SANTA CRUZ, Calif. – Without an official explanation for the apparent sonic boom that rattled residents of California’s Central Coast residents on Wednesday, Santa Cruz Sentinel readers have speculated on everything from earthquakes to extraterrestrials. But experts say most of these theories leave a lot to be desired.

The boom occurred at 9:15 a.m. PST Wednesday, 12 hours after a similar unexplained rattle hit Orange County, Calif.

“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,” Aptos, Calif., resident Julie Drysdale told the Sentinel after Wednesday’s event.

Speculation about the boom has been rampant online, with commenters implicating the weather, the military, even space debris. So how do these theories hold up?

Wednesday’s shaking was no earthquake, said U.S. Geological Survey seismologist David Oppenheimer.

Seismic stations around Monterey Bay recorded a compression wave at 9:15 a.m. Wednesday, but the wave lacked the up-and-down shear that usually characterizes an earthquake. And, Oppenheimer said, it was moving too slowly to be passing through rock.

“It looks like an air wave,” he said.

Oppenheimer said he couldn’t pinpoint the wave’s source, because USGS software is specialized for ground waves and too few seismic stations picked up the shake.

When lightning discharges, the surrounding air rapidly expands, producing a supersonic shock wave. Could Wednesday’s rumble be thunder?

Not likely, said Warren Blier, science officer for the National Weather Service in Monterey, Calif.: “There was nothing that was producing lightning or thunder at that time, in my recollection.”

Even if lightning had been present, thunder is only heard over a few miles, Blier said. Reports of the shaking came from cities ringing the Monterey Bay, and seismic monitors in Big Sur picked up the event.

It would seem most online speculation about alien invaders is tongue-in-cheek: The National UFO Reporting Center in Washington state received no calls from the Central Coast Wednesday morning. Even Peter Davenport, director of the center, was skeptical.

“My guess would be that it’s not UFO-related,” he said. “But I can’t be certain.”

Aliens aren’t the only things that come from outer space; in theory, a meteor could cause a sonic boom, said Jon Giorgini, senior analyst at NASA’s Jet Propulsion Laboratory.

A falling meteor compresses the air below it and creates a vacuum above.

That pressure difference could create a sonic boom, but it’s more likely to tear the meteor apart in the upper atmosphere. For the sound wave to hit the ground, the meteor would have to be large – and low.

“To have enough energy to create a shock wave to rattle things on the ground, somebody would have had to have seen it,” Giorgini said. “It probably was not a meteor.”

The Federal Aviation Administration searched Wednesday’s records but found no sign of supersonic airplanes in civilian airspace. Ian Gregor, FAA spokesman for the Western-Pacific region, said early Friday that he had heard anecdotally about a military exercise in the training area off the Northern California coast. But later he said he could not confirm that story.

“It’s possible that a pilot …” he began, before stopping himself to say, “If there was higher speed flight going on in there, we wouldn’t know about it.”

A spokesman for the Vandenburg Air Force Base told the Santa Cruz Sentinel on Wednesday the Air Force did not have jets flying off the coast that morning. Vandenburg officials could not be reached to comment further Friday.

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