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The ground beneath the Dillsburg area isn't completely still.

Every now and then, local geologist Jeri Jones gets a call from a resident reporting a boom or something else to indicate seismic activity.

Recalling the thousands of small earthquakes that shook the area from 2008 to 2010, Jones said it's still a mystery as to what caused them.

He said researchers determined the quakes resulted from the fracturing of an igneous rock called diabase three-quarters to one mile below the earth's surface.

"We don't know why (the fractures happened)," he said Thursday. "Maybe it was from tensions deep in the earth, or something to do with groundwater in the area โ€” we don't know."

There are no permanent stations monitoring seismic activity in York, Jones said.

Because the closest networked active station measuring seismic activity was about 25 miles away at Millersville University and inaccuracy grows with distance, researchers had to wait 10 months to find out where the quakes' epicenter was, Jones said.

"We didn't have any good data when these started. We had to install our own temporary network, and then we got some good data," he said.

More accurate and immediate data wouldn't have helped solve the mystery of why the quakes happened, he said, but it would have helped them determine the quakes' true epicenter faster.

So, Jones was glad to hear that the Department of Conservation and Natural Resources and the Department of Environmental Protection plan to fund a network of stations and make the data live, so it's immediately available to researchers. "The more stations you have recording the same event, the more accuracy you'll get," he said.

Increased efforts: The DCNR announced last week that the two agencies will step up efforts to monitor seismic activity in Pennsylvania, spending about $531,000 over three years to create a real-time network of 30 stations monitoring seismic activity around the state, plus up to five temporary stations that can be used to investigate specific events.

The public will have access to the data through a website that Penn State will create, according to a news release.

"Most of the work (to measure seismic activity) is being done at Penn State," said Helen Delano, a senior geologic scientist with the DCNR Bureau of Topographic and Geologic Survey.

The number of seismic monitoring stations in the state has increased to 30 from just six in 2009, she said. As of now there are no plans to expand the network beyond its current size.

The way it works now, Delano said, researchers can't read instruments remotely: they have to go out into the field to get measurements.

"We're hoping that with this new development, data will be live online to allow for faster analysis and more awareness of what's going on, especially for smaller events," she said.

According to the DCNR's website, Pennsylvania has a relatively low rate of seismic activity compared to more active states like Alaska and California, but the state does get earthquakes, and quakes that originate elsewhere can be felt here.

Fracking: One reason for the intensification of efforts to monitor seismic activity has to do with Pennsylvania's natural gas industry, Delano said.

Natural gas companies drill into the Marcellus Shale rock formation mainly in the southwest, north-central and northeastern parts of Pennsylvania, according to a DEP fact sheet. Marcellus Shale does not extent into York County.

The companies extract the gas by drilling into the shale both vertically and horizontally and use a process called hydraulic fracturing, or "fracking."

Through this process, after the well is drilled, cased and cemented, the drillers pump a mixture of water, sand and other fluids into the well in order to fracture the shale around it, allowing the gas to flow to the wellbore. Each well typically requires anywhere from 1 million to 5 million gallons of water, according to the fact sheet.

"There have been a couple of recent reports of increased seismic activity (due to hydraulic fracturing) in several different states," Sam Earman, professor of geology and hydrology at Millersville University, said on Thursday.

With the growth of the natural gas industry in our state, "it seems like a good idea to see if we can observe activity caused by that in Pennsylvania," he said.

"Hydraulic fracturing itself does not generate the potential for earthquakes," Earman said. But the process generates large amounts of toxic wastewater, which companies often inject into deep wells, sometimes all the way down into the earth's bedrock โ€” a practice known to cause seismic activity.

When earthquakes are caused by non-natural factors, it's called induced seismicity, he said.

A better understanding of Pennsylvania's seismic activity "may cause regulators to stop or limit injections in certain wells to eliminate the chance of problems in the future," he said.

โ€” Reach Julia Scheib at jscheib@yorkdispatch.com.

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