Introduction

Human activities, such as dam building, mining, and injecting fluids via underground wells, may cause earthquakes (also known as induced earthquakes or induced seismicity).¹ Underground fluid injection activities that may induce earthquakes include hydraulic fracturing oil and gas production wells (HF), enhanced oil and gas recovery wells, wastewater disposal wells, some enhanced geothermal energy production wells, and geologic sequestration wells for liquid carbon dioxide storage.² HF, recovery, and disposal well activity has increased in the central and eastern United States since 2008 (CEUS, defined as the area on the inset map of Figure 1), in part due to advances in horizontal drilling to recover oil and gas from unconventional resources.³ A small fraction of tens of thousands of HF, recovery, and disposal wells induce tens to hundreds of earthquakes in the CEUS.⁴ Wastewater disposal accounts for the majority of induced earthquakes in the CEUS since 2009.⁵ Some of these induced earthquakes have caused damage and led to lawsuits against well operators, motivating a variety of stakeholders to consider ways to mitigate induced earthquakes.⁶

The number of induced earthquakes in the CEUS since 2009 correlates in space and time with some underground fluid injection activities, such as oil and gas activities and wastewater disposal, and in many cases research has identified a fault prone to slipping that may be the source of these earthquakes near these injection activities (Figure 1).⁷ The number of magnitude 3.0 or larger (M 3.0+) earthquakes increased every year from 2009 to 2015 and reached a peak of 1,010 events in 2015 compared to an average number of M3.0+ earthquakes in the CEUS of 25 or fewer events per year from 1973 to 2008.⁸ Since 2015, the annual number of M 3.0+ earthquakes has declined from this peak, but remains above 25 earthquakes per year. In addition, there has been a small increase in M 3.0+ earthquakes in 2020 and 2021 (Figure 1). As the annual number of earthquakes in the CEUS has increased since 2009, so too has the magnitude of some of these events, with more earthquakes greater than M 4.0. Some communities are feeling ground shaking from these events and five earthquakes of M 4.8+ caused property damage in Colorado, Oklahoma, and Texas between 2011 and 2016.

Figure 1. Number of Magnitude 3.0 of Greater Earthquakes in the Central and Eastern United States, 1973-2021

Source: U.S. Geological Survey (USGS), "Induced Earthquakes," at https://www.usgs.gov/programs/earthquake-hazards/induced-earthquakes. Notes: The inset map defines the area included as the central and eastern United States (CEUS) for the purposes of this report. The USGS notes the rate of M 3.0+ earthquakes per year in this area was 25 or fewer events from 1973 to 2008 (blue bars), and this may be considered the average geologic rate (i.e., the expected average rate of earthquakes in the CEUS related to natural geologic processes based on recorded events). Since 2009, the annual rate of earthquakes in the CEUS has increased to at least 58 events per year from 2009 to 2012 and at least 100 events per year since 2013 (orange bars). The peak annual number in 2015 was 1,010 events. The purple dots on the map (which correspond to blue bars on the graph) are earthquakes of magnitude 3.0 or greater that occurred between 1973 and 2008. The red dots on the map (which correspond to the orange bars on the graph) are earthquakes of M 3.0+ that occurred between 2009 and 2021.

Under the Safe Drinking Water Act (SDWA; 42 U.S.C. §§300f-300j), EPA is authorized to regulate underground injection activities (except for most HF activities) to prevent endangerment of underground sources of drinking water (USDW). EPA has issued Underground Injection Control (UIC) regulations for six classes of wells, including wastewater disposal, enhanced oil and gas recovery, some geothermal energy, and carbon sequestration.

SDWA does not require EPA to address seismicity directly; however, EPA UIC program regulations include seismicity-related siting and testing requirements for hazardous waste and carbon dioxide sequestration injection wells.⁹ Such requirements are not included in regulations governing oil and gas wastewater disposal wells, although regulators (either EPA or a state) have discretionary authority to add conditions to individual permits. In 2015, EPA published technical recommendations and best practices for minimizing and managing the impacts of induced seismicity from oil and gas wastewater disposal wells.¹⁰

The USGS, universities, and state agencies have conducted seismic monitoring and research to identify the causes and assess the risks of induced seismicity in the CEUS. These studies, in addition to EPA guidance and state mitigation measures, may have contributed to decreasing the annual number of earthquakes in the CEUS since the peak in 2015.¹¹ Even so, the annual number of induced earthquakes (M 3.0+) in the CEUS remains high (e.g., eight times higher in 2021 than the historic annual rate before 2009) and more research and mitigation may help to reduce earthquake risks.

