Though many Central Coast residents felt a rattle Wednesday morning, the source of the shaking was not under their feet, according to the U.S. Geological Survey. At 9:15 a.m., USGS sensors detected ground movement, but the signals did not resemble an earthquake, said Leslie Gordon, a USGS spokeswoman.

The movement appeared to originate off the Monterey Bay coast, Gordon said. "Our best guess is that it was a sonic boom from a jet off the coast," Gordon said. "That's all we can say scientifically." Jet airplanes create sonic booms when they break the sound barrier. As a jet rips through the air at supersonic speeds, it emits shock waves that produce a tell-tale bang and rattle buildings.

But the Air Force did not have jets flying off the coast Wednesday morning, said Steve Bauer, a spokesman at Vandenberg Air Force Base. After receiving calls about a similar boom in Southern California, the Federal Aviation Administration said it was searching through flights its employees monitored Wednesday morning to find the supersonic jet. "We haven't found anything ... that would explain the sonic boom," said Ian Gregor, a spokesman for the administration.

Gregor said the FAA would finish its search today. Officials at the NASA Ames Research Center at Moffett Field had not heard about the possible sonic boom, said Bruce Buckingham, the center's chief public affairs officer. According to the Orange County Register, a sonic boom shook Southern California at 9:15 p.m. Tuesday, almost exactly 12 hours earlier than the Central Coast rattle. Bob Dollar of the USGS told the Register that Caltech scientists reviewed seismograms from Tuesday night's event in Orange County.

"These data are consistent with a sonic event coming onshore near Dana Point and traveling northward inland," Dollar said. "The energy traveled across our seismic sensor network at the velocity of a compressional wave in air rather than the velocity of a similar wave through the ground, which is much faster," Dollar said.

The F/A 18 Super Hornet fighter jets used by the Navy and Marines in Southern California are capable of breaking the sound barrier and producing a sonic boom felt on the ground, the Register reported. When the space shuttle lands at Edwards Air Force Base it too produces a sonic boom. There are currently no shuttles in flight.

The Santa Cruz County Sheriff's Office started receiving phone calls from residents about a possible earthquake just after 9 a.m. Wednesday morning, said a lead dispatcher. Julie Drysdale, who lives in Aptos, reported feeling a sonic boom or earthquake at 9:17 a.m. "I was outside and heard two loud booms. My husband said the house shook quickly, like a truck hit it, not the typical earthquake shaking, much quicker," Drysdale said.

Robert Diller, who lives on Glen Haven Road in Soquel, said he heard four loud booms this morning -- two before 10 a.m. and another two around noon. "They made our windows rattle," Diller said. "It was like a blast, it sounded like a dynamite blast almost." Residents in Salinas and Monterey also reported feeling the boom.

To confuse matters, the ground did move Wednesday morning as well. The USGS Web site reported four minor earthquakes in the region. A magnitude 2.0 earthquake hit near Los Altos Hills at 8:40 a.m. Two quakes struck outside Tres Pinos: a 1.3 magnitude at 5:42 a.m. and a 1.6 at 7:52 a.m.

Gordon said the shaking detected at 9:15 a.m. was not posted on their site, because it was not classified as an earthquake. At 11:12 a.m., a 1.7 movement was measured in a quarry near Portola Valley. The USGS attributed that to a probable quarry explosion.