* Originally dated 17 Sep 1992, 20:43 ***************** Los Angeles Times ************************ Mysterious tremors widely noticed from Los Angeles to San Diego on Wednesday at 8:55 a.m., and again about 10 a.m. definitly 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 said they knew nothing about a plane purportedly called the Aurora that is said to periodically streak over the Los Angeles Basin at about 4000 m.p.h. on a northeasterly path. There have been at least six episodes of morning sonic boom since June of last year. Sources have told the Times that the Air Force is said to be using a secert base called S-4 near the dry Groom Lake, north of Las Vegas, to test the plane, which once was inadvertently listed on an official budget report but which now the Air Force denies exists.