A new study published today in the journal Science has uncovered even more evidence that there is a direct link between the process used for hydraulic fracking of underground natural gas and small earthquakes. Interestingly enough, the earthquakes are not triggered by the drilling or removal of gas and oil, but by even larger earthquakes reverberating from the other side of the world. However, the injection of wasterwater fluids can be the the mechanism that primes the area for a quake.
According to a review of the study by Mother Jones, areas with "high subsurface fluid pressures" (e.g., underneath volcanoes and geysers) are already known to see an increase in seismic activity when massive earthquakes strike, even when the pressure occurs naturally and those earthquakes are very far away. The effect takes months to develop as the shockwaves from the bigger quake move slowly under the Earth's surface, so the connection between tremors in say, China and Wyoming, won't be obvious to casual observers.
But since one of the byproducts of fracking is wastewater, that is often injected back into the ground as a means of disposal. (It should be noted that not all wastewater is injected into disposal wells, and not all disposal wells contain fracking wastewater, and many other industrials process produce wastewater, too.) The presence of these disposal wells can increased fluid pressure in certain areas, which in turn, can make fault lines near the
fracking well site more unstable. (Check out the cool Mother Jones .gif for a demonstration.) Once that has happened, the trigger of a large enough seismic event somewhere else in the world can make a previously stable fault, weakened by fracking fluid injection, starts to shake.