Understanding induced earthquakes caused by underground fluid injection in the CEUS may help the U.S. Geological Survey (USGS), the Department of Energy (DOE), and other stakeholders understand fault mechanisms and the potential for induced earthquakes caused by similar underground fluid injection processes used for geothermal energy production or geologic carbon sequestration.¹² Some in Congress are interested in increasing these activities for energy production and for reducing the amount of carbon dioxide in the atmosphere.

In the 117^th Congress, there was support for more research on induced seismicity to understand the causes and reduce the risks. The House Committee on Appropriations, for example, in its report accompanying the FY2023 Department of the Interior, Environment, and Related Agencies appropriations bill, called for $3.1 million for the USGS Earthquake Hazards Program for induced seismicity.¹³ Other measures introduced in the 117^th Congress would have changed the federal role in the regulation of some underground fluid injection activities. Congress may consider whether the federal government should play any role in regulating induced seismicity from underground fluid injection activities.

Understanding, Monitoring, and Assessing the Risk of Induced Seismicity

Scientists and others have known, since the 1920s, that pumping fluids in and out of Earth's subsurface has the potential to cause earthquakes.¹⁴ Some wells pump fluids into a target rock formation in the subsurface to permanently contain waste products deep underground, such as wastewater disposal wells and carbon sequestration wells (see text box titled "A Historical Example: The Rocky Mountain Arsenal"). Other wells pump fluids into a target rock formation in the subsurface to extract oil and gas (e.g., enhanced oil and gas recovery wells or HF wells) or to extract energy as heat (e.g., enhanced geothermal wells).¹⁵ The underground fluid injection may change the amount of local stress in Earth's crust, and the forces that prevent faults from slipping may become unequal. Once those forces are out of equilibrium, the fault may become unstable and may slip. The sudden movement on the fault leads to an earthquake, which releases energy and sends seismic waves out from the fault; these waves may reach the surface with enough energy to cause shaking at the surface and the shaking may cause damage.

Horizontal (or directional) drilling techniques combined with HF helped spur an increase in oil and gas activities in the CEUS since about 2008,¹⁶ where most of these economically viable unconventional resources are concentrated.¹⁷ HF accounts for most of the onshore oil and gas production in the United States in 2021.¹⁸ HF typically targets an impermeable shale formation that contains oil or gas trapped in the rock (i.e., an unconventional resource); and works by horizontal drilling into the formation (Figure 2).¹⁹ Fluids injected under high pressure into a shale formation fracture the rock and enhance release of the oil and/or gas for extraction from a well.²⁰ The fracturing of the rock during the HF process induces micro-earthquakes of M < 1.0 that do not cause any human-felt shaking at the surface. In rare cases, where HF operations inject fluid close to a preexisting fault (e.g., a fault in the deeper crystalline basement rocks below the shale formation), the fluid may activate the fault and induce an earthquake of higher magnitude (e.g., M 3.0+) that may be felt at the surface, and if strong enough may actually cause damage.²¹

Figure 2. Illustration of the Possible Relationship Between Underground Injection and Induced Seismicity

Source: North Carolina General Assembly, presentation by the Arkansas Oil and Gas Commission, Fayetteville Shale Overview, for the North Carolina Delegation, slide 33, prepared by Southwestern Energy, November 21, 2013, at http://www.ncleg.net/documentsites/committees/BCCI-6576/2013-2014/5%20-%20Feb.%204.%202014/Presentations%20and%20Handouts/Arkansas%20Site%20Visit%20Attachments/Att.%205%20-%20AOGC%20Presentation%2011-21-13%20%283%29.pdf. Notes: The figure is for illustrative purposes only and does not depict any specific location or geological formation, which may be more complex than shown. The term triggered in the figure is synonymous with the term induced as used in this report. Likewise, horizontal shale well is synonymous with hydraulic fracturing, an unconventional oil and gas production technique, and water disposal is synonymous with wastewater disposal of waste products from oil and gas activities. Shale, sand (or sandstone), and limestone are sedimentary rocks formed from different sediments. Shale, formed from muds or clays on lakebeds or seabeds, may contain organic matter and oil or gas. Igneous rocks, such as granite, formed from magma and are sometimes called crystalline rocks because you can see crystals in the rock without a microscope.

