After an initial couldn't-have-been-us denial, it turns out it was the military's fault after all.
A Navy spokesman confirmed late Friday that two Navy F/A-18 fighter jets went "supersonic," rattling doors and windows ---- and nerves ---- like an earthquake along San Diego County's coastline about 12:45 p.m. Friday.
The jets were showing off for about 2,000 family members and invited guests of sailors aboard the Carl Vinson during a daylong family cruise, said Lt. Aaron Kakiel, a spokesman for Naval Air Forces Pacific, at North Island Air Station.
"Those two aircraft went supersonic about 35 miles from the coast," Kakiel said. "Usually you don't hear the side booms travel that far. It was kind of surprising to us."
And that surprise led to initial denials, but then officials looked closer, he said.
"Sorry for the bad information, and sorry for the inconvenience for people," Kakiel said.
People across North County reported the rattling, but confusion as to what happened followed. Earthquake-tracking systems didn't report any seismic activity, said a geophysicist with the National Earthquake Information Center.
The event bore the hallmarks of a sonic boom, and military officials said they knew of no aircraft that were operating at supersonic speed in the region. One expert said there was evidence that the shaking came from an extreme atmospheric tremor called a "skyquake."
The first report of earthquakelike activity in North County came in at 12:43 p.m., said Bruce Presgrave of the U.S. Geological Survey, National Earthquake Information Center. More reports followed from people who noticed structural shaking in Del Mar, Oceanside, Carlsbad, Escondido and elsewhere.
The information center recorded no earthquake activity at the time people reported feeling the disturbances, Presgrave said. But he said that a sonic boom could have produced the phenomenon.
A sonic boom is a thunderlike sound associated with atmospheric disturbances created when an object moves faster than the speed of sound (about 770 mph).
No aviation activity that would result in a sonic boom had been reported to the information center, Presgrave said.
Military officials from Miramar Marine Corps Air Station to Camp Pendleton to the U.S. Navy all fielded calls from curious media, and all said they knew of nothing going on to cause a big boom or shaking.
So the shaking may have been a "skyquake," recorded Friday by microphones near Miramar Marine Corps Air Station, according to Kristoffer Walker, a project scientist for Scripps Institution of Oceanography in San Diego.
Military activity in designated zones at least 50 miles off the coast can cause a skyquake, or atmospheric tremor, which can rattle windows and created the accompanying sound that some residents reported hearing, Walker said in an email.
Certain atmospheric conditions, such as winds blowing toward San Diego County at more than 100 mph, can carry a loud, low-frequency noise far inland, bouncing the sound waves between the ocean and the fast-moving wind to rattle windows.
A similar event shook San Diegans' windows about 9 a.m. April 12, according to Walker.
While skyquakes are caused by sound, the frequencies are often too low for humans to hear, according to Walker.
On nctimes.com, Facebook and Twitter, speculation among commentators about possible causes ranged from secret military projects to a meteor or an incursion into local airspace by aliens.