There were earthquakes in Greeley, Colorado this summer: a place more associated with dairy farms and hail than earthquakes. USGS investigated and found there is a link between these new earthquakes and injection wells (wells that inject the left-over waste fluid from fracking deep into the earth). This map shows the location of injection wells in Colorado and earthquakes that have occurred since 1973 with a magnitude of 2.0 or greater. You'll see the occurrence of earthquakes has increased since the 1990's. If you want further information, check out USGS and this article from the Wall Street Journal.
