The boom that shook buildings and rattled residents over downtown Hillsborough on Thursday morning a few minutes before noon was caused by a 2.2 magnitude earthquake centered 2.6 miles southwest of Hillsborough, according to the U.S. Geological Survey.
The noise was so loud the Orange County Courthouse was evacuated and at least one elementary school in Hillsborough hustled their students off the playground and went on a brief lockdown while security personnel attempted to determine the cause.
The U.S. Geological Survey confirmed the earthquake. The quake occurred at a depth of 5.4 kilometers (about 3.3 miles) beneath the surface.
Orange County Emergency Services received 28 calls about the noise, said Emergency Services Director Kirby Saunders. Orange Rural Fire Department and the Orange County Sheriff's Office immediately deployed UAVs to look for plumes of smoke, wreckage or anything else that could have caused the boom but found nothing, Saunders reported.
"Given the heightened state of security, it was important for us to assess and attempt to determine the cause," Saunders said. "We were able to quickly rule out any threats/risk to the safety of our community."