Many South Bay residents on Tuesday afternoon reported hearing a loud boom that rattled windows. The incident caused confusion throughout the region, with some calling the NBC Bay Area newsroom asking if an earthquake occurred.
"I noticed there was a lot of movement and vibration of the windows," said Richard Reyna, a witness to the loud boom.
The United States Geological Survey said the incident was not earthqake related. But experts at the USGS said their seimographs did detect motion in the area roughly at 2:30 p.m.
USGS Geophysicist Rufus Catchings said the incident appears to be a sonic boom.
A sonic boom is caused when an aircraft reaches the speed of sound.
The California National Guard confirmed to NBC Bay Area that the 144th Fighter Wing is conducting a statewide earthquake disaster drill, and the timing of the boom is consistent with their F-15s flying up and down the coast.
"To me it sounded like a boom, a trembling," Reyna said.