Loud 'boom' that shook Hillsborough was small earthquake, USGS decides

The United States Geological Survey (USGS) has shifted course, now saying a loud boom noise heard Thursday in Hillsborough was, in fact, an earthquake.

The small 2.2 magnitude earthquake had a shallow depth of 3 miles. It was reported Thursday at 11:50 a.m. near Hillsborough.

"It was so small that our computer algorithms did not catch it," said Thomas Pratt with the USGS. "When we got felt reports we looked and thought it was likely a mine blast because there is a large quarry right near it. We relooked this morning, though, and confirmed it indeed was an earthquake."

Earlier on Friday, the USGS thought the noise was likely a sonic boom, which is defined by the United States Air Force as "an impulsive noise similar to thunder" that is "caused by an object moving faster than sound."

However, the Air Force told WRAL News it couldn’t attribute a sonic boom to its Thursday aircraft operations.

The Hillsborough Police Department was investigating Thursday after people reported hearing a "boom" noise so powerful it shook their homes.

At 3 p.m. on Thursday the town of Hillsborough posted an update: "We have yet not been able to determine the source of the noise this morning. So far we have ruled out any obvious causes (bombs, explosions, crashes, planes, building failures, etc.)."

Multiple WRAL viewers said their houses shook, including a woman working from home.

"I heard a huge boom and my whole house shook," the woman said. "I live in a wooded neighborhood, so initially I thought that a tree had fallen on my house."

The woman said two other neighbors came outside and shared similar experiences.

"It literally caused our homes to shake," another viewer told WRAL News.

Mark Kleinschmidt, the former mayor of Chapel Hill, said the Orange County courthouse was evacuated due to the sound.

WRAL meteorologist Mike Maze also heard the boom in Apex.