The notion exists that "fracking" for natural gas or enhanced oil extraction directly caused the earthquakes observed in Oklahoma. There is some evidence for this, but by far the most likely cause is deep wastewater injection.
Sharp increase in central Oklahoma seismicity since 2008 induced by massive wastewater injection
K. M. Keranen, M. Weingarten, G. A. Abers, B. A. Bekins, S. Ge
Unconventional oil and gas production provides a rapidly growing energy source; however, high-production states in the United States, such as Oklahoma, face sharply rising numbers of earthquakes. Subsurface pressure data required to unequivocally link earthquakes to wastewater injection are rarely accessible. Here we use seismicity and hydrogeological models to show that fluid migration from high-rate disposal wells in Oklahoma is potentially responsible for the largest swarm. Earthquake hypocenters occur within disposal formations and upper basement, between 2- and 5-kilometer depth. The modeled fluid pressure perturbation propagates throughout the same depth range and tracks earthquakes to distances of 35 kilometers, with a triggering threshold of ~0.07 megapascals. Although thousands of disposal wells operate aseismically, four of the highest-rate wells are capable of inducing 20% of 2008 to 2013 central U.S. seismicity.
http://www.sciencemag.org/content/345/6195/448.short
Some evidence and some publications have pointed to the simple act of fracking causing the problem, but the big culprit is wastewater injection. A similar cluster of earthquakes was seen in Denver in the late 1960s.
http://earthquake.usgs.gov/research/induced/pdf/Healy-et-al-1968-Science-(New-York-NY).pdf
(NOTE: the clickable portion of the link above is incomplete. You'll need to copy and paste the entire link to read the article)
The distinction is important because if the US EPA quits giving the fracking industry a pass on deep wastewater injection, this all ends.
