For at least the third time in 2021, occupants of homes and businesses in San Diego were startled late Tuesday morning by an unexplained, abrupt shaking.
Residents reported feeling the brief, violent shake at 11:36 in the Mission Hills and San Carlos neighborhoods of San Diego, as well as in Santee and El Cajon in East County.
Shortly after the jarring incident, many San Diegans took to Twitter and other social-media platforms. One person tweeted out that "san diego is cool bc im like oh wow just felt an earthquake but no actually it was a sonic boom." Another tweeted, "I'm just logging in to confirm if that really was an earthquake or if I was losing my mind lmao. San Diego acting real sus right now, quaking while its raining."
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Official confirmation of the incident was slow in coming, but an official with United States Geological Society said that there no quakes locally at that time. Spokesman Robert DeGroot said USGS sensors did not detect anything seismic activity at that time.
Representatives for San Diego Sheriff's and San Diego Police said Tuesday morning that they had not received any calls reporting the incident, and the National Weather Service said they were unaware of anything that might have caused the event.
NBC 7 put in a request for a statement with officials at Marine Corps Air Base Miramar and received the following reply:
"MCAS Miramar does not have the ability to track aircraft outside of its immediate airspace directly in and around the base. The FAA is the federal agency responsible for monitoring flight throughout San Diego and would best be able to answer your query."
NBC 7 asked the FAA if it could confirm whether any aircraft, military or otherwise, was operating at speeds in or around San Diego County at the time that could cause such an event?
"A sonic boom would only come from military aircraft operating in military airspace," FAA public-affairs specialist Steve Kulm replied. "I recommend contacting the military."
Asked if any military aircraft were operating offshore from San Diego at approximately 11:36 a.m. on Tuesday, Marine Capt. Matthew 'Red Bull' Gregory replied, "No, I cannot."
Of course, the Marines are not the only branch of the military to operate locally; the Department of Defense, U.S. Air Force and U.S. Navy also fly in the offshore training ranges, a military spokesman told NBC 7.
NBC 7 also contacted the U.S. Navy regarding Tuesday's incident.
“We have no indications that naval aircraft caused a sonic boom in or around the San Diego area today,” Cmdr. Zach Harrell, spokesperson for Commander, Naval Air Forces, said via email on Tuesday afternoon.
"Weird things happen all the time, and we find perfectly scientific explanations for them," the USGS's DeGroot said, adding later, "we find things that pique our interest and then we figure it out."
In June, an official with MCAS Miramar said a similar incident that occurred on June 8 was possibly due to aircraft training over the Pacific.
"While MCAS Miramar cannot account for every sound event that occurs within the area, in this case the cause is possibly due to aircraft training occurring in the W-291 range, approximately 30 miles southwest of San Diego over the Pacific Ocean," MCAS said in a release. MCAS Miramar said two aircraft departed the base on June 8 and were conducting air-to-air combat training.
Since the mid-1970s, the Federal Aviation Administration (FAA) has largely restricted supersonic flight over land. However, over the Pacific and at that distance, supersonic speed is within all FAA statutes and military regulations, MCAS Miramar explained.
USGS Duty Seismologist Jonathan Tytell said the "event" in June was picked up by three sensors: one in Rosarito, Mexico; another in Pala; and a third on Barrett Mountain. Tytell said the "event" definitely wasn't an earthquake.
'Unknown Event' in February
Another mysterious boom took place in February. The "unknown event," as the U.S. Geological Survey referred to it, had many San Diegans posting on social media that they heard a boom and felt sudden shaking.
Whatever it was, it was noticed at around 5:10 p.m. on Feb. 16, with many who reported the event concentrated in the southern half of San Diego County.
An on-duty seismologist for the USGS confirmed to NBC 7 at the time that there was no earthquake detected on sensors in the county during the same time frame.