Oklahoma does experience earthquakes, and the state has recently become one of the most seismically active regions in the contiguous United States. Before 2009, Oklahoma’s seismic activity was relatively low, averaging only a handful of minor earthquakes each year. The dramatic increase in the frequency and magnitude of earthquakes since then has fundamentally changed the state’s seismic profile. This surge in ground shaking has prompted significant scientific investigation and a range of regulatory responses. Understanding this phenomenon requires examining both the state’s natural geology and the recent human activities that have drastically altered the seismic landscape.
Natural Seismic History
The central United States, including Oklahoma, is not located near a major tectonic plate boundary, but the region still possesses a history of natural, though infrequent, seismic activity. This natural background seismicity is primarily related to ancient, deep-seated geological structures that represent zones of weakness in the Earth’s crust. One such feature is the Nemaha Uplift, a buried ridge of Precambrian basement rock that extends across the state from Oklahoma City northward into Kansas. Movement along the faults associated with this uplift contributed to the minor earthquakes recorded in Oklahoma before 2009.
Another significant geological structure is the Meers Fault in southwestern Oklahoma, which is recognized as the only Quaternary-active fault in the state. Geological evidence indicates that movement on the Meers Fault produced large, prehistoric earthquakes. These structures show that the state’s crust contains ancient fault systems that are susceptible to reactivation. The natural tectonic rate of activity was historically low, with Oklahoma recording only about 128 known earthquakes between 1897 and 1976.
Understanding Induced Earthquake Activity
The sharp increase in Oklahoma’s earthquake rate starting around 2009 is primarily attributed to induced seismicity, which is caused by human activity. This phenomenon is strongly linked to the disposal of massive volumes of wastewater into deep underground injection wells, a byproduct of oil and gas operations. This process is distinct from hydraulic fracturing, but the water that flows back from production, often called “produced water,” must be disposed of.
This produced water is injected deep into the Arbuckle Group, a porous rock formation that lies directly above the crystalline basement rock where most earthquakes occur. The injection increases the fluid pressure, known as pore pressure, deep beneath the surface. This elevated pressure can act to “lubricate” pre-existing, dormant faults, reducing the effective stress that holds them in place. When the pressure exceeds a certain threshold, it can cause the faults to slip, triggering an earthquake.
The state saw a peak in activity in 2015, with 888 earthquakes of magnitude 3.0 or larger occurring that year. This surge included the 2011 Prague earthquake (Magnitude 5.6) and the 2016 Pawnee earthquake (Magnitude 5.8), the largest recorded in Oklahoma’s history. The temporal and spatial correlation between high-volume, deep wastewater injection and the location of the earthquakes provides strong evidence for this induced connection, highlighting the importance of injection depth.
State Monitoring and Regulatory Responses
The state’s response to the seismic surge has involved both enhanced scientific monitoring and significant regulatory action. The Oklahoma Geological Survey (OGS) plays a central role by maintaining a network of seismic monitoring stations across the state to track and report earthquake activity in real-time. This data collection is essential for understanding the scope of the problem and guiding mitigation efforts.
The Oklahoma Corporation Commission (OCC), which regulates the state’s oil and gas industry, has implemented a series of directives to mitigate induced seismicity. A primary focus has been on disposal wells injecting into the Arbuckle formation, particularly those in designated “Areas of Interest” (AOI). Regulatory actions have included mandatory volume reductions for disposal wells, such as a 2016 directive calling for a 40% reduction in injection volume in some areas.
Additionally, the OCC has mandated that operators “plug back” certain wells to limit injection to shallower depths, preventing the water from reaching the deep Arbuckle formation near the basement rock. These efforts, which include the closure of some wells in high-risk zones, have been effective in significantly lowering the rate of induced earthquakes. The state also adopted protocols for operators, lowering the magnitude threshold for required action and mandating the use of seismic arrays for real-time monitoring in certain areas.
Earthquake Safety and Future Risk Assessment
For residents, the primary safety protocol during an earthquake is to “Drop, Cover, and Hold On.” This involves dropping to the floor, taking cover under a sturdy piece of furniture, and holding on until the shaking stops. If no sturdy cover is available, one should crouch against an interior wall and protect the head and neck with their arms. It is important to stay away from windows, glass, and anything that could fall.
While regulatory actions have successfully reduced the frequency of larger induced earthquakes, the underlying geological risk remains, and monitoring continues. The state’s infrastructure, which was not initially designed for frequent seismic activity, is still vulnerable to damage from moderate-magnitude events. Homeowners should consider purchasing earthquake insurance, as standard home insurance policies typically do not cover earthquake-related damage. Ongoing preparedness includes assembling an emergency kit and having a family plan to ensure safety during and after an event.