Edgefield County’s recent earthquakes may have startled many in South Carolina, but experts say the state experiences many minor earthquakes each year.
“We have hundreds of these a year,” said John Wagner, a retired Clemson University professor of geosciences. “Most of them are too small for people to feel.”
Stephen Moysey, an associate professor of hydrogeophysics and geostatistics at Clemson, said people can expect earthquakes of some magnitude to continue.
“It has been going on for millions of years, and it will continue to go on,” he said. “It’s just a natural part of geology.”
A 3.2 magnitude earthquake shook Edgefield County on Sunday, two days after a 4.1 magnitude earthquake rattled windows, shook walls and jolted residents in the county and across Upstate South Carolina, western North Carolina and northern Georgia.
Less than an hour after the Friday quake, more than 2,500 people from 451 zip codes had reported feeling the tremors.
Reports on the U.S. Geological Survey web site extended as far west as Atlanta, as far east as Charlotte, and from Columbia to the south and beyond Asheville to the north.
Moysey said seismic activity in this part of the country isn’t unusual.
“Historically, this is a very active zone,” he said. “This was an area of active mountain-building millions of years ago. Today, we’re not as active as the West Coast. But this is an area that has faults and can experience seismic activity.”
Moysey said very minor earthquakes are measured around the world every day, but no one hears about them or sees them.
In fact, on the USGS website, during the past 24 hours, six earthquakes of 2.5 or greater magnitude have been reported in Alaska and another six in Oklahoma. The website lists 29 that have occurred worldwide in the past 24 hours.
Wagner said earthquakes with a magnitude below 2 are mostly unnoticed by people but are picked up by instruments.
Wagner said the earth’s crust is constantly adjusting and those adjustments are felt as earthquakes.
“It’s just like you walk across the floor and you hear things creak,” he said. “Really almost every place has minor earthquakes from time to time.”
He said one factor adding to those adjustments in South Carolina has been the damming of rivers in the state, which has replaced valleys formerly filled with air with much heavier water now squatting on the earth’s crust.
“Just like when you walk across the floor, you add weight and your floor readjusts a little bit,” he said. “When you create reservoirs, the crust has to readjust to those pressures.”
The only active deep fault in South Carolina, he said, runs from Summerville to Charleston.
The Charleston earthquake of 1886 which killed 60 people measured 7.3, according to USGS, which describes it as “the most damaging earthquake to occur in the Southeast United States and one of the largest historic shocks in Eastern North America.”
According to the USGS web site, “Moderately damaging earthquakes strike the inland Carolinas every few decades, and smaller earthquakes are felt about once each year or two.”