earthen portion of the dam helps avoid failures from liquefaction, he said.
The Corps also has seismographs at Thurmond Dam and other Savannah River projects, Mr. Parker said. The devices are wired to an alarm in the powerhouse that is set off by excessive vibrations.
The equipment is maintained by the Waterways Experiment Station in Vicksburg, Miss., which notifies the Corps each time there is an earthquake, prompting a physical inspection of the entire dam, he said.
So far, he added, there has never been any damage from a quake.
"There was no impact at all from the '74 incident, and that was as large and as close of an earthquake we've had since the project was built," he said.
ALTHOUGH A DAMfailure because of earthquake -- or any other cause -- is deemed nearly impossible, Augusta authorities maintain a detailed evacuation plan just in case the impossible happens.
According to the Emergency Evacuation Plan for Dam Failure and Flooding, prepared by the Emergency Management Agency, the failure of Thurmond would put most of downtown Augusta under as much as 25 feet of water for 13 hours.
Although the initial wall of water would hit downtown in an hour and 50 minutes, it would take several hours for the waters to reach peak destructive flow, the report said. Almost 70,000 people would be endangered.
Overall, however, experts say Augusta is unlikely ever to experience a serious earthquake. The only major quake in the Southeast occurred in 1886 in Charleston, S.C., claiming 110 lives.
ACCORDING TOan account prepared by the U.S. Geologic Survey, the Charleston quake sent major tremors as far as Augusta, where damage was significant.
"In Augusta, the shaking was the most severe in the state," the report said. "An estimated 1,000 chimneys and many buildings were damaged. The business and social life were paralyzed for two days."
The Corps' Mr. Parker said the Savannah River dams are safe from almost any type of failure.
"The last studies we did were 10 years ago, based on a requirement to review the seismic stability of Thurmond Lake," Mr. Parker said. "Based on our conservative criteria, our analysis showed the project has no problems."