HF and other oil and gas activities produce a large amount of wastewater (i.e., about 10 barrels of wastewater for every barrel of product).²² Disposal wells inject wastewater into a sedimentary formation (typically sandstone or limestone) below shallower underground water resources. Disposal wells typically inject larger volumes of fluids for longer periods (months to years) than HF wells, so disposal wells may be more likely than HF wells to induce seismicity.²³

Fluid injection from a disposal well may induce an earthquake on a preexisting fault. After fluid injection into a sedimentary layer (i.e., a target rock formation), the increase in pore pressure in the sedimentary layer could propagate into preexisting fault(s), most commonly located in the crystalline basement rocks underlying the sedimentary formation (Figure 2).²⁴ Slip along a fault creates an earthquake; the magnitude of the earthquake depends on the amount of slip on the fault and other factors.

Not all induced earthquakes stem from faults in the crystalline basement rocks. Some studies have identified induced earthquakes on shallower unstable faults at or above the fluid injection depth. Also, induced earthquakes may occur months or years after fluid injection (similar to the induced earthquakes that occurred years after injection at the Rocky Mountain Arsenal, see text box titled "A Historical Example: The Rocky Mountain Arsenal").²⁵ Induced earthquakes which occur near the Earth's surface may transfer more of their energy into ground shaking at the surface than earthquakes originating at greater depths.²⁶ For this reason, an M 3.0 earthquake near the surface can produce more shaking and thus may cause more damage at the surface than a deeper M 3.0 event.

Underground injection in wastewater disposal wells has induced five damaging earthquakes in the United States (Table 1) between 2011 and 2016.²⁷ Stakeholders in Oklahoma sued various well operators to recover the costs of damages from an M 5.8 earthquake near Pawnee, OK, and an M 5.0 earthquake near Cushing, OK.²⁸ (See text box titled "Magnitude 5.8 Earthquake near Pawnee, OK, on September 3, 2016" for more details about the earthquake and subsequent mitigation efforts.)

Seismic Monitoring of Induced Earthquakes

Researchers say they lack sufficient data on well operations, geologic conditions underground and in some cases sufficient monitoring of areas of concern for earthquakes with seismic instruments to identify any critically stressed faults and analyze which underground fluid injection activities may induce earthquakes on these faults.²⁹

Seismic monitoring is a primary tool for estimating the state of stress and identifying faults underground.³⁰ The USGS Earthquake Hazards Program monitors and reports on earthquakes in the United States and globally, assesses earthquake hazards, and conducts research on the causes and effects of earthquakes under the authority of the National Earthquake Hazards Reduction Program.³¹ The USGS has deployed seismic instruments to understand induced earthquakes related to oil and gas activities in Kansas, Oklahoma, Ohio, and Texas.³² The USGS also deployed seismic instruments near Decatur, IL to understand induced earthquakes related to an experimental geologic carbon sequestration project.³³

In addition to the USGS, state agencies and universities have enhanced or established short- and long-term seismic monitoring to understand induced seismicity and to mitigate seismic risks in the CEUS. For example, Arkansas, Kansas, Ohio, Oklahoma, and Texas have seismic networks operated by state agencies and/or universities to study natural and induced seismicity.³⁴ Researchers using these networks seek to understand which operations may cause induced seismicity and to improve the capability of regulators and operators to mitigate induced earthquakes.

One example of a state-run seismic monitoring network to understand earthquakes is the Oklahoma Geological Survey's (OGS's) Seismic Monitoring Program.³⁵ OGS has operated the program since 1961 and increased the size of the network and its seismic monitoring capabilities beginning in 2009 to understand the increasing number of earthquakes per year in the state. Figure 3 shows the number of M 3.0+ earthquakes in Oklahoma from 2009 to 2021 from the OGS earthquake catalogs. The figure shows a peak in the annual number of earthquakes in 2015 and a corresponding peak in higher-magnitude events (i.e., M 4.0+). The Oklahoma Corporation Commission's (OCC) Induced Seismicity Department correlated most of these earthquakes with underground injection activities. The OCC regulates oil and gas activities and the Underground Injection Control program (on behalf of the EPA) in Oklahoma (See text box titled "Magnitude 5.8 Earthquake near Pawnee, OK, on September 3, 2016"). According to a 2018 OCC report, the annual number of induced earthquakes (M 3.0+) in Oklahoma has decreased since 2015, primarily due to regulations and directives to mitigate induced seismicity.³⁶

Figure 3. Earthquakes of Magnitude 3.0 or Greater in Oklahoma, 2009-2021

Source: Oklahoma Geological Survey, "Earthquake Catalog Download Tool," at https://www.ou.edu/ogs/research/earthquakes/catalogs. Notes: CRS downloaded the earthquake catalogs for the years 2009-2021. The plots show the number of earthquakes of the indicated magnitude range for each year. There were no earthquakes of M 4.0 or greater in 2009 in the Oklahoma Geological Survey earthquake catalog.

