The Monday quake was a magnitude 1.9 and was centered 2.8 miles west of the town, said D.T. Townsend, a spokesman at Savannah River Site.

The event, which callers to the Barnwell County Sheriff’s Office reported as a “loud boom,” was recorded by the site’s seismology instruments but was not related to activities at SRS, he said. “It was strictly an off-site event.”

The cause of the boom remains a mystery. Site officials even checked with the Federal Aviation Administration to determine whether supersonic flights occurred in the region, but there were none.