In August 2012, the Proceedings of the National Academy of Sciences published my article surveying two years of seismicity in the Barnett Shale of Texas. The data showed that virtually all well-located epicenters were situated within 3 km of high-volume injection wells. This article summarizes the results of that publication. Recently there has been concern about earthquakes possibly caused by the disposal of hydrofracture flowback fluids in injection wells. Most investigations of earthquakes caused by human activity take place only after an earthquake occurs that is severe enough to be felt by nearby residents and receive media attention. Such events usually have magnitudes ∼M3 or greater and occur in populated areas. Limiting research only to these events does not help us understand why some injection wells trigger seismic activity and others do not.