SUMMERVILLE -- The fourth small earthquake in a month rattled Summerville early Wednesday, but no need to crouch in a doorway just yet.
The cluster isn't too unusual for an active fault zone, such as the Lowcountry, and it's way short of an "earthquake swarm" that potentially signals a bigger eruption.
What would be alarming? A hundred quakes or more over a few months, said Erin Beutel, S.C. Earthquake Education and
Preparedness Program director at the College of Charleston.
The recent cluster "is a reminder (of the earthquake hazard here), not a precursor," she said.
The quake measured 2.6 on the Richter scale, something that would rumble the ground about like a truck passing by. Preceding it have been three quakes: One measuring 1.4 on the scale struck Sunday. That followed a 2.6 quake Dec. 21 and a 2.2 temblor in the Lincolnville area Dec. 7.
All four were centered in the Summerville area.
The quake struck shortly before 3 a.m., evidently centered under the Ashborough East neighborhood off Dorchester Road in the Oakbrook area. But Beutel said analysts couldn't be sure.
Only one instrument picked it up, among an array deployed across the area. That one is a University of South Carolina sensor deployed in a bore hole in Summerville.
"There's no indication these are somehow a precursor of the Big One. People don't need to move out of town," said Jim Knapp, Earth and Ocean Science professor at USC who has studied Lowcountry seismic faults.
Ironically, small quakes release the stress on faults that create earthquakes, so they could also be helping ward off a bigger quake, he said.
The Big One, of course, is the catastrophic 1886 earthquake that devastated the Lowcountry. Thought to have been centered under the Ashley River near Middleton Place, it killed 100 people, leveled nine of every 10 brick buildings in Charleston and damaged nine of every 10 in Summerville.
It is thought to have been a 7.3 on the earthquake-magnitude Richter scale and considered the worst recorded in the eastern United States.
A cluster similar to this most recent one occurred in Summerville a few years back, Beutel said. They might occur more often and are just now being picked up because of advances in the sensors and deployment, she said.
Dozens of small quakes occur around the world each day, and it's not unusual for the Lowcountry to have a dozen in a year. A swarm of "foreshocks" can signal a bigger earthquake, but sometimes don't.
A major quake can occur with few or no precursors. Analysts couldn't be sure at first if the 5.8 earthquake in Virginia in August was the major temblor or a precursor to a larger one, Knapp said.
Studies are under way on "precursor" factors, such as foreshocks, water table rises or radio frequency changes that can occur ahead of an earthquake, Knapp said.
But when it comes to predicting temblors, there's really just no telling so far.
"Unfortunately, earthquake prediction is a horribly imprecise science," Beutel said.
Reach Bo Petersen at 937-5744 or follow him on Twitter at @bopete.
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