(CNN) -- A 4.3-magnitude earthquake struck central Oklahoma Wednesday morning, the U.S. Geological Survey reported, causing minor damage and injuries.
The Oklahoma Department of Emergency Management said in a statement it had "received a few reports of minor damage, primarily to windows and due to items falling from shelves." Two people required medical care after falling, the statement said.
"At first I thought it could be an explosion," said Capt. Tom Easley, of the Norman, Oklahoma, police. "You think of the worst possible scenario."
Posters on the website of CNN affiliate KOCO in Oklahoma City reported feeling the temblor in numerous locations.
"Our office building shook for nearly 30 seconds," one poster wrote. "I heard it and felt it. I watched the drink on my desk shake and the liquid slosh around in my cup."
Other posters reported they thought a vehicle had struck the building they were in. One poster said the wall of his home had minor damage.
The USGS initially classified the quake at magnitude 4.5 but later downgraded it to 4.3. It struck at 9:06 a.m. and was centered 5 miles east of Norman and 20 miles south-southeast of Oklahoma City, the survey said.
The quake was relatively shallow, at 3.1 miles deep. The closer a quake's epicenter is to the Earth's surface, the more shaking it can cause.
Oklahoma City Mayor Mick Cornett said emergency dispatchers were receiving calls from people who said they felt the quake, but "this is not necessary. We are currently not doing anything. There are no reports of damage."
Cornett noted that an earthquake is extremely unusual for the area and said he had never before felt one, but he added there was no reason for people to be alarmed.
The quake was felt in 19 counties, according to the Oklahoma Department of Emergency Management.
Although no damage was found, the University of Oklahoma, located in Norman, called in a team of structural experts to evaluate buildings and structures, university President David Boren said in a statement.
The police department received several calls from people reporting things falling off shelves or plaster coming down, but nothing major, said Patricia Nolen of the Norman police.
"This is the largest one that I've certainly ever felt in Oklahoma, and I've been here my whole life," said Jim Bailey, deputy fire chief in Norman.
No power outages were reported, said Randy Simmons of the Oklahoma Electric Cooperative. "The wall sure shook, though."
CNN's Vivian Kuo and Divina Mims contributed to this report.