Around 4 p.m., callers flooded switchboards at local emergency call centers, inquiring whether there had been some sort of explosion or earthquake. Fearing the latter, firefighters at a fire station in the northern suburb of Clairemont evacuated their building and moved the fire engines outside.
The United States Geological Survey later determined the noise was a sonic boom. Similar booms have been heard and felt in the area before; in 2006, seismologists at the Scripps Institute of Oceanography in La Jolla traced another such boom to an area in the Pacific Ocean about 120 miles west of San Diego that is routinely used for military training flights. USGS seismologists suspect that today's boom probably originated somewhere well away from San Diego, bounced off the upper atmosphere, and reverberated over the city.
Officials at local military bases, however, were also at a loss to explain the boom, as they told callers that no training flights were taking place at the time.
Written by Sandy Smith