When a small earthquake rattled Lancaster County on Sunday, local seismic expert Charles Scharnberger had one immediate thought.
“I had no doubt, I knew right away it was an earthquake,” he said Monday.
“And I knew, oh my God, I’m going to have to spend the next day doing interviews.”
A 2.3 magnitude earthquake struck at 4:49 p.m. and was centered about two miles southeast of Millersville, according to the U.S. Geological Survey.
The brief tremor fooled many area residents into thinking there’d been an explosion nearby and triggered a flood of calls to 911.
Scharnberger, a professor emeritus of the Department of Earth Services at Millersville University, has been talking about earthquakes for several decades.
While Lancaster County has no active fault lines and is by no means a hotbed of seismic activity, it’s the most active seismic region in Pennsylvania, Scharnberger said. In fact, he said, experts in the field refer to it as the Lancaster Seismic Zone.
Jay Parrish, a former Pennsylvania state geologist and Geographic Information System director for the county, agreed the region is pretty active.
“We are in a relatively active area of Pennsylvania, although, in terms of Pennsylvania as a whole, there’s not much activity here,” he said.
“They aren’t big quakes, and there aren’t a lot of them,” Parrish added. “But in an area that doesn’t have a lot of quakes, we have more activity than most.”
Unfortunately, Parrish said, little is known about the underlying structure of the local bedrock.
Earthquakes originate in the deep earth, far below the relatively shallow limestone deposits, Parrish explained. And no one has gone deep enough into the local substrate to learn much about its makeup.
“It usually has very little to do with the surface geology, and we’re semi-blind when it comes to saying what’s happening down there,” he said. “The wonders of geology are still open for discovery.”
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Scharnberger said the “crystalline basement” — the hidden geology underlying the limestone and sedimentary rock — is “very old metamorphic rock” about three miles beneath the surface.
“The basement rock is highly fractured,” he said. Tectonic forces — caused, among other things, by the gradual migration of North America toward the Pacific Ocean — create stress on those fractures, he explained, and when a fracture slips to relieve the stress, the earth quakes.
There are plenty of fault lines lying deep below Lancaster County soil, Scharnberger said, but “they are very ancient,” dating to the formation of the Appalachians.
“They are not very likely to be active,” he said.
“There are faults everywhere,” Parrish agreed. “Most are inactive as far as we know, and very, very old. But there are lots of them.”
Nothing suggests an increase in local seismic activity, Parrish added — and there’s no reason to believe a bigger quake is likely to occur here.
But it’s not impossible, Scharnberger warned.
“I don’t see why not,” he said, noting that a 5.8 magnitude quake was measured not long ago in Virginia.
The U.S. Geological Survey measured a 5.8 magnitude earthquake in central Virginia on Aug. 23, 2011.
“Magnitude 6 or greater earthquakes are rare — very rare — in the East, but they’re not impossible,” he said. “They could happen anywhere.”
It’s also possible, he noted, that a second quake could follow close on the heels of Sunday’s event.
“It’s not unusual for earthquakes to occur in pairs,” Scharnberger said. “It wouldn’t surprise me if we had another one in a couple of days.”