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TERESA BOECKEL | YORK DAILY RECORD Updated 6 hours ago
JACK GRUBER, USA TODAY
An earthquake that shook York County recently is one of the largest to have originated in York County, said Charles Scharnberger, professor emeritus of earth sciences at Millersville University.
Measuring 2.8 on the Richter scale, the quake struck at 9:11 p.m. Sept. 14, according to the U.S. Geological Survey. It originated just north of Route 30 in the area of Farmers in Paradise Township.
Rumbling could be felt throughout central Pennsylvania and northern Maryland. Some who reported feeling it were as far away as Carlisle, the Philadelphia suburbs, and Frederick, Md.
No damage was reported, York County spokesman Mark Walters said.
York County has experienced earthquakes in the past, including a swarm in the Dillsburg area about a decade ago. However, this was one of the larger ones to have originated in or near York County, Scharnberger said.
Here’s what you need to know about earthquakes in the area:
What’s the potential for a big earthquake in York ?
Based on the record of relatively small earthquakes in the Lancaster Seismic Zone, which would include Lancaster, York and Berks counties, it’s possible to calculate the probability of larger earthquakes occurring, Scharnberger wrote in an email.
“I have done this for a magnitude 6.0 earthquake, which is not unreasonable in light of the magnitude 5.9 earthquake in Virginia a few years ago,” he said. “I find that the probability of a 6.0 quake here is about one in 200 each year.”
In other words, a magnitude 6.0 is the “200-year earthquake” for the area, he said.
More: How big was Pennsylvania’s worst earthquake?
More: Earthquakes in Pa. aren’t new, but recent Mifflintown shake has ‘unusual’ qualities
More: What was that loud boom in northern York County Friday night?
What caused the most recent earthquake?
It’s not entirely clear.
The Lancaster Seismic Zone has a record of earthquakes dating at least to the early 19th century, Scharnberger.
“The rock of the Earth’s crust is under stress — probably arising from the movement of the North American Plate away from the Mid-Atlantic Ridge,” Scharnberger. “The crust relieves this stress by fracturing or, more likely, by slipping along a pre-existing fracture. If that fracture has slipped previously, we call it a fault.”
Two faults are known in the Farmers area, said Jeri Jones, owner of Jones Geological Services.
Where do most earthquakes happen in York County?
York County has been rather quiet in the history of earthquakes, Jones said.
One earthquake in eastern York County in 1978 registered 3.0 on the Richter scale.
The greatest number of earthquakes have been in the Dillsburg area, but they have been a magnitude 2.5 or less, Scharnberger said.
The swarm started with a 2.0 tremor on Oct. 5, 2008, and the booming and rattling occurred periodically until early 2010. The tremors were centered in an area along Old York Road and Brandon Lane in Carroll Township.
Could the area experience another one soon?
“Of course we ‘could,’ but earthquakes are notoriously unpredictable, except for long term probabilities,” Scharnberger said.
What are some things people might not know about local earthquakes?
Earthquakes are not all that rare, Scharnberger said.
Based on their depth, an earthquake can be a simple rumble with no shaking, Jones said. Fracking also causes quakes, but none of that is happening locally.
What other locations have the potential for earthquakes that might affect York County?
In 1881, an earthquake with a magnitude of 5.0 originated in or near New York City. That one was felt in York and Lancaster counties, but did not do any damage, Scharnberger said.
Jones pointed out that Lancaster County is on the edge of a higher seismic zone extending northeast into New Jersey. There could be a larger earthquake in the Big Apple that could be felt in York County.