From yahoo.com news:
In a study he recently published in the journal Proceedings of the National Academy of Sciences, Frohlich analyzed 67 earthquakes recorded between November 2009 and September 2011 in a 43.5-mile (70 kilometers) grid covering northern Texas' Barnett Shale formation. He found that all 24 of the earthquakes with the most reliably located epicenters originated within 2 miles (3.2 km) of one or more injection wells for wastewater disposal.
The injection well just south of DFW airport has been out of use since September 2011, according to Frohlich, but he says that doesn't rule it out as a cause of the weekend's quakes. He explained that, though water is no longer being added, lingering pressure differences from wastewater injection could still be contributing to the lubrication of long-stuck faults.
"Faults are everywhere. A lot of them are stuck, but if you pump water in there, it reduces friction and the fault slips a little," Frohlich told Life's Little Mysteries. "I can't prove that that's what happened, but it's a plausible explanation."
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Rig Count Interactive Map by Baker Hughes, an energy services company.
Shale Sheet Fracking, a Youngstown Vindicator blog.
The Ohio Environmental Council, a statewide eco-group based in Columbus.
Earthjustice, a national eco-group.
People's Oil and Gas Collaborative-Ohio, a grass-roots group in Northeast Ohio.
Concerned Citizens of Medina County, a grass-roots group.
No Frack Ohio, a Columbus-based grass-roots group.
Fracking: Gas Drilling's Environmental Threat by ProPublica, an online journalism site.
Pipeline, blog from Pittsburgh Post-Gazette on Marcellus shale drilling.
Allegheny Front, environmental public radio for Western Pennsylvania.