The quake was centered eight miles southeast of Norman, south of Lake Thunderbird, near E Post Oak Road and 84th Avenue SE, Oklahoma Geological Survey research seismologist Austin Holland said. He estimated the magnitude of the 9:06 a.m. quake at 5.1 and called it a "small to moderate earthquake," while the U.S. Geological Survey put it at 4.3.

Although there were only two reported minor injuries in Oklahoma, people were caught off guard by a jolting earthquake in the land of tornadoes.

Readers of The Oklahoman reported the quake felt or sounded like "a 747 landing in the neighbors' yard," "a semi hit our house" or "my house had been hit by a trash truck." People from seven states reported to the U.S. Geological Survey that they felt the quake, including from as far away as Brentwood, Tenn.

"That sucker it rattled my whole house, it literally shook the whole thing," said Charles Parnell, who lives in east Norman near Lake Thunderbird. "The chandelier jumped up and down a little bit, but there was no damage. It just surprised the heck out of me."

said his agency's 5.1 estimate likely will be reduced.

"I'm certain that we'll probably come down in our magnitude measurement because the USGS's 4.3 is so much lower than our 5.1. We keep track of the USGS magnitudes in our catalogs so we can improve. It's always good to compare your work to the foremost experts in the field of earthquake measurement."

Phone lines flooded

On Wednesday, Emergency Medical Services Authority in Oklahoma City said dispatchers received numerous calls relating to the earthquake. Most callers were having anxiety and worry over the earthquake, spokeswoman Lara O'Leary said, but two patients fell and needed medical care and were taken to Oklahoma City hospitals.

Oklahoma City resident Kenneth Tankersley, 52, was one of them. He broke his left ankle when he fell from a ladder outside his house as the earthquake struck. He said he was trying to take down an antenna when the ground shook.

"I guess my ladder wasn't set very solid, and it just shook me off of it," Tankersley said by phone.

Randy Keller, director at the Oklahoma Geological Survey, said the quake "came in loud and strong on all of our seismic stations."

Norman police Capt. Tom Easley said there were no known reports of injuries or damage in the city, but residents flooded the 911 lines.

Rick Smith, warning coordination meteorologist for the National Weather Service, Norman Forecast Office, was in his office at the National Weather Center in Norman.

"It was a rumble. It didn't last long, but there was a jolt at the end," Smith said.

At the University of Oklahoma, a section of glass in the ceiling at Dale Hall was shattered, though it remained in place, and there was no immediate confirmation it was caused by the earthquake.

In Purcell, about 12 miles south of Norman, the sensation was intense, said Erica Lippel, a spokeswoman for the Purcell Police Department.

"Everybody pretty much went out in the street," she said.

There have been about 570 earthquakes in the state this year, many too small to be felt. A 4.1 magnitude quake was reported on Feb. 27 near Sparks along the Turner Turnpike northeast of Oklahoma City.

The cause of Wednesday's quake and other recent ones could result from a buried fault or even oil and gas drilling activity, said Estella Atekwana, professor of hydrogeology at Oklahoma State University. The exact cause wasn't determined.

A 5.5 magnitude earthquake on April 9, 1952, is the largest ever recorded in Oklahoma, according to the U.S. Geological Survey. That one at El Reno, Oklahoma City and Ponca City cost thousands of dollars.

Earthquakes that wouldn't be felt in places like California, where subsurface conditions are different, can be felt here, Randy Keller, the Oklahoma Geological Survey director said in 2009. The hard rock beneath the Sooner State is better at conducting the seismic waves.