Oklahoma has experienced a significant increase in earthquake activity since the early 2000s, transforming a state once considered seismically quiet into one of the most active in the continental United States. Historically, the region rarely recorded large magnitude events, with seismic activity primarily limited to the southwestern corner of the state. This sudden rise in the frequency and magnitude of earthquakes, particularly in central Oklahoma, began to accelerate around 2009. This established a new pattern of shaking in an area long accustomed to geological stability.
Oklahoma’s Underlying Geological Susceptibility
The state’s capacity to host significant earthquakes stems from its deep, ancient geological structure. Beneath the surface sedimentary layers lies the Precambrian crystalline basement rock, a rigid foundation fractured by numerous pre-existing, dormant fault systems dating back hundreds of millions of years.
These faults are remnants of past tectonic events and are concealed by younger rock, making them difficult to map. Although Oklahoma is far from a modern plate boundary, the North American continental plate is under constant, slow-moving stress that compresses and strains these ancient faults. This means that many of these deep faults are already critically stressed and primed for movement.
The Nemaha Ridge and the Meers Fault exemplify the region’s inherent seismic framework. The mechanical stability of these features is precarious, meaning the deep geology is highly sensitive to even small changes in subsurface pressure, making the region susceptible to an external trigger.
The Primary Trigger: Wastewater Injection
The surge of earthquake activity in central Oklahoma is overwhelmingly linked to the practice of disposing of massive volumes of wastewater deep underground. This involves pumping saline water, known as “produced water,” into disposal wells, primarily targeting the Arbuckle Group, a deep sedimentary formation atop the crystalline basement rock. Produced water is a byproduct of oil and gas extraction that must be separated and disposed of.
This disposal process is distinct from hydraulic fracturing, or “fracking.” Fracking involves injecting small volumes of fluid over a short period to release oil or gas. While fracking can cause micro-seismicity, it is not the dominant cause of the larger, damaging earthquakes in Oklahoma. The problem lies with the sheer volume and prolonged duration of wastewater disposal.
The mechanism by which fluid injection triggers earthquakes is the increase in pore pressure. As billions of barrels of water are injected into the Arbuckle formation, the fluid pressure within the rock pores increases significantly. This elevated pressure diffuses downward into the underlying crystalline basement rock, where the pre-existing, stressed faults are located.
The increase in pore pressure effectively pushes against the fault surfaces, reducing the friction that holds the fault blocks in place, a process known as “un-clamping.” When frictional resistance is lowered, the slow-building tectonic stress overcomes the remaining resistance, causing the fault to slip suddenly. This forced movement prematurely activates the dormant faults, generating induced earthquakes. The largest events, including the magnitude 5.8 earthquake near Pawnee in 2016, have been definitively linked to these high-volume disposal practices.
State Actions and Earthquake Reduction Efforts
In response to the scientific consensus linking wastewater disposal to the surge in earthquakes, Oklahoma regulators began implementing mitigation strategies. The Oklahoma Corporation Commission (OCC), which oversees oil and gas operations, worked with the Oklahoma Geological Survey (OGS) to enforce new rules starting around 2015. These measures focused on reducing the fluid volumes and injection depths of disposal wells in the most seismically active areas.
The most impactful regulatory action was the mandated reduction of injection volumes, particularly for wells targeting the deeper Arbuckle formation. The OCC also directed operators to perform “plug-backs,” which involves cementing the lower portion of disposal wells to prevent fluid from reaching the deep crystalline basement rock where the largest faults are located. These directives covered thousands of square miles in central Oklahoma and required a significant reduction in the total volume of injected wastewater.
The regulatory actions have correlated with a substantial decrease in the state’s seismicity rate. Studies indicate that the combination of volume reductions and mandated plug-backs was highly effective, reducing the rate of induced earthquakes significantly. While the risk has not been eliminated, the proactive regulation of wastewater disposal has allowed the state to move toward a more historically typical background level of seismic activity.