American Petroleum Institute estimates that about one million wells have been hydraulically fractured in the United States. Among those, only a few cases of felt induced seismicity from hydraulic fracturing has been documented.

In recent years, seismic activity tied to hydraulic fracturing has been documented in England, the Horn River basin of British Columbia and Ohio.

The Oklahoma Geological Survey (OGS) has now documented a temporal connection between hydraulic fracturing injection stages and nearby felt earthquakes, although more research is needed to explain the location of the quakes, some more than 5 km from the well. What’s more, OGS seismologist, Austin Holland recently reported there is growing data suggesting that as many as 2 percent of hydraulically fractured wells in Oklahoma may induce felt seismic events. 

The University of Oklahoma and OGS hope to take advantage of the extensive subsurface data and real-time monitoring of hydraulic fracturing injections to improve understanding of induced seismicity. These monitoring opportunities are rarely available for wastewater injection wells.

Oklahoma Earthquakes and Wastewater Injection

The U.S. Geological Survey (USGS) reports that the number of earthquakes greater than magnitude 3 in Oklahoma is increasing exponentially. There were 20 in 2009 and 427 through the first 10 ½ months of 2014. Before 2008, earthquakes were rare, averaging 2 to 6 per year. These recent quakes are assumed to be caused by wastewater injection wells, although that is often difficult to confirm.

The largest earthquake in Oklahoma—magnitude 5.6— happened on November 5, 2011. It destroyed 14 homes and injured two people. The earthquake was located near Prague, about 30 miles east of Oklahoma City. USGS has linked the earthquake to wastewater disposal wells. Hydraulic fracturing is not associated with the Prague quake.

Large earthquakes did happen before 2008. The second largest Oklahoma earthquake—magnitude 5.5—occurred in the Oklahoma panhandle in April 1952.