Seismic monitoring provides details about earthquakes that may help to identify and reduce earthquake risks.³⁷ For example, in Oklahoma seismic monitoring shows a slight increase in M 4.0+ events after some injection activities ceased or changed. This may signal that the fluid injections may affect geologic conditions further from the injection location (see text box titled "A Historical Example: Rocky Mountain Arsenal"). Similarly, the recently established Texas Seismic Network (TexNet, started in 2017 by the Texas state legislature) identified an annual increase in the number of M 3.0+ and M 4.0+ earthquakes from 2019 to 2021.³⁸ Texas regulators in consultation with seismologists at TexNet determined that wastewater disposal induced these earthquakes and some of the earthquakes occurred at or above the fluid injection site on shallower faults.³⁹ The occurrence of induced earthquakes on shallower faults contrasts with the illustrative model in Figure 2 and with the identification of deeper faults in Oklahoma and Ohio, highlighting the importance of understanding the different geologic conditions in different locations.

Evaluating the Risk of Induced Earthquakes

The USGS Earthquake Hazards Program conducts research, monitors, reports, and assesses the risks of induced seismicity from many underground injection activities as a small component of its overall program.⁴⁰ The increase in seismicity since 2009 in the CEUS (Figure 1) is caused by underground fluid injection (primarily from wastewater disposal), according to USGS and other studies.⁴¹ In addition to the increase in the number of earthquakes since 2009, the number of induced earthquakes of M 4.0+ increased over the same period. Larger magnitude events (4.0+) in the shallow crust increase the risk of damage from more intense ground shaking.

The USGS prepares and regularly updates U.S. Seismic Hazard Maps to assess earthquake hazards and their associated risks across the country.⁴² These maps typically exclude the seismic hazard posed by induced earthquakes, because researchers are unsure how to treat potentially induced earthquakes in their seismic hazard analysis.⁴³ The natural tectonic processes causing earthquakes do not change much over geologic timescales of thousands to millions of years. For example, the seismic hazard in portions of California, Alaska, and other states experiencing these tectonic forces do not vary much from year to year.⁴⁴ In contrast, induced seismicity can vary over short timescales because underground fluid injection activities often change over short times (i.e., weeks, months, or a few years). Those characteristics mean assessing risk from combining natural seismic hazards with induced seismic hazards is difficult, in part because induced earthquakes are a short-term hazard, compared with the perennial seismic hazard from natural tectonic forces.⁴⁵

Despite the difficulty, the USGS released one-year seismic hazard forecasts for the CEUS for 2016, 2017, and 2018 that included contributions from induced and natural earthquakes.⁴⁶ The 2018 map in Figure 4 shows two main areas of earthquake hazard in the CEUS: the natural seismic zone near New Madrid, MO, and the induced seismic zone extending around Oklahoma City, OK.⁴⁷

The risks from induced earthquakes is ultimately dependent on local geologic conditions and local fluid injection activities.⁴⁸ Oklahoma state agencies monitor induced earthquakes at the local level and understand that fluid injections may cause earthquakes on preexisting faults in the deeper crystalline basement below the injection site soon after injection or the earthquakes may be delayed for reasons that are not fully understood. Oklahoma's monitoring identifies different fault risks for Oklahoma regulators to consider and they may adjust their response regarding fluid injection activities that may induce earthquakes. Similarly Texas state agencies recognize that induced earthquakes may occur on preexisting faults in the shallow layers above or at the same level as the injection site as well as in the deeper crystalline basement based on TexNet monitoring. Texas regulators have posted plans to respond to the seismicity in shallow and deep areas to reduce risks.⁴⁹

Figure 4. Chance of Damage from an Earthquake in the Central and Eastern United States in 2018

Source: USGS, "Hazard Estimation for Induced Earthquakes," at https://www.usgs.gov/programs/earthquake-hazards/science/hazard-estimation-induced-earthquakes. Modified by CRS. Notes: One-year forecast for 2018 of potential earthquake shaking in the central and eastern United States based on past induced and natural earthquakes. The natural seismic zone near New Madrid, MO, and the induced seismic zone near Oklahoma City, OK, had the highest potential for shaking in 2018.

Overview of the Current Regulatory Structure Regarding Induced Seismicity

According to a National Research Council report, conventional oil and gas production and hydraulic fracturing combined generate more than 800 billion gallons of fluid each year. Underground injection control (UIC) Class II injection wells dispose of more than one-third of this volume deep underground.⁵⁰ 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 and growth of HF production has contributed significantly to a growing volume of wastewater requiring disposal and has created demand for disposal wells in new locations. Recent incidents of seismicity near disposal wells have drawn renewed attention to laws, regulations, and policies governing wastewater management and have generated various responses at the federal and state levels. This section of the report reviews the current regulatory framework for managing underground injection and identifies several federal and state initiatives in response to concerns surrounding Class II disposal and induced seismicity.

