Region not used to quake activity

Earthquakes unusual here, but not unheard of, geologist says

When Juniata College geology professor Katy Johanesen heard a low rumbling and felt a vibration while at her Huntingdon home, the former California resident said she knew immediately it was from a mild earthquake and not caused by a passing train or something else.

"I was very excited and immediately took to social media to tell people what it was," she said Thursday afternoon. "I was sitting at a computer with my hands on the desk when I heard a low rumble similar to thunder and felt the vibration. Then, there was a pause of about a second or two followed by another quiet rumbling."

Based on her education as a geologist and her experiences in California -- a more earthquake-prone area -- she knew it was a very mild earthquake that wouldn't cause damage to buildings or injuries. One of her students told her something fell off his bookshelf, but he didn't know why until she shared the news with him. If traveling in a car or playing music loudly with the bass up, area residents would have likely missed it, Johanesen said.

"It would have been easy to dismiss if you didn't know to look for it," she said, describing the rumble as similar to slow, rolling thunder.

The United States Geological Survey reported that the magnitude 3.4 earthquake occurred about 8:30 p.m. Wednesday in Juniata County. The epicenter of the earthquake was about 11 miles southwest of Mifflintown. About 1,500 people reported the tremor to the USGS. It was felt in portions of Juniata, Huntingdon, Centre and Blair counties.

WTAJ-TV Meteorologist Mike Doyle said an earthquake in central Pennsylvania is a "pretty rare occurrence." The USGS website reported no aftershocks recorded as of late Thursday afternoon.

Doyle said he didn't expect any more as it was a low intensity quake, but aftershocks may occur as long as 48 hours afterward.

"Earthquakes in this area are not super common be­cause our mountains were formed by geologic crunching a long time ago," Johanesen said. "But stress along these old faults can build up."

Generally speaking, she said such stress may build up along older fault lines when erosion has removed material causing rocks to relax and expand naturally.

The biggest earthquake experienced in Pennsyl­vania occurred in the Pymatunig area near Erie in 1998 and registered as a 5.2 magnitude.

Two earthquakes oc­curred in 1991; Centre Hall, had a 3.0 magnitude and Mount Union a 2.5 magnitude. Many area residents likely remember feeling the after shocks that followed a 2011 earthquake in Virginia that registered as a 5.8 magnitude, Doyle said.

Mirror Staff Writer Patt Keith is at 949-7030.