I'm at the fracking workshop at Duke today and someone just asked a question about the connection between fracking activities and earthquakes, such as those that occurred last year near fracking operations in Arkansas and Ohio.
Michael Parker of ExxonMobil, who is among the speakers, was quick to mention that the wells potentially related to the quakes are injection wells—not drilling wells. Companies use injection wells, also known as disposal wells, to dump the material that is extracted with the gas or oil. This can include drilling chemicals and corrosion inhibitors. Think of the wells as a place to dump your acid reflux.
These are not innocuous operations, even if the earth is not moving under your feet. In 2006, I wrote a story about a historically African-American community in DeBerry, Texas, where the private wells had been contaminated by material from disposal wells. You can read that story here: www2.sacurrent.com/news/story.asp?id=61632
So even if injections wells are found not guilty of causing earthquakes, they pose other hazards.