With the increase in the use of horizontal drilling and hydraulic fracturing to extract oil and gas from shale, and the concomitant increase in the amount of fluids that are injected for high-volume hydraulic fracturing and for disposal, there are several indications of a link between the injected fluids and unusual seismic activity. However, the precise relationships between earthquake activity and the timing of injection, the amount and rate of fluid injected, and other factors are still not well understood, although a better understanding of these complex relationships appears to be emerging. Several studies have pointed out that, of the more than 30,000 wastewater disposal wells classified by the Environmental Protection Agency (EPA) as Class II, only a small fraction appears to be associated with damaging earthquakes. The potential for damaging earthquakes caused by injection of fluids for hydraulic fracturing operations, as opposed to deep-well injection of wastewater from fracking and other oil and natural gas production, appears to be much smaller. The federal Safe Drinking Water Act (SDWA) authorizes EPA to regulate underground injection activities to prevent endangerment of underground sources of drinking water. The SDWA does not address seismicity; however, EPA underground injection control (UIC) program regulations include seismicity-related siting and testing requirements for hazardous waste and carbon sequestration injection wells. In February 2015, EPA released a document outlining technical recommendations and best practices for minimizing and managing the impacts of induced seismicity from oil and gas wastewater disposal wells. Deep-Well Injection of Oil and Natural Gas Wastewaters

As stated above, the number of earthquakes of M greater than 3.0 in the central and eastern United States has increased dramatically since about 2009 ( Figure 1 ). In the past, Oklahoma has experienced earthquakes large enough to be felt at the surface, including two earthquakes in the range of M 5.0 or greater in 1918 and 1952. Despite the difficulty, and because of the sharp uptick in seismic activity in the central and eastern United States since 2009, on March 28, 2016, the USGS released a one-year seismic hazard forecast for 2016 that includes contributions from both induced and natural earthquakes (revised June 16, 2016). Fracking induces microseismicity, mostly less than M 1.0—too small to feel or cause damage. In some cases, fracking has led to earthquakes larger than M 2.0, including at sites in Oklahoma, Ohio, and England. In western Canada earthquakes larger than M 3.0 have been correlated to fracking. The study indicated that most of the seismic activity in the WCSB since 1985 seems to be associated with oil and gas activity, although only a small portion of fracking operations appear to be linked to seismic activity (0.3% of wells used for hydraulic fracturing). Deep-well injection has long been the environmentally preferred option for managing produced brine and other wastewater associated with oil and gas production. However, the development of unconventional formations using high-volume hydraulic fracturing has contributed significantly to a growing volume of wastewater requiring disposal and has created demand for disposal wells in new locations. Most oil and gas states and some tribes have assumed primacy for Class II wells under this provision. For example, EPA Region III now evaluates induced seismicity risk factors when considering permit applications for Class II wells. The vast majority of Class II disposal wells (and hydraulic fracturing wells) do not appear to be associated with significant seismic events; however, due to the growing volumes injected by these wells and increased seismicity in some disposal areas, an increasing concern in the United States is that injection of these fluids may be responsible for increasing rates of seismic activity. Congress may be interested in oversight of EPA's UIC program or, more broadly, in federally sponsored research on the relationship between energy development activities and induced seismicity. Although only a small fraction of the more than 30,000 U.S. wastewater disposal wells appears to be problematic for causing damaging earthquakes, such incidents may raise questions as to whether other energy-related activity—specifically, underground injection for carbon dioxide sequestration—may present similar risks.
The development of unconventional oil and natural gas resources using horizontal drilling and hydraulic fracturing has created new demand for disposal wells that inject waste fluids into deep geologic formations. Deep-well injection has long been the environmentally preferred method for managing produced brine and other wastewater associated with oil and gas production. However, an increasing concern in the United States is that injection of these fluids may be responsible for increasing rates of seismic activity. The number of earthquakes of magnitude 3.0 or greater in the central and eastern United States, where there are many injection wells, has increased dramatically since about 2009. For example, over 60 earthquakes of magnitudes 4.0 to 4.8 have occurred in central Oklahoma from 2009 to mid-year 2016. Some of these earthquakes may be felt at the surface. The largest earthquake in Oklahoma history (magnitude 5.8) occurred on September 3, 2016, near Pawnee, causing damage to several structures. Central and northern Oklahoma were seismically active regions before the recent increase in the volume of waste fluid injection. However, the sharp uptick in earthquake activity does not seem to be due to typical, random changes in the rate of seismicity, according to several studies.
The relationship between earthquake activity and the timing of injection, the amount and rate of waste fluid injected, and other factors are still uncertain and are current research topics. Despite increasing evidence linking some deep-well disposal activities to human-induced earthquakes, only a small fraction of the more than 30,000 U.S. wastewater disposal wells appears to be associated with damaging earthquakes. However, the U.S. Geological Survey (USGS) deemed the increase in earthquake hazard in the central United States—likely from deep-well injection—sufficient to release a new one-year seismic hazard forecast for 2016 that includes contributions from both induced and natural earthquakes.
The potential for damaging earthquakes caused by hydraulic fracturing, as opposed to deep-well injection of wastewater from oil and gas activities, appears to be much smaller. Hydraulic fracturing intentionally creates fractures in rocks to increase the flow of oil and gas. The technique induces microseismicity, mostly of less than magnitude 1.0—too small to feel or cause damage at the surface. In a few cases, however, hydraulic fracturing has led directly to earthquakes larger than magnitude 2.0, including at sites in Oklahoma, Ohio, and England. In western Canada, earthquakes greater than magnitude 3.0 have been associated with hydraulic fracturing activities, although only from a very small percentage of hydraulic fracturing wells.
The Environmental Protection Agency's (EPA) Underground Injection Control (UIC) program under the Safe Drinking Water Act (SDWA) regulates the subsurface injection of fluids to protect underground drinking water sources. EPA has issued regulations for six classes of injection wells, including Class II wells used for oil and gas wastewater disposal and enhanced recovery. Most oil and gas producing states administer the Class II program. Although the SDWA does not address seismicity, EPA rules for certain well classes require evaluation of seismic risk. Such requirements do not apply to Class II wells; however, EPA has developed a framework for evaluating seismic risk when reviewing Class II permit applications in states where EPA administers this program.
Although only a small fraction of U.S. wastewater disposal wells appears to be problematic for causing damaging earthquakes, the potential for injection-related earthquakes has raised an array of issues and has affected oil and gas wastewater disposal in some areas. In response to induced seismicity concerns, both EPA and state work groups have issued recommendations for best practices to minimize and manage such risks. Several states have increased regulation and oversight of Class II disposal wells. Congress may be interested in oversight of EPA's UIC program or in federally sponsored research on the relationship between energy development activities and induced seismicity.