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Hard ground makes earthquakes rare
Eastern US has fewer temblors than West
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An earthquake occurring in Georgia is about as likely as a hurricane in California.

Although Tuesday's 5.8-magnitude temblor that occurred near Richmond, Va., wasn't centered in Georgia, the shock traveled hundreds of miles causing the ground to shake in Gainesville and even further south.

Julian Gray, resident geologist at the Tellus Science Museum in Cartersville, said the Virginia quake seems to be the result of a fault line that runs from Alabama to New York.

"Whenever stress is built up usually the rock will break in some zone of weakness," Gray said. "It's like if you had a broken bone, it heals, but you have another accident in the future that's more than likely where it's going to break."

Although rare, Georgia does have a history of earth movement as a result of fault lines below the surface of the state.

In fact, there are three major fault lines that travel though Georgia, including one that is located directly under Atlanta and passes through Hall County — the Brevard Fault Zone. Those fault lines, however, "haven't been active in a serious way for hundreds of millions of years," Gray said.

"There are a number of (faults) that parallel the Blue Ridge and Appalachian Mountains," he said.

The last earthquake to occur in Georgia was in April 2003 when a magnitude-4.3 tremor occurred in Menlo. Georgia has experienced 13 earthquakes since 1811.

The epicenter of an earthquake is measured using magnitude, but in areas outside the epicenter, the strength is measured using the Mercalli intensity scale. Using a scale of I to XII, the Mercalli scale measure the stress of an earthquake on the Earth's surface, objects of nature and man-made structures.

Tuesday's earthquake created an intensity of VII on the Mercalli scale at the epicenter and an intensity between II and III on the scale in Georgia, Gray said. A seismic reading that size will occur in the state once every two to three years, he said.

"In an intensity VII there are buildings shaking, things are coming off shelves, stuff like that," Gray said. "What we felt in Georgia, you can detect it. People report it feels like a truck went by."

The makeup of the earth's crust along the East Coast allowed waves from the earthquake to travel hundreds of miles, even reaching as far as Canada, Gray said. He said geologists refer to this crust as "Coal's Crust."

"It's not mobile, it's not active and so it's completely solid and therefore waves are transmitted greater distances," Gray said.

The crust along the West Coast, he said, is much thicker than on the East Coast.

"In the West Coast where they have many more faults, the faults damp the seismic waves from going across because there's a lot of different blocks, so it sort of muffles the signal more than it would here on the East Coast where we have a solid slab of rock," Gray said.

Less than a day before Tuesday's earthquake, another tremor occurred in Colorado. That quake measured 5.3 magnitude - the largest to strike Colorado in four decades.

Gray said there was no relation between the two tremors.

"It's a remarkable coincidence that you have two newsworthy earthquakes in the same day, so they're not connected in any way," he said.


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