Oklahoma Earthquake: 4.3 Temblor Rattles State

"Since 2009, more than 200 magnitude-3.0 or greater earthquakes have hit the state's midsection, according to the Geological Survey. Scientists are not sure why seismic activity has spiked, but one theory is that it could be related to wastewater from oil and gas drilling that is often discarded by injecting it deep into underground wells."