Denton County sheriff's dispatchers received nine calls from residents between 11:45 and 11:51 p.m. asking whether the shaking was an earthquake, a sonic boom, an explosion or something else altogether.

The earth moved Tuesday night for many Denton-area residents, but what caused it remains a mystery.

"I was laying in bed, reading a book when it happened. It seemed like a loud noise and the whole house shook for a second or two," said Susan Seaborn of Corinth.

She looked at her watch when the windows rattled. It read 11:34 p.m.

"But my husband said, 'I don't remember a bang.'"

Seaborn said she felt another bump a few minutes later, although her husband said he didn't feel that either.

The couple looked around their house and found no damage, but they did find their neighbors outside doing the same thing. Their neighbors thought a tree had fallen on their roof.

Denton County sheriff's dispatchers received nine calls from residents between 11:45 and 11:51 p.m. asking whether the shaking was an earthquake, a sonic boom, an explosion or something else altogether.

University of Texas professor Cliff Frohlich, an expert on Texas earthquakes, said that he, too, received calls from Denton-area residents curious whether what they felt was an earthquake.

There were no reports from the National Earthquake Information Center, but that seismic data is limited to quakes that register magnitude 3 or more on the Richter scale, Frohlich said.

"And that would be felt across the state," Frohlich said.

A seismograph in Hockley, north of Houston, showed no activity either, but a small tremor, measuring 2 or less on the Richter scale, would be felt only locally. Such a quake could be measured only if there were instruments in the area, he said.

The most recent measurable earthquake in North Texas shook Commerce on May 13, 1997, with a 2.9 magnitude temblor. The nearest quake shook Valley View on Sept. 18, 1985, measuring 3.3 on the Richter scale. The biggest quake in North Texas was centered just north of Paris and measured 4.2 back in 1934, Frohlich said.

The biggest quakes on record in the nation's midsection occurred in Missouri in 1811 and 1812. Based on damage reports, experts estimate those quakes measured about 8 on the Richter scale.

The kinds of reports people have made about Monday night's shake are consistent with a temblor of 2 or less, Frohlich said.

"People report a loud noise or a jolt. They hear them more than they feel them," Frohlich said.

But, he said, the reports are also consistent with other earth-shaking events, such as storms and sonic booms.

Jody Gonzalez, Denton County emergency management coordinator, said that the southwestern corner of the county is in a Fort Worth-Wichita Falls flight path where pilots are allowed to fly their craft at supersonic speeds.

If an airplane does pass the speed-of-sound barrier, the boom could be felt anywhere in the county, depending on the wind, Gonzalez said. But emergency management officials aren't typically told when such a flight has come through.

"We never know," he said.