Old sand geyser shows quakes part of life here

Researcher's digs indicate regular pattern

SUMMERVILLE — A curving line of white thumbtacks stuck in the clay wall of a trench shows further evidence that we're living on an earthquake fault.

It also might provide proof that big ones similar to the devastating 1886 quake happened centuries ago and likely will happen again.

The scene is just north of the old tabby fort wall at Colonial Dorchester State Historic Site. The tacks outline probable evidence of a sand blow, a geyser of sand and water that shoots from the ground during a major earthquake.

"We had no idea what we would find," said Pradeep Talwani, director of the South Carolina Seismic Network at the University of South Carolina.

The sand geyser could have happened during the quake of 1886, which left cracks in the nearby tabby walls and laid waste to much of Charleston. It's more likely, though, that it happened 500 years earlier, Talwani said. He will take samples for carbon dating to find out.

Talwani has dug 115 of these trenches around the Lowcountry in the last 25 years. This is his first in the Summerville area. A pattern has emerged. Earthquakes strong enough to cause sand blows, which include the 1886 earthquake, hit the Charleston area about every 500 years.

While that might seem comforting, Talwani's research should serve as a warning, said Steven Jaume, director of the Earthquake Education and Preparedness Center at the College of Charleston.

"It's more confirmation that we're in earthquake country," Jaume said. "This doesn't just happen in California."

While the big earthquakes are likely every 500 years, quakes are fairly common in the Lowcountry, where faults converge beneath the Ashley River. A dozen or more tiny temblors are recorded each year. A magnitude 4.1 quake shook houses and cracked sidewalks in Summerville in 1995.

The 1886 earthquake, thought to have been a magnitude 7.6 quake, killed 100 people and destroyed or damaged most of the buildings in Charleston and Summerville.

Experts are predicting a smaller but significant quake any time within the next 20 years.

"A smaller quake can still cause a lot of damage," Jaume said.

The trench is about 3 wide and almost 4 feet deep. The top layer was excavated by hand and sifted for artifacts before it was dug, said park manager Ashley Chapman, an archaeologist.

Workers found glass from bottles and windows, pottery made in England and Germany, Chinese porcelain, brick fragments and iron nails, all dating to the 1700s.

"Luckily for us, people did not haul away their trash to a landfill then," Chapman said. "We can tell a lot about people from their trash."

Reach Dave Munday at dmunday@postandcourier.com or 745-5862.

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