According to this article from MSNBC, a report from the National Research Council has come out finding little reason for concern that earthquakes and hydraulic fracturing are causally connected: "The controversial practice of hydraulic fracturing to extract natural gas does not pose a high risk for triggering earthquakes large enough to feel." Also interesting: "The man-made quakes that [U.S. Geological Survey seismologist William] Ellsworth has been seeing are almost all related to wastewater injection, he said. Ellsworth said he agreed with the research council that 'hydraulic fracturing does not seem to pose much risk for earthquake activity.'" (Emphasis is ours.)