Oklahoma, traditionally not associated with major seismic activity, has experienced a profound change in its geological profile over the last decade. The state now has earthquakes, and the frequency of these events dramatically increased, briefly making it one of the most seismically active states in the contiguous United States. This surge, known as induced seismicity, is directly linked to industrial activity deep beneath the surface. This shift from a quiet interior region to an earthquake hot spot prompted urgent scientific investigation and regulatory action.
Documenting the Shift in Oklahoma’s Seismicity
Historically, Oklahoma averaged only about one or two earthquakes of Magnitude 3.0 or greater annually between 1970 and 2008. This low baseline activity was typical for a region far from tectonic plate boundaries. A distinct shift began around 2009, with the number of earthquakes rapidly increasing year over year. By 2014, the annual rate of Magnitude 3.0+ earthquakes in Oklahoma surpassed that of California, the most seismically active state in the nation.
The peak occurred in 2015, which saw over 900 earthquakes of Magnitude 3.0 or higher—a nearly 900-fold increase compared to the historical average. While most tremors ranged from Magnitude 2.0 to 4.0, the state also saw larger events, including the Magnitude 5.8 earthquake near Pawnee in 2016, the largest ever recorded in Oklahoma. This data established a clear temporal correlation between industrial activity and the seismic surge.
The Scientific Mechanism Behind the Quakes
The scientific consensus links the dramatic increase in Oklahoma’s earthquakes to the deep injection