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The seismic network in southern California has provided the first opportunity to study the size and shape of indirect sonic boom carpets over a large area. The high density of the sites and large ground coverage allow analysis of the direct and indirect boom patterns on both sides of the flight trajectory, and the development of the booms can be followed over several hundred kilometers. The recent addition of pressure transducers at selected TERRAscope sites remedies the only significant weakness of the seismic data, the difficulty of predicting amplitudes.
From analysis of the space shuttle STS-42 reentry, the ground patterns are extremely complex. Ray theory fails to predict indirect sonic boom arrival times, observed multiple booms within the first shadow region, and extensive overlap of the multiple refracted sonic booms. The extensive ground coverage of the ‘‘mystery boom’’ and shuttle reentry booms suggest exposure under the real atmosphere is much larger than previously expected.
The inverse problem of predicting the aircraft trajectory from the ground arrival times is more difficult. Nonetheless, using the seismic network data, we were able to identify the source of the ‘‘mystery booms’’ as indirect booms propagated from offshore operations. However, careful study of the seismic data is required to identify direct and indirect sonic boom carpets before attempting to make predictions about the trajectory.
Strange Sound Reported in San Diego
Residents from Chula Vista to Oceanside reported a large rumble around 12:45 p.m. Friday.
The mysterious sensation was described by some people as sounding like a door slamming while others said it was strong enough to rattle windows.
A check of the U.S. Geological Survey website showed no earthquake activity.
NBC 7 San Diego's Dagmar Midcap was in Del Mar at the time and described it as a "Sonic 'rumble'" She tweeted, "according to my contacts at USGS, not seismic but rather sonic."
Two months ago, when San Diegans heard a similar sound, there was evidence of chaff on weather radar. Chaff is a material sometimes emitted during military exercises.
On Friday, however, Tina Stall with the National Weather Service said there was no visible chaff in the area at the time the noise was reported. The mysterious sound had both residents and experts scratching their heads. Scripps Institution of Oceanography scientist Kristoffer Walker said he felt it too, and looked into microphones recorded from MCAS Miramar. Evidence from his research revealed an answer. "There was indeed an atmospheric tremor, or 'skyquake,'" Walker said. "The likely cause of these 'skyquakes' is routine military activity very far off the coast of San Diego (at least 50 miles away) in zones that are designated military training zones." Typically, we don't hear these "skyquakes." But when the wind reaches speeds of over 100 miles per hour, the sound can reach parts of San Diego, Walker said. A spokesperson from Camp Pendleton said Marines are not training with anything unusual. They often train with various military equipment and will be training with tanks both Saturday and Sunday. On Friday evening, the U.S. Naval Air Forces official Facebook page posted the following message regarding the mysterious boom heard around San Diego: “San Diego, it looks like the boom that was heard and felt today was likely due to some aircraft associated with the USS Carl Vinson (CVN 70) family day cruise. As part of a flight demonstration two F/A-18 aircraft went supersonic about 35 miles off the coast. Sorry for any inconvenience this may have caused. -- LT Aaron Kakiel, media officer.” So, according to the Navy, it appears Friday's San Diego boom mystery has finally been solved.