To report feeling an earthquake, go to the U.S. Geological Survey's "Did You Feel It?" site at http://earthquake.usgs.gov/earthquakes/dyfi/ and click on "report unknown event." To sign up for the U.S. Geological Survey's earthquake notification service, go to https://sslearthquake.usgs.gov/ens.
Two magnitude 2.0 earthquakes struck early Friday morning about four miles southwest of Ringgold, Ga.
The first struck at 1:57 a.m. at a depth of 6.3 miles and the second hit at 2:26 a.m. at a depth of 7.8 miles., according to the Memphis-based Center for Earthquake Research and Information.
Mitch Withers, a research professor with the earthquake center, said that 2.0 magnitude quakes hardly would be noticeable.
"If somebody was right on top of them, I suppose it's possible," Withers said. For example, someone who was lying in bed in a quiet house might have noticed it, he said.
Seismometers in Tennessee and Georgia detected the quakes, said Paul Caruso, a geophysicist at the U.S. Geological Survey National Earthquake Information Center in Golden, Colo.
"Sometimes one quake will trigger another quake," Caruso said of Ringgold's back-to-back temblors. "It's hard to say what the cause is, especially with really small quakes."
Caruso estimated that a million magnitude 2.0 quakes strike worldwide in a year.
"In California, they're just rolling in continuously," he said.
Withers said one way that people learn about small earthquakes these days is by signing up for the U.S. Geological Survey's Earthquake Notification Service. Users receive notifications via email or cellphone texts. The service is customizable, so those who sign up can get notice about earthquakes in their area or only a certain magnitude, Withers said.
Ringgold is in the East Tennessee Seismic Zone that stretches from Fort Payne, Ala., to southwestern Virginia. The zone gets about 70 to 100 earthquakes per year, mostly in the magnitude 2.0 range, Withers said. The zone is about half as seismically active as the 150-mile-long New Madrid Seismic Zone that extends into five states, including western Tennessee.