A series of 10 “seismic events” occurred in the Bellefonte area Thursday, scientists with the Pennsylvania State Seismic Network determined.
Andy Nyblade, professor of geosciences at Penn State, said his team of researchers at the Pennsylvania State Seismic Network detected two 1.1 magnitude events near Bellefonte, one on Axemann Road near the Centre County Correctional Facility and the other on the end of the Purdue Mountain.
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The other eight events “we can’t locate,” he said, because the magnitudes were smaller and the data quality was “not sufficient.” The magnitudes of the tectonic events ranged from 0.3 to 1.1.
On Thursday, residents of Bellefonte and its surrounding areas, including students, teachers and staff at Bellefonte Area high and middle schools, reported feeling “tremors” and hearing loud “blasts” between the hours of 11:30 a.m. and 3 p.m.
“The waveforms in the data are consistent with small, shallow earthquakes, but they’re not necessarily conclusive of them being earthquakes,” Nyblade said.
Scientists can’t rule out other possibilities, he said, such as a mine collapse or sinkhole.
The last earthquake that originated in this area and was felt by people happened in Centre Hall in 1991. The quake registered between a 2.7 and 3 on the Richter scale. It was caused by the movement of a small fault located in the upper crust of the earth, most likely west of Centre Hall, according to two Penn State professors who studied the quake at the time.
Brian Dunst, geologist supervisor at the state Department of Conservation and Natural Resources’ Bureau of Topographic and Geologic Survey, said that the data they have point to Thursday’s event having been a natural occurrence.
Dunst said that having consulted with a state Department of Environmental Protection representative, he does not believe the tremors are connected to collapsing mines.
“There’s nothing in that area, or they have no reports of anything nearby, that would indicate that that’s caused by a mine collapse,” he said.
Both Dunst and Nyblade said there was a possibility of a small fault line near Axemann Road that might have had a bit of movement Thursday.
“You find rivers and streams, they follow fault lines,” said Nyblade.
Bellefonte borough Assistant Manager Don Holderman said that when he spoke to officials at Graymont quarry in Pleasant Gap, they gave him no indication that the tremors stemmed from activity there.
Similarly, a spokesperson for Glenn O. Hawbaker in Pleasant Gap said they were not performing any activities Thursday that would have caused tremors or loud noises.
The Bellefonte Police Department was so overwhelmed with phone calls from people who were hearing noises and feeling tremors that it issued a statement asking residents not to call the school district, police department or 911.
“This ties up phone lines for people that are truly having an emergency. In future events we will get the information out to the public as soon as we find out the real cause,” the Facebook post said.
Dunst said it was understandable that many people took notice of the tremors and noises and panicked. The booming noises, he said, were “an indication that that earth movement was very shallow.”
“If they’re shallow — and this is one theory — the high-frequency waves intersect with ground surface and are converted to soundwaves. So that’s what people hear,” he said.
That Bellefonte residents reported walls shaking and pictures falling down during a 1.1 magnitude event — it usually takes something over 2 magnitude for that sort of activity — is also an indication that the event was not too far below the ground, Dunst said.
As to whether residents can expect something like this to happen again, Dunst said it’s hard to tell.
“Earthquakes are very difficult to predict,” he said. “These are very small events, and because of that, they’re very difficult to measure. Those signatures have to be seen on multiple seismographs before we can get an idea. And at this magnitude, we’re pushing the limits of the instrumentation.“
Anyone who does continue to experience tremors and bangs from below the ground is encouraged to report what they feel and hear to the U.S. Geologic Survey website: Did You Feel It? or contact the Pennsylvania Geologic Survey, Dunst said.