Mysterious Tremors Rattle Southland : Puzzle: Experts rule out earthquakes. Speculation that they are sonic booms from an exotic spy plane flying out of a secret Air Force base in Nevada cannot be confirmed.

Mysterious tremors widely noticed from Los Angeles to San Diego on Wednesday at 8:55 a.m. and again about 10 a.m definitely weren’t earthquakes, experts said, and speculation that they stemmed from an exotic spy plane that has reportedly been flying out of a secret Air Force base in Nevada could not be confirmed.

Military spokesmen at Edwards Air Force Base near Mojave and Nellis Air Force Base at Las Vegas said they knew nothing about a plane purportedly called the Aurora that is said to periodically streak over the Los Angeles Basin at about 4,000 m.p.h. on a northeasterly path. There have been at least six episodes of morning sonic booms since June of last year.

Sources have told The Times that the Air Force is said to be using a secret base called S-4 near the dry Groom Lake, 120 miles north of Las Vegas, to test the plane, which once was inadvertently listed in an official budget report but which now the Air Force denies exists.

A few visitors to the remote area have told The Times of seeing flying saucer-type aircraft parked on the desert in what appeared to be an Air Force attempt to duplicate flying saucer aerodynamics.

On Wednesday, Air Force Capt. Donna Eggleston, a spokeswoman at Nellis AFB, said the base at Groom Lake is so secret it has no telephone number she can make public.

As to Aurora, Eggleston said, “We’re at a loss. . . . We do get calls about it periodically and there’s no one to refer them to.” Groom Lake, she said vaguely, “is somewhere north of here near the nuclear testing range.”

At Edwards AFB, spokesman Dennis Shoffner said, “When it comes to confirming any kind of plane like that, I just don’t have anything for you.”

Calls from civilians to police, fire and news media outlets Wednesday found most people saying the rattling of windows at their locales resembled that of an earthquake. But Kate Hutton, a Caltech seismologist, said no ground motion had been recorded at the times in question on seismographic instruments.

Ironically, earlier in the morning, an authentic aftershock of the Landers earthquake did occur, although it wasn’t felt in urban areas around Los Angeles or San Diego. The magnitude 3.6 temblor was centered five miles southeast of Yucca Valley, in the same area shaken by hundreds of earthquakes in recent months.

Hutton attributed the mysterious tremors to sonic booms, but said she did not know their origin.