Seismograph printout  - Millersville

This image came from a Millersville seismograph. It shows activity in the 10 p.m. hour Thursday and a weak earthquake in New Jersey around 3:40 a.m. Friday.

Some Conestoga-area residents heard a “boom” or thunder-like sound, but it wasn’t storming.

At least a few even felt the ground shake.

Turns out, seismic activity late Thursday night was likely caused by "a very small earthquake," an expert says.

Charles Scharnberger, professor emeritus of earth sciences at Millersville University, measured the magnitude of the suspected quake at just .8 magnitude on the Richter scale.

One person from Conestoga called 911 shortly after 10 p.m. after hearing a “boom” and feeling a “little bit of shake” in the ground, a dispatch supervisor said. No injuries were reported and no one else called 911 with any similar reports.

The Millersville seismograph showed seismic activity occurred shortly after 10 p.m. Thursday, as well as at around 3:40 a.m. Friday.

Scharnberger said the latter activity was from a small earthquake that struck in central New Jersey. The U.S. Geological Survey confirmed a 2.7-magnitude earthquake rattled homes and businesses in Bernardsville, New Jersey, at 3:41 a.m., reported.

That couldn’t be felt in Lancaster County, but Monday night’s activity could, he said.

Solanco School District spokesman Keith Kaufman lives south of Willow Street and said he and his son that night heard "a very distinct rumble" like thunder, but the skies were clear and conditions were nice.” They did not feel the ground shake.

John Johnson, of Millersville, looked up the seismic activity on the Millersville University online site after his daughter and a friend said they heard a “loud boom” and felt something. Others also reportedly noticed the small earthquake, according to news reports.

Minor earthquakes are not unusual in the county, Scharnberger said. The 72-year-old started at the university in 1973 and retired in 2003, but he still operates the seismograph station at the university as a volunteer.

There have been dozens of minor quakes here since a significant 4.1-magnitude earthquake struck Marticville on Easter Sunday on April 22, 1984, he said.

The two most-active earthquake areas in the county through the years have been Conestoga and from East Petersburg to Landisville, Scharnberger said. Several have occurred in the Conestoga area, but like all earthquakes on the East Coast, experts don’t know why.

“There are no faults that can be shown to be active,” he said. “It is a puzzle why we have earthquakes” in the East.

A report of a possible earthquake a week ago turned out to be a blast at a quarry, Scharnberger said. It’s possible work at a quarry could trigger a small earthquake, he said, but that wouldn’t explain earthquakes in Conestoga as no quarry has operated there since the Safe Harbor Dam was built nearly a century ago.

Small earthquakes have been shown to correlate with exceptional rainfall events, but he said there haven’t been huge rains this year.

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