Christopher Joyce, NPR's science reporter, investigates how wells used for disposal of contaminated water used in fracking may cause the quakes (note that the gas companies disavow the relationship) and what can be done to prevent them. Earthquakes caused by mining is documented in South Africa, and these are apparently not the first associated with wastewater wells.
"Recent quakes reported in Ohio and Arkansas are associated with wastewater wells, not fracking wells. The water first used in fracturing rock is retrieved and pumped into these waste wells, which take in lots of water. And at more than 9,000 feet deep, the water is under high pressure that can build up over months or years. It's this pressure that can actually create earthquakes.
In the 1960s, a wastewater well in Colorado at the Rocky Mountain Arsenal is believed to have been the trigger for a magnitude-4.8 quake.
One way to avoid creating earthquakes is not to inject fracking wastewater into waste wells, but to recycle it instead. The state of Pennsylvania tried that, but they found that wastewater treatment plants couldn't get all of the toxic material out of fracking water, and the "cleaned up" water returned to rivers wasn't clean enough. So well operators in the state decided to ship wastewater to Ohio, where it has been going down into wells".