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Tree rings bespeak massive U.S. quake

Growth rings harvested from the trunks of bald cypress trees in a western Tennessee lake show dramatic evidence of a powerful earthquake 179 years ago, a researcher says. Roy VanArsdale of the University of Arkansas told the American Geophysical Union last week that corings removed from the trunks of bald cypresses growing in Reelfoot Lake prove the lake formed in the New Madrid earthquake series of 1811 to 1812.

"This gives us physical documentation that the historical accounts of that earthquake are correct," VanArsdale said.

The study also has yielded a bald cypress tree ring signature that could help scientists look for evidence of earlier earthquakes that may have occurred in the area.

The New Madrid earthquake, thought to be the largest east of the Mississippi in recent centuries, was centered along the Mississippi River near where Arkansas, Missouri and Tennessee come together. There are thought to have been at least three ground movements during the winter of 1811-1812.

Modern geologists estimate the New Madrid quake, felt as far as New England, would have registered 8 or higher on the Richter scale.

But because the New Madrid area was then wilderness, historical accounts of what happened in the quake center are sparse and scientists must search for indirect evidence of the event.

VanArsdale said that in the early 1800s, a stream now called Reelfoot River flowed into the Mississippi, meandering through a vast hardwood forest in northwest Tennessee.

When the earthquake occurred, it caused a massive uplift of land that blocked the Reelfoot River and created what is known as Reelfoot Lake.

"This would have occurred in a matter of seconds," VanArsdale said.

As the lake rose, most of the hardwood trees covered by water died. But the bald cypress, which thrives in water, continued to grow.

"These bald cypress are spectacular trees," VanArsdale said. "Some of them are 800 to 1,000 years old."

A team of scientists took trunk corings from 48 of the ancient trees, yielding a cross-section of the annual growth rings. The rings were then dated and VanArsdale said there was a distinctive pattern for the years of the New Madrid quake.

Several of the trees showed a stunted growth for 1812, the growing season just after the quake.

"Some of these trees would have been traumatized by the shaking ground," he said. They would have suffered broken limbs and ripped out roots, thus slowing growth.

But the following years, the trees showed a powerful growth spurt, laying down rings much wider than in previous years.

VanArsdale said the trees grew new benches, the so-called cypress knees that form on the tree at the water level.

Both the growth surge and the stunted growth pattern of 1812 are unique tree rings, he said, and may provide a marker to search for evidence of earlier earthquakes in cypresses growing at other lakes.

"Now that we know what we're looking far, we are checking cypress trees at St. Francis and at Big Lake," VanArsdale said.

Those lakes, south and west of Reelfoot, may also have been shaped by earthquakes, he said.