EPA Regulation of Underground Injection

The principal law authorizing federal regulation of underground injection activities is the Safe Drinking Water Act (SDWA), as amended.⁵¹ The law specifically directs EPA to promulgate regulations for state UIC programs to prevent underground injection that endangers drinking water sources.⁵² Historically, EPA has not regulated oil and gas production wells. Further, as amended in 2005, SDWA explicitly excludes the regulation of underground injection of fluids or propping agents (other than diesel fuels) associated with hydraulic fracturing operations related to oil, gas, and geothermal production activities.⁵³

SDWA authorizes states and Indian tribes to assume primary enforcement authority (primacy) for the UIC program for any or all classes of injection wells.⁵⁴ EPA must delegate this authority, provided the state or tribal program meets certain statutory and EPA requirements.⁵⁵ If EPA does not approve a state's UIC program plan or if a state chooses not to assume program responsibility, then EPA implements the UIC program in that state.

For oil and gas-related injection operations (e.g., produced water disposal through Class II wells), the law allows states or tribes to administer the UIC program using state rules rather than meeting EPA regulations, provided a state demonstrates it has an effective program that prevents underground injection that endangers drinking water sources.⁵⁶ Most oil and gas states and some tribes have assumed primacy for Class II wells under this provision.

Under the UIC program, EPA, states, and tribes regulate more than 700,000 injection wells. To implement the UIC program as mandated by SDWA, EPA has established six classes of underground injection wells based on categories of materials injected by each class

In addition to the similarity of fluids injected, each class shares similar construction, injection depth, design, and operating techniques. The wells within a class are required to meet a set of appropriate performance criteria for protecting USDW.⁵⁷

Class II injection wells include wells (1) to inject fluids to enhance recovery of oil and gas from conventional fields (Class IIR), (2) to dispose of brines (saltwater) and other fluids (wastewater) associated with oil and gas production (Class IID), and (3) to store liquid hydrocarbons. There are more than 156,000 Class II wells across the United States. Based on historical averages, roughly, 80% of the Class II wells are enhanced recovery wells and 20% are disposal wells.⁵⁸ Class II injection wells, specifically Class IID disposal wells, have caused the most induced earthquakes in the CEUS since 2009 (see Table A-1 in the Appendix for the number of oil and gas wells and Class II wells by state in operation in 2019 or 2020).

Consideration of Seismicity in EPA UIC Regulations

SDWA does not mention seismicity; rather, the law's UIC provisions authorize EPA to regulate underground injection to prevent endangerment of USDW. Seismicity has the potential to affect drinking water quality through various means (e.g., by damaging the integrity of a well or creating new fractures and pathways for fluids to reach groundwater). EPA UIC regulations include various requirements aimed at protecting USDW by ensuring injected fluids remain in a permitted injection zone. Some of these measures also could reduce the likelihood of inducing seismic events. For example, injection pressures for Class II (and other) wells may not exceed a pressure that would initiate or propagate fractures in the confining zone adjacent to a USDW.⁵⁹ As a secondary benefit, limiting injection pressure could prevent fractures that may act as conduits through which injected fluids could reach an existing fault.

EPA regulations for two categories of injection wells—Class I hazardous waste disposal wells and Class VI wells for geologic sequestration of CO₂—specifically address evaluation of seismicity risks with siting and testing requirements. For Class I wells, EPA regulations include minimum criteria for siting hazardous waste injection wells, requiring that wells be limited to geologically suitable areas. The UIC director (i.e., EPA or the delegated state or tribe) is required to determine geologic suitability based on an "analysis of the structural and stratigraphic geology, the hydrogeology, and the seismicity of the region."⁶⁰ Testing and monitoring requirements for Class I wells state that "the Director may require seismicity monitoring when he has reason to believe that the injection activity may have the capacity to cause seismic disturbances."⁶¹

For Class VI CO₂ sequestration wells, EPA regulations similarly require evaluation of seismicity risks through siting and testing requirements. In determining whether to grant a permit, the UIC director must consider various factors, including potential for seismic activity:

Prior to the issuance of a permit for the construction of a new Class VI well or the conversion of an existing Class I, Class II, or Class V well to a Class VI well, the owner or operator shall submit ... and the Director shall consider ... information on the seismic history including the presence and depth of seismic sources and a determination that the seismicity would not interfere with containment.⁶²

