Between 1991 and 1993, southern California became one of the many locations where reports of mystery booms began to occur with consistency. The press, at the time, had leaped on the idea that a top secret aircraft, the Aurora, had been responsible, and that the noises reported in the region had been the sonic booms this aircraft produced as it flew over land.

With all the public interest in the California mystery booms and their possible causes, the California Institute of Technology at Pasadena became involved, resulting in an investigation of sonic booms that culminated in a paper by Joseph E. Cates and Bradford Sturtevant, appropriately titled “Seismic detection of sonic booms.”

In essence, the paper relied on signals detectable through ground motion to map sonic boom “carpets”, covering an area of about 50,000 square kilometers around the vicinity of the southern California seismic network. Using this data, the researchers determined a much greater relationship existed between sonic booms and ground exposure, which was utilized in part to make determinations based on seismic records with hope of tracing the origins of “mystery boom” reports around that time.

“In the latter half of 1991 and early 1992, the U.S.G.S. office in Pasadena received a number of calls from the general public concerning ‘‘mystery booms’’ heard in southern California,” the authors of the paper wrote. “Initially the events were assumed to be earthquakes, but further analysis of the seismograph records suggested sonic booms as the most likely source. An initial analysis of the seismic signals by the U.S.G.S. by attempting to fit hyperbola to the arrival time data for 25 sites near the coast attributed the sonic booms to a source flying at high altitude and high Mach number. These reports were picked up in the popular press and attributed to a top-secret hypersonic Aurora spyplane.”

However, researchers noted one key trait about the incidents being reported: they all had occurred on a Thursday morning at around 7 AM local time. The USAF then commissioned MIT Lincoln Labs to carry out their own investigation, which determined that sonic booms produced by a pair of F-4 Phantoms en route to Edwards Air Force Base had caused the noises, as they passed over land traveling at about Mach 1.

However, contrary to what Lincoln Labs had determined, the Caltech researchers noted that a third boom report had not been accounted for in their assessment. “The Lincoln Lab theory of two aircraft flying essentially down the center of the boom pattern fails to explain the three events detected,” the Caltech authors wrote. “The aircraft would have to be flying at a speed of approximately Mach 1 relative to ground sound speed which would place the aircraft at or near the cutoff velocity for their altitude.”

“The analysis of the complete set of data eliminates both of the early theories for the source of the mystery booms,” they noted, thus ruling out the twin jets theory, as well as the previous theory involving “a high-speed aircraft flying north off the coast.”

Cates and Sturtevant’s final analysis suggested that the cause of the booms had, in fact, likely been the same sort of anomalous weather phenomenon that contributed to offshore sonic booms from Concorde aircraft being carried further inland, as determined in relation to the mystery boom wave on the East Coast between 1977 and 1978:

“From the complete analysis, all the observed booms appear to be indirect booms from a source offshore propagated inland by high winds. Southern California typically has strong jet stream winds and stratospheric winds blowingfrom west to east. Such anomalous sound propagation is well known, and mystery booms attributed to aircraft are not a new phenomenon. In the late 1970s, a series of East Coast mystery booms occurred. Although a wide range of phenomena were grouped into the ‘‘mystery booms,’’ the majority were attributed to indirect sonic booms from the Concorde (Rickley and Pierce, 1980) and sonic booms from military aircraft maneuvering offshore.”

By implementing the seismic network data, Cates and Sturtevant identified the source of the mystery sounds in Southern California as “indirect booms propagated from offshore operations.” However, it should be noted that toward the end of their paper, the authors note that a clear determination of the specific source of the booms had eluded them:

“An attempt to associate the mystery booms with specific flight operations from any of the local military bases has been unsuccessful. Local military bases reported no unusual activity on the dates of the mystery booms; in particular, the Pacific Missile Test Range, which operates offshore from Point Mugu, reported no supersonic flight operations on the mornings of the October 1991 or January 1992 events.”

Hence, while the seismic data was consistent with previous models of observed sonic booms carried inland by anomalous sound propagation, the final source of the flight operations that led to the California mystery booms appears to remain indistinct.