Man made earthquakes in Arkansas?

For the past few day's, most of the media attention has been on the earthquake and tsunami disaster in Japan. There is however, a bit of a mystery involving earthquakes right here at home. For the past 6 months, the state of Arkansas has experienced over 700 earthquakes, some measuring 4.7 in magnitude. In fact, the region near the small town of Greenbrier has been having tremors since 2009. The culprit? It isn't known for sure, but the main suspect is a practice  known as hydraulic fracturing, in which water is injected into the ground at high pressure to fracture rock and release natural gas trapped within it. Salt water is a common by-product of the fracking process, and the easiest way to deal with it is to inject the toxic waste water back into the ground. But that can lubricate the surrounding rock, possibly leading to the quakes. So we can now cause our own earthquakes? Can't wait for some militant country to try and perfect that one.