A minor earthquake apparently caused a boom and tremor felt strongest in Cape Elizabeth at midday Thursday.

The U.S. Geological Survey, which tracks earthquakes in the country, said the magnitude-2.0 earthquake was centered about 3.7 miles south-southeast of Portland.

Don Blakeman, a geophysicist with the USGS, said people who are within five to 10 miles of a magnitude-2.0 earthquake are likely to feel it, but it isn’t strong enough to be felt by those farther away.

“It’s just people really near the epicenter,” he said.

The boom that people reported hearing, Blakeman said, is the sound of rocks breaking and, since earthquakes like the one Thursday are typically not very deep, a boom is often associated with earthquakes near the surface.

Blakeman said 95 percent of earthquakes occur near the boundaries of the Earth’s massive plates, on fault lines where plates are rubbing together, such as in California. Quakes near the center of those plates – Maine is near the middle of the North American plate – are usually caused by the stresses created by the movement of the plate, rather than friction with another plate.

There’s also a theory that some parts of the Earth nearer the poles are still rebounding from the retreat of the glaciers of the Ice Age and so the earthquakes represent a slight lurch upwards, although Blakeman said he’s not sure about the veracity of that theory.

The boom and vibration were felt strongly in Cape Elizabeth and also reported in parts of adjacent South Portland and was a mystery until the USGS reported the quake Thursday afternoon.

Cape Elizabeth Police Chief Neil Williams said he and officers at the police headquarters on Ocean House Road felt what sounded like an explosion and a vibration around noon. He said the department also got calls on the department’s “business line” with residents reporting hearing and feeling what seemed like an explosion, although no 911 calls came in.

“We heard the loud noise and it also shook the ground,” Williams said.

Police initially suspected that roadwork being conducted nearby on Scott Dyer Road might have been the source, Williams said, but officers who went there said workers told them they weren’t the source of the noise and tremor.

Maine had seven minor earthquakes in June, according to the USGS.

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