There are 181 injection wells in the Oklahoma county where most of the weekend earthquakes happened, said Matt Skinner, spokesman for the Oklahoma Corporation Commission, which oversees oil and gas production in the state and intrastate transportation pipelines.

http://www.ajc.com/news/nation-world/big-quake-follows-increase-1218826.html

A previously unreported study out of the Oklahoma Geological Survey has found that hydraulic fracturing may have triggered a swarm of small earthquakes earlier this year in Oklahoma. The quakes, which struck on Jan. 18 in a rural area near Elmore City, peaked at magnitude 2.8 and caused no deaths or property damage.

http://www.eenews.net/public/eenewspm/2011/11/02/1

Geologists have tied fracking wastewater disposal wells in central Arkansas to an outbreak of more than 1,200 so-called “minor earthquakes.” According to the Arkansas Democrat-Gazette, the state’s Oil and Gas Commission has voted to ban fracking wastewater disposal wells within a 1,150-square-mile area north of Conway in the Fayetteville Shale region. According to the Arkansas Geological Survey (AGS), the fracking operations were taking place on top of an active fault line.

http://www.stuarthsmith.com/earthquake-outbreak-arkansas-bans-fracking-operations-inside-thousand-square-mile-area

Injection well operators say there is no clear link between earthquakes and fluid being pumped underground. No operating Ohio wells have been linked to earthquakes, but a now-closed well in Ashtabula County was. The operation, run by Resource Environmental Services Inc. from 1986 to 1994, was linked to numerous small earthquakes between 1987 and 2003.

http://www.newsandsentinel.com/page/content.detail/id/148088/Pa–gas-drilling-waste-pumped-under-Ohio-.html?isap=1&nav=5072