(CNN) The Oklahoma Corporation Commission is telling operators to shut down 35 disposal wells that may have played a role in a 5.6-magnitude earthquake that shook at least six states Saturday, Gov. Mary Fallin said.

The disposal wells, which are linked to fracking and other industries that need to dispose of toxic waste water by injecting it deep into the earth, have recently drawn concern that they may actually induce earthquakes.

"This is a mandatory directive," Fallin said.

The commission, which regulates fuel, oil, gas, public utilities and transportation industries, is investigating to determine the epicenter of the quake that hit Pawnee, said commission spokesman Matt Skinner.

The Environmental Protection Agency is also investigating, Fallin said.

Skinner says earthquakes in Oklahoma are generally not directly caused by fracking, but rather by pressure from the disposal wells, which are used by the industry to get rid of the toxic waste water that comes out of the earth along with oil and gas.

"The disposal wells dispose into the state's deepest formation, the Arbuckle formation, which is right above what we call the basement," Skinner said. The basement is above where the critical faults lie that shift and make earthquakes."

Those disposal wells within five miles of the 10-mile section of the fault in question are to be shut down within seven days, and all the other wells must be shut down within 10 days, Fallin said.

Controversial disposal method

Oil and gas drillers and other industries use the disposal wells to inject toxic waste water deep into the earth, which raises questions about possible pollution of the water supply. The wells are used to dispose of fracking waste.

Saturday's earthquake, which some people described as the most powerful in memory, rendered six buildings in Pawnee Nation uninhabitable, Fallin said. Rural Pawnee County and the city of Pawnee had at least six buildings damaged, with one homeowner suffering minor injuries, Fallin said.

The earthquake damaged at least one historic building in Pawnee, city officials told CNN.

"It's an old historical building about 100 years old. It's still standing but some of the outer layers of sandstone fell. It could be cosmetic damage. We don't know yet," said Pawnee Mayor Brad Sewell.

Fallin issued a declaration of emergency for Pawnee County to start the process of "helping individuals, families and businesses impacted by the earthquakes" and help them request any necessary assistance.

The earthquake was also felt in Kansas, Arkansas, Missouri, Texas, Nebraska and Iowa, according to the US Geological Survey.

"A quake this size would shake for about 15 seconds and it was a shallow quake, about 7 kilometers (4.35 miles) deep," said Randy Baldwin, a USGS geophysicist.

With earthquakes in the central and eastern United States, "the ground is softer, so the seismic waves have a much farther travel distance then an earthquake in California or Nevada," said another USGS geophysicist, Robert Sanders.

Oklahoma Corporation Commission staff is at the office reviewing disposal wells in the vicinity of the earthquake near Pawnee. — Governor Mary Fallin (@GovMaryFallin) September 3, 2016

The Oklahoma Department of Transportation is checking bridges for damage and structural engineers are assessing building safety, according to Fallin. The governor also said no "concerning structural problems" in the state's highways and bridges have been found.

There have been a total of 11 earthquakes in the same area Saturday, the USGS said. The largest was at 7:02 a.m. (8:02 a.m. ET), with the preliminary 5.6 magnitude. The other quake magnitudes ranged from 2.7 to 3.6.

Despite minimal damage, the quake set off anxious speculation on social media.

Wow! Felt the earth move in Rogers, Arkansas. Had to Google to find out what was going on. #Earthquake — CarolLangston (@CarolLangston) September 3, 2016

We survived #Earthquake 2k16. Hashtag KCforever. We will rebuild. We will press on. pic.twitter.com/teTlwixZEn — Mike Brooks (@MikeBrooks) September 3, 2016

This morning wake up call brought to by Oklahoma's oil and natural gas industry. — Clifton Adcock (@cliftonhowze) September 3, 2016

Some tweeters saw humor in the quake, with no serious casualties.

That wasn't an earthquake that hit Oklahoma this morning. That was Sooner fans champ hopes crashing to Earth on the 1st Saturday of season. — Eric (@illstr8r) September 3, 2016

"Oooooooooklahoma, where the fault lines rumble like a train..." Yeah, NO. I like our theme song the way it is. #earthquake — Ree Drummond (@thepioneerwoman) September 3, 2016

But a March report released by the USGS showed that people in parts of Texas and Oklahoma now face similar ground-shaking risks from human-induced activity, such as fluid injection or extraction, as residents face from natural earthquakes in California.

The agency outlined the risk of these so-called "induced" earthquakes, noting that Oklahoma City and the surrounding region face a 5-12% chance of damage from an earthquake in 2016.

Seismic activity is also on the rise in certain energy-intensive states after a relatively stable period of about 30 years, according to the USGS report. Earthquake rates have "recently increased markedly" in multiple areas of the Central and Eastern United States, especially since 2010, the report said. Its 2014 model of seismic risks did not consider man-made quakes.

The agency said several damaging quakes have occurred recently near injection wells. For instance, a magnitude 5.6-earthquake caused minor injuries and damage to homes in 2011 near Prague, Oklahoma. Other tremors in these fracking regions include a 5.3-magnitude quake near Trinidad, Colorado, in 2011 and a 4.8-magnitude quake near Timpson, Texas, in 2012.

As to whether the injection wells are what causes the earthquakes, and this quake in particular, Skinner would only say, "There's all kinds of theories as to why it puts pressure on the basement faults, but somehow that is happening and we have taken many actions based on that."