EPA regulations for oil and gas wastewater disposal wells (or other Class II wells) do not include these provisions or otherwise address seismicity. However, the regulations give discretion to UIC directors to include in individual permits additional conditions as needed to protect USDW (including requirements for construction, corrective action, operation, monitoring, or reporting).⁶³ Again, for the purpose of protecting drinking water sources, permits for all Class I, II, and III wells must contain specified operating conditions, including "a maximum operating pressure calculated to avoid initiating and/or propagating fractures that would allow fluid movement into a USDW."⁶⁴ Regulations for Class I wells further specify that "injection pressure must be limited such that no fracturing of the injection zone occurs during operation."⁶⁵

Outside of regulations, EPA has taken steps to address induced seismicity concerns associated with Class II disposal wells. For example, EPA Region III, which directly implements the UIC program in Pennsylvania and Virginia, evaluates induced seismicity risk factors when considering permit applications for Class II wells.⁶⁶ In responding to public comments on a Class II well permit application in 2013, the regional office noted the following:

Although EPA must consider appropriate geological data on the injection and confining zone when permitting Class II wells, the SDWA regulations for Class II wells do not require specific consideration of seismicity, unlike the SDWA regulations for Class I wells used for the injection of hazardous waste.... Nevertheless, EPA evaluated factors relevant to seismic activity such as the existence of any known faults and/or fractures and any history of, or potential for, seismic events in the areas of the Injection Well as discussed below and addressed more fully in "Region 3 framework for evaluating seismic potential associated with UIC Class II permits, updated September, 2013."⁶⁷

Recommendations to Mitigate Induced Seismicity Related to Class II Disposal Wells

As discussed above, SDWA does not directly address seismicity; rather, the law authorizes EPA to regulate subsurface injections to prevent endangerment of drinking water sources. In 2011, in response to earthquake events in Arkansas and Texas, EPA asked the Underground Injection Control National Technical Workgroup to "develop technical recommendations to inform and enhance strategies for avoiding significant seismicity events related to Class II disposal wells." The workgroup was specifically asked to address concerns that induced seismicity associated with Class II disposal wells could cause injected fluids to move outside the containment zone and could endanger drinking water sources. EPA requested that the report contain the following specific elements:

• Comparison of parameters identified as most applicable to induced seismicity with the technical parameters collected under current regulations.
• Decisionmaking model/conceptual flow chart to
• provide strategies for preventing or addressing significant induced seismicity,
• identify readily available applicable databases or other information,
• develop a site characterization checklist, and
• explore applicability of pressure transient testing and/or pressure monitoring techniques.
• Summary of lessons learned from case studies.
• Recommended measurement or monitoring techniques for high-risk areas.
• Applicability of conclusions to other well classes.
• Defined specific areas of research, as needed.⁶⁸

In February 2015, EPA released the National Technical Workgroup's final report, Minimizing and Managing Potential Impacts of Injection-Induced Seismicity from Class II Disposal Wells: Practical Approaches, which addressed the above tasks.⁶⁹ The report does not constitute formal agency guidance, nor has EPA initiated any rulemaking regarding this matter. Rather, the document includes practical management tools and best practices to "provide the UIC Director with considerations for addressing induced seismicity on a site-specific basis, using Director discretionary authority."⁷⁰

Among other findings, the report identifies three key components that must be present for injection-induced seismic activity to occur:

• 1. Sufficient pressure buildup from disposal activities.
• 2. A fault of concern.
• 3. A pathway allowing the increased pressure to communicate from the disposal well to the fault.⁷¹

As discussed, current Class II regulations give discretion to UIC directors to include in individual permits additional conditions and requirements as needed to protect USDW.⁷² The Practical Approaches document notes that, although EPA is unaware of any USDW contamination resulting from seismic events related to induced seismicity, potential USDW risks from seismic events could include loss of disposal well mechanical integrity, impact to various types of existing wells, changes in USDW water level or turbidity, or USDW contamination from a direct communication with the fault inducing seismicity or contamination from earthquake-damaged surface sources.⁷³

The report includes a decision model to inform regulators on site assessment strategies and recommends monitoring, operational, and management approaches to manage and minimize suspected injection-induced seismicity. Among the management recommendations, the report suggests that, for wells suspected of causing induced seismicity, managers should take early actions (e.g., requiring more frequent pressure monitoring and reducing injection rates) rather than requiring definitive proof of causality.⁷⁴

The technical workgroup also identified research needs to better understand the potential for injection-related induced seismicity, including research regarding geologic siting criteria for disposal zones in areas with limited or no data. As a general principal, the workgroup recommended that future research be conducted using a holistic, multidisciplinary approach, combining expertise in petroleum engineering, geology, geophysics, and seismicity.⁷⁵

