Human-induced earthquakes, also known as induced seismicity, are an increasing concern in regions of the United States where the produced fluids and wastewaters from oil and natural gas production are being injected into the subsurface through deep disposal wells. The immediate concern is that injection of these fluids underground may cause damaging earthquakes in regions that typically do not experience much seismic activity. Induced seismicity has garnered increased attention partly as a result of the rapid development of unconventional oil and gas resources using hydraulic fracturing (often referred to as "fracking"). Specifically, the use of high-volume hydraulic fracturing has contributed significantly to the volume of wastewater requiring disposal and has also created demand for disposal wells in new locations. In examining these issues, it is important to distinguish between seismic activity possibly related to hydraulic fracturing itself and the possibility of seismic activity related to injecting fluids down disposal wells, which may not be located near where wells were fracked. Human activities have long been known to have induced earthquakes in some instances: impoundment of reservoirs, surface and underground mining, withdrawal of fluids such as oil and gas, and injection of fluids into subsurface formations. 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. The principal seismic hazard that has emerged from increased oil and gas production in the United States appears to be related to disposal of wastewater using deep-well injection in some regions of the country. For example, one study showed the central United States has experienced a sharp increase in seismicity since 2009, increasing from an average of 24 earthquakes per year of magnitude 3.0 (M 3.0) or greater prior to 2009, to an average of 193 earthquakes of M 3.0 or greater through 2014. The number of M 3.0 or greater earthquakes in the central United States has continued to increase; Figure 1 shows that the central United States experienced an average of 330 earthquakes of M 3.0 per year or greater from 2009 through January 2016. Many studies suggest that the most important contributing factor to this rising trend likely is deep-well injection of oil-and-gas-related wastewater. 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. However, even though it is only a small fraction, the overall number of disposal wells is so large that the total seismic hazard, at least for the central United States, appears to have increased measurably (see " National Issues—Changes to the U.S. Earthquake Hazard Maps "). 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. Multiple studies indicate that the vast majority of fluid injections into production wells for hydraulic fracturing cause microearthquakes—the results of fracturing the rock to extract oil or natural gas—which are typically too small to be felt or cause damage at the surface. There are some cases where fracking caused detectable earthquakes felt at the surface, but most were too small to cause damage (see " Hydraulic Fracturing " below). This report reviews the current scientific understanding of induced seismicity, primarily in the context of Class II oil and gas wastewater disposal wells. The report also outlines the regulatory framework for these injection wells and identifies several federal and state initiatives responding to recent events of induced seismicity associated with Class II disposal. How deep-well injection is linked to induced seismicity, and state and federal efforts to address that linkage, are of interest to Congress because of the potential implications for continued development of unconventional oil and gas resources in the United States. If the current boom in onshore oil and gas production continues, then deep-well injection of waste fluids is likely to also continue and may increase in volume. Also, what Congress, the federal government, and the states do to address and mitigate possible human-caused earthquakes from deep-well injection of oil-and-gas-related fluids may provide some guidance for the injection and sequestration of carbon dioxide. Carbon dioxide sequestration—intended to reduce greenhouse gas emissions—would involve ongoing, long-term, high-volume, high-pressure injection via deep wells. Several large-scale injection experiments are currently underway; however, the relationship between long-term and high-volume carbon dioxide injection and induced earthquakes is not known. 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. Such requirements are not included in regulations governing oil and gas wastewater disposal (Class II) wells, although regulators have discretionary authority to add conditions to individual permits. 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. In the 114 th Congress, no legislation has been introduced to address induced seismicity associated with oil and gas wastewater disposal or other injection activities. The Fracturing Responsibility and Awareness of Chemicals (FRAC) Act of 2015 was introduced in the House ( H.R. 1482 ) and the Senate ( S. 785 ). Among other things, the bills would amend the SDWA term "underground injection" to include the injection of fluids and proppants used in hydraulic fracturing operations, thus authorizing EPA to regulate this process under the SDWA. It is unclear whether the legislation would affect EPA regulatory authority regarding the potential for induced seismicity from hydraulic fracturing, unless it could be shown that induced seismicity caused by the hydraulic fracturing process posed a threat to underground sources of drinking water. As discussed, the potential for damaging earthquakes caused by hydraulic fracturing appears to be much smaller than deep-well injection of wastewater from oil and gas production activities. Since about the 1920s, it has been known that pumping fluids in and out of Earth's subsurface has the potential to cause earthquakes. In addition, a wide range of other human activities have been known to cause earthquakes, including the filling of large reservoirs, mining, geothermal energy extraction, and others. The mechanics of how human industrial activities may cause earthquakes are fairly well known: The human perturbation changes the amount of stress in Earth's crust, and the forces that prevent faults from slipping become unequal. Once those forces are out of equilibrium, the fault ceases to be locked and the fault slips, sending shock waves out from the fault that potentially reach the surface and are strong enough to be felt or cause damage. Even knowing that human activities can cause earthquakes, and the mechanics of the process, it is currently nearly impossible to discriminate between man-made earthquakes and those caused by natural tectonic forces through the use of modern seismological methods. Other lines of evidence are required to positively link human activities to earthquakes. That linkage is becoming increasingly well understood in parts of the United States where activities related to oil and gas extraction—deep-well injection of oil and gas wastewater and hydraulic fracturing—have increased significantly in the last few years, particularly in Oklahoma, Texas, Arkansas, Ohio, Colorado, and several other states. Nevertheless, the majority of deep-well injection and hydraulic fracturing activities are not known to cause earthquakes; most are termed "aseismic" (i.e., not causing any appreciable seismic activity, at least for earthquakes greater than M 3.0). Scientists currently have limited capability to predict human-caused earthquakes for a number of reasons, including uncertainty in knowing the state of stress in the earth, rudimentary knowledge of how injected fluids flow underground after injection, poor knowledge of faults that could potentially slip and cause earthquakes, limited networks of seismometers (instruments used to measure seismicity) in regions of the country where most oil-and-gas-related activities are occurring, and difficulty in predicting how large an earthquake will grow once it is triggered. Despite these uncertainties, a simple conceptual model for understanding how deep-well injection may be triggering earthquakes is starting to evolve, particularly for Oklahoma, which has experienced the greatest change in seismicity since about 2009. In the conceptual model, injecting oil and gas wastewater into the sedimentary formation increases its pore pressure. Over time the increase in pore pressure propagates into pre-existing faults in the crystalline basement rocks underlying the sedimentary formation (see Figure 2 ). Some of the faults in the crystalline basement rocks are "critically stressed," meaning that any change to the pre-existing pressure regime has the potential to initiate "slip" along the fault. Slip along a fault creates an earthquake; the size of the earthquake is generally related to the amount of slip and the length of the fault. Furthermore, studies indicate that even small pressure perturbations have the potential to cause relatively large earthquakes along these critically stressed pre-existing faults in crystalline basement rocks. This may have been the case for the large, M 5.6 and M 5.8 earthquakes in Oklahoma in 2011 and 2016, respectively. Prior to the M 5.6 earthquake that occurred on November 6, 2011, in central Oklahoma, an M 4.8 earthquake that struck northeast Denver, on August 9, 1967 was generally accepted as the largest recorded human-induced earthquake in the United States.