It happened again Thursday morning. There was a brief rumble, like somebody moving bulky furniture around, and the ground lurched, San Gabriel Valley residents said. Then, just as they were about to head for their door frames, it was gone.
The mysterious rumble and bounce has occurred five times since last June, always about 7 a.m. on Thursdays. These were no earthquakes, but a passing aircraft, Caltech seismologists say.
"All I can say is that it's something that's traveling through the atmosphere at several times the speed of sound in a generally northeasterly direction," said Jim Mori, a seismologist with the United States Geological Survey at Caltech.
Analysts from Jane's Defense Weekly, the widely respected London-based military periodical, say that the Thursday morning phenomenon could be a top-secret new spy plane, dubbed the Aurora, that can fly up to six times the speed of sound, or about 4,000 m.p.h.
The Air Force denies that any such aircraft exists.
But Jane's editor Bill Sweetman says there are indications that the airplane is one of the Defense Department's so-called "black" programs, a top secret project whose funding is disguised in the defense procurement budget as one of a number of "selected activities."
USGS seismologists, who have recorded similar rumbles in June, October, November and January, say the aircraft's turbulence has a distinctive signature, or wave pattern, on the seismograph unlike that of any known aircraft, Mori said.
"We know it's not, say, the space shuttle, whose wave forms look different," Mori said.
The Geological Survey, which is primarily concerned with earthquakes, has been monitoring such aerial phenomena only because of the numerous calls Caltech receives about recent tremors, Mori said. The rumbles have been reported from the South Bay to Yuba County, north of Sacramento.
"We get calls from people who think it's an earthquake, and we check our records," Mori said.
Los Angeles area residents described Thursday morning's rumble as something like a small tremor.
"As a Californian, you get tuned into that kind of thing," said Nell Crawford, a producer's assistant from Mt. Washington.
"It was kind of a rumble--very short," said Caryn Eaves, a public affairs director for the Tournament of Roses in Pasadena.
"It rattled the glass on the glass door," said Altadena restaurant manager Dan Bridal. "And there was a roar, a slight roar."
Sweetman said the knowledge of the mysterious aircraft first came from a 1986 Defense Department budget report, in which the Aurora was listed next to the SR-71 Blackbird and the U-2, two Air Force spy planes. That was the last official reference to the Aurora, said Sweetman, an aircraft technology specialist as well as the North America technology editor for Jane's.
"It got in there through an accidental slip," he said. "It should have been edited out."
Ever since, military analysts have associated reports of mysterious sonic booms with experimental supersonic aircraft, Sweetman said.
Jane's analysts believe that the Aurora is being prepared as a replacement for the Blackbird, which was retired from service in 1989. Sweetman said the new plane is believed to be long and sleek, with a 66-foot wingspan and a distinctive bat-wing design.
The publication Aviation Week and Space Technology last month reported recent nighttime sightings of an unknown aircraft with a diamond-shaped lighting pattern near Beale Air Force Base in Yuba County. The aircraft, with a red light near the nose, an amber light near the tail and white lights near the wingtips, was seen in tight formation with a pair of F-117 Stealth fighters and a KC-135Q refueling tanker, the publication said.
The aircraft, which quickly extinguished its lights after joining the formation, had a distinctive engine noise, like "air rushing through a big tube," the publication said.
Sweetman said the speeds indicated by Geological Survey instruments ruled out conventional aircraft. "It's too fast for any aircraft that we know about," he said.
He speculated that the aircraft had been on a test run from Groom Lake, an Air Force research and development base near Las Vegas, accelerating over the South Pacific and then veering back, in a northeasterly direction, toward Nevada.
"Its descent would take it across Los Angeles at supersonic speeds but not very high," Sweetman said. "Those are the kinds of conditions under which you get a boom."