The magnitude-5.6 earthquake in Oklahoma this past weekend was not caused by hydraulic fracturing, experts said. The ground tremors caused by fracking are "equivalent to a gallon of milk falling off the kitchen counter," said Mark Zoback, a Stanford University geophysicist. "There's a fault there. You can have an earthquake that size anywhere east of the Rockies. You don't need a huge fault to produce an earthquake that big. It's uncommon, but not unexpected," said Paul Earle, a seismologist at the U.S. Geological Survey.