A massive earthquake rocked Oklahoma on Saturday morning, with residents as far as Des Moines and St. Louis reporting they felt the tremors. The United States Geological Survey said it was a 5.6 magnitude earthquake, one of the largest ever to strike Oklahoma. It was centered about 9 miles from the town of Pawnee, and struck at about 7 a.m. People from Nebraska to North Texas said they felt the earthquake, which will likely fuel discussion about the underground disposal of wastewater from oil and natural gas production. Oklahoma, a major energy-producing state, has suffered numerous earthquakes this year, prompting federal regulators to intervene and limit wastewater disposal activity. Certain parts of the state are now on par with California in terms of earthquake activity.