Earthquake Swarm Continues in Dillsburg, PA

Experts try to calm residents

related: Dillsburg Earthquake Swarm Could be Record-Setting
3 More Earthquakes Rock Penn.




November 11, 2009
Carlisle Sentinel

The people gathered in Dillsburg Wednesday night to learn more about the local earthquake swarm had one burning question: “Are we working up to a big one?”

The answer is probably not, according to the panel of experts who spoke and then took questions.

“I think you can be fairly assured that we’re not building up to something catastrophic here,” said Charles Scharnberger, a retired geology professor from Millersville University. Pennsylvania is a pretty stable region tectonically speaking, he said, and those don’t tend to have big earthquakes.

But, Scharnberger added, the fact remains that there’s a lot that isn’t known about this series of small earthquakes, of which 817 have been reported to date, several of a magnitude about 2.9 but most registering at less than 1 on the Richter scale.

Image: Just a few of the hundreds of tremors that have hit near Dillsburg , PA

He reminded the crowd of about 75 people that when he spoke to them last October, not long after the first noticeable tremor, he told them he expected them to stop soon. That prediction was based, he said, on reports of similar earthquake swarms in the northeastern United States, ranging from New Jersey and Maryland to Connecticut and up through Ontario. All lasted from six to nine months, he said; Dillsburg is going on 14, and so far he hasn’t found reports of any other earthquake swarm that lasted as long.

“How long it’s going to go on, I don’t know,” he said.

Someone asked how big the largest quakes in those swarms were.

“Generally about 3 (on the Richter scale), which can be exciting but isn’t really going to do any damage,” Scharnberger said.

Still Not Sure

Scharnberger and Jeri Jones of Jones Geological Services in Spring Grove said they suspect there have been at least several hundred tremors that were not recorded, possibly starting as early as August 2008. Local quakes have generally been less than a kilometer below the surface of the earth, compared to the 5 to 10 kilometers below of quakes in California’s San Andreas fault. Most epicenters are clustered in a small area on Old York Road near Orebank Road and Mandy Lane just south of Dillsburg, they said, but data indicates that a number have originated significantly outside that area.

As for what prompted the earthquakes, and why they’re happening now, the men said they don’t have a definitive answer. It’s possible it’s a cyclical thing and the last occurrence was too long ago for us to know about, they said, and it’s also possible that it has something to do with the kind of rock found in the region and continental plate stresses.

One theory they’re especially interested in has to do with the effect of rainfall. Jones said they’ve started pulling precipitation data and comparing notes, as other areas have documented a connection between how much rain they have and how many quakes are recorded. So far there doesn’t seem to be a direct connection here, they said, but they’re going to keep looking into it.

More technical information about Dillsburg’s earthquakes is available at the following Web address: www.dcnr.state.pa.us/topogeo/openfile/dillsburg.aspx.

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