New Jersey was shaking more than the crowd at a Bruce Springsteen concert Thursday afternoon, ABC News reports. Tremors—initially believed to be an earthquake—shook houses from southern Jersey to Long Island starting around 1:30pm local time. "It almost sounded like an airplane was coming and then the whole house was shaking," an Atlantic County babysitter tells NBC Philadelphia. And police had to ask people to stop calling 911 to report the tremors. According to NJ.com, things didn't settle down for more than an hour. The United States Geological Survey tweeted that the shaking was caused by a sonic boom, not an earthquake. The agency states nine total booms were felt in the region Thursday afternoon, ABC reports.
But what caused the sonic booms? Initially, there was plenty of head-scratching, with everyone from NASA to a Delaware military base denying responsibility. But, finally, mystery solved: Military fighter jets were the culprit. A Navy rep confirmed an F-35C (top speed almost 1,200mph) and an F-18 from Naval Air Station Patuxent River in Maryland were running supersonic tests over the Atlantic Thursday afternoon. What's wild is that these test flights occur nearly daily in the "Test Track," a region that runs parallel to the coast of parts of Delaware, Maryland, and Virginia. The AP notes that "certain atmospheric conditions can increase the chance of hearing the booms."