State Initiatives Regarding Induced Seismicity

Several organizations and states in the CEUS are monitoring, assessing, guiding, and regulating Class II wells (i.e., waste and recovery) and HF operations that may induce seismicity.⁷⁶ In 2014, the Interstate Oil and Gas Compact Commission and the Ground Water Protection Council formed an Induced Seismicity Work Group (ISWG) with state regulatory agencies and state geological surveys to "proactively discuss the possible association between recent seismic events occurring in multiple states and injection wells."⁷⁷ The ISWG issued its first primer about induced seismicity in 2015, a second primer in 2017, and a guide in 2021. The 2015 and 2017 primers focused on induced seismicity associated with Class II wells. The 2021 guide updated its summary of the scientific understanding of induced seismicity and "expanded on the topic of induced seismicity related to hydraulic fracturing."⁷⁸

States regulate oil and gas activities within their state boundaries, and in some cases the state oil and gas activities program includes the state UIC program. In other cases, a different state agency runs the UIC program.

States in the CEUS that have experienced increased induced seismicity related to oil and gas activities have instituted mitigation strategies. Strategies may include requiring underground injection operators to (1) assess seismic risks before beginning operations, (2) provide details about their operations, and (3) monitor injection sites with seismic instruments for any earthquakes. If induced seismicity is attributed to certain wells, regulators may ask or require the operators to change their operations (e.g., reduce the volume or pressure of fluid injections or change the depth of the injections), or to stop their operations.⁷⁹ For example, Oklahoma has regulations and directives to mitigate induced seismicity while allowing oil and gas activities in the state.⁸⁰ In addition, some states have banned the drilling of underground injection wells in geologic zones of known seismic risk. The 2021 guide provides examples of state efforts to regulate activities to reduce potential seismic risks and mitigate induced seismicity related to oil and gas activities and concomitant wastewater disposal.

Options for Congress

Interest in policies or regulations to mitigate induced seismicity caused by underground fluid injection in the CEUS may change in accordance with the number of induced earthquakes per year, which peaked in 2015, then decreased from 2016 to 2019, and increased again in 2020 and 2021 (Figure 1). Other reasons for renewed interest include several M 5.0 or larger events in Texas in 2020 and 2022 and an M 4.5 earthquake near Clyde, OK, in 2022; news reports and public concern about induced seismicity damage and related litigation; increasing oil and gas activities related to higher oil prices; and increasing interest in the development of enhanced geothermal systems and geologic carbon sequestration.⁸¹ Underground fluid injection activities associated with advancing geothermal energy systems, and injecting liquid carbon dioxide for geologic sequestration may induce earthquakes and may require mitigation strategies similar to those for oil and gas activities to reduce earthquake risks.

Congress may consider whether to support federal agency efforts in research, risk assessment, response, and/or mitigation strategies to understand and reduce induced seismicity caused by underground fluid injection activities. In the past, the USGS has studied induced seismicity caused by underground fluid injections (e.g., oil and gas activities, wastewater disposal, and geologic carbon sequestration) in the CEUS and issued one-year earthquake hazard forecasts for the CEUS for 2016, 2017, and 2018 (see "Understanding, Monitoring, and Assessing the Risk of Induced Seismicity").⁸² The USGS has partnered with states in the CEUS to monitor earthquakes with USGS and state-led seismic networks.⁸³ The FY2022 and FY2023 President's budget requests called for increased funding for the USGS to study induced seismicity caused by geothermal or geologic carbon sequestration activities, and for a one-year earthquake hazard forecast for the CEUS similar to those produced in 2016-2018.⁸⁴ The House Appropriations Committee agreed with some of the Administration's proposals to support induced seismicity research at the USGS in FY2023 (H.Rept. 117-400).⁸⁵ The Senate Appropriations Committee did not express support for any specified induced seismicity work by the USGS in FY2023.⁸⁶ The FY2023 Consolidated Appropriations Act does not include any specific funding for induced seismicity research or assessment by the USGS (P.L. 117-328). Congress may consider whether to direct the USGS to specifically study or provide earthquake hazards assessment for induced seismicity and whether to specify any funding for such work.

