It is concluded that a "plausible cause" of a series of small earthquakes in Texas during 2008 - 2009 is saltwater pumped deep into the earth to recover natural gas, though this explanation is not definitive. In a process known as "hydraulic fracturing", shale layers are cracked by injecting water mixed with sand under high pressure, in order to liberate trapped natural gas. According to the USGS (United States Geological Survey), there may be 200 trillion cubic feet of gas trapped in shale across America.
Seismologist, Brian Slump of the Southern Methodist University, analysed data from 11 earthquakes and by a process of triangulation managed to place the origin to around one tenth of a mile south of Dallas-Fort Worth airport, on top of a geological fault located about 15,000 feet below the surface. Slump commented that although this is an old fault, stresses upon it remain that could trigger earthquakes.
Since 2002, 13 fracture wells have been drilled in proximity to it, but the study led by Slump found that the epicentre is almost exactly on the point of a reinjection well into which 9,000 barrels of seawater/day were pumped at a depth of 10,000 - 14,000 feet. The saline "flowback" water was then pumped to the surface and disposed of by injecting it into deep rock formations, in order to avoid further treatment.
Caution has been advised by Shaopeng Huang, from Michigan University, who said that: "a causal link between a given earthquake with a particular borehole is debatable," considering the huge amount of energy implicit in even small quakes. Slump stresses that his team are saying merely that such a link is "plausible not definitive", while noting that since the cessation of saltwater injection following a third set of tremors in June, the quakes have stopped.
I am reminded of a Swiss Geothermal energy project, in which the injection of water into naturally hot rock to recover heat, also caused quakes. Evidently, one should proceed with some trepidation in mixing geology with water that is not naturally part of it.
"Texas earthquakes may be linked to wells for gas mining," By Dan Vergano, USA Today. http://www.usatoday.com/tech/science/2010-03-11-quakes11_ST_N.htm