Congress may consider whether EPA or DOE requirements, reports, or guidance regarding induced seismicity from underground fluid injection activities are sufficient to reduce the risks of induced earthquakes. Especially given earthquake activity in the CEUS in the past few years and ongoing research and seismic monitoring by federal, state, or local entities (e.g., USGS, state geological surveys, state regulators, and universities). The EPA requires induced seismicity risk assessment for Class I hazardous waste disposal wells and Class VI geologic carbon sequestration wells, but not for Class II wells. EPA issued a report in 2015 on induced seismicity risk assessment and management for Class II wastewater disposal wells (see "Consideration of Seismicity in EPA UIC Regulations"), but the report has no regulatory authority and did not discuss Class II enhanced oil and gas recovery wells. In addition, DOE supports research and demonstration projects for enhanced geothermal systems and geologic carbon sequestration; these underground fluid injection activities may induce earthquakes.⁸⁷ DOE issued a report in 2012 on induced seismicity protocols to mitigate earthquake risks associated with the development of enhanced geothermal systems.⁸⁸ Congress may provide oversight as to whether these past requirements, reports, and guidance are sufficient to reduce the risks of induced earthquakes.

Congress may consider the federal role in regulating underground fluid injection activities to mitigate induced seismicity. EPA has some regulatory authority regarding induced seismicity for Class I and VI wells, while USGS and DOE have no regulatory authority regarding induced seismicity for any underground injection activities (see "Overview of the Current Regulatory Structure Regarding Induced Seismicity"). The EPA allows states and Indian tribes to operate the UIC program in their state or tribal land if they meet the program criteria (see "EPA Regulation of Underground Injection"). Most induced earthquakes in the CEUS are correlated with Class II wastewater disposal and HF wells (excluded from the EPA UIC program, except when diesel fuel is used) that are not regulated for induced seismicity by EPA. In the 117^th Congress, some Members introduced bills that would have regulated HF wells through EPA or the Bureau of Land Management (BLM). Such measures could potentially include a federal role for regulating induced earthquakes. For example, the Fracturing Responsibility and Awareness of Chemicals Act of 2021 (H.R. 2202) proposed to repeal the exemption for HF activities in SDWA including the underground injection of fluids for HF activities under SDWA Section 1421(d)(1). The Restoring Community Input and Public Protections in Oil and Gas Leasing Act of 2021 (H.R. 1503) proposed to give the BLM authority to regulate HF activities on federal lands. The Safe Hydration is an American Right in Energy Development Act of 2021 (H.R. 2164) and the CLEAN Future Act (H.R. 1512) proposed to amend SDWA to require EPA to revise regulations for state UIC programs to require testing of USDW that are within access of HF activities. None of these measures would have required EPA to regulate Class II wells for induced seismicity.

Congress may consider allowing states to continue to regulate oil and gas and underground fluid injection activities with support for research, monitoring, hazard assessment, and risk management from federal agencies. Another bill introduced in the 117^th Congress would have specified state primacy in regulating HF wells. The Fracturing Regulations Are Effective in State Hands Act (S. 2393) proposed to clarify that the state has sole authority to regulate HF activities on federal land within state boundaries. Oklahoma provides an example of a state regulating oil and gas activities and the UIC program in the state while working with federal agencies.⁸⁹ The Oklahoma Corporation Commission (OCC) regulates oil and gas activities and has primacy for the UIC program in the state. As the number and magnitude of earthquakes on annual basis increased in Oklahoma (Figure 3), the state began to address the issue. Oklahoma agencies enhanced seismic monitoring, established regulations and protocols for induced seismicity, and issued directives requiring well operators to change their operations or cease operations where induced earthquakes posed a risk (see "Seismic Monitoring of Induced Earthquakes", "State Initiatives Regarding Induced Seismicity", and the text box titled "Magnitude 5.8 Earthquake near Pawnee, OK: September 3, 2016"). Oklahoma worked with the USGS on seismic monitoring and earthquake science and with the EPA on managing the UIC program. In addition, Oklahoma worked with EPA, which has primacy over the Osage Nation UIC program in Oklahoma to deal with the aftermath of the M 5.8 earthquake near Pawnee, OK.⁹⁰ The EPA issued directives similar to the OCC directives, requesting well operators within the Osage Nation to change their operations or cease operations near the M 5.8 event. The OCC attributed a decrease in the number of induced earthquakes per year in Oklahoma after 2015 to their monitoring, research, regulations, and directives.⁹¹

Appendix. Onshore Oil and Gas Production and Disposal Wells By State

The U.S. Energy Information Administration estimates the amount of oil and gas production and the number of oil and gas wells accounting for this production for the country. Table A-1 shows that most of the onshore oil and gas production per state by total number of oil and gas wells and by HF wells is in the central and eastern United States. The Environmental Protection Agency estimates the number of Class II disposal and recovery wells for the country. Table A-1 shows that most of the Class II wells are in the central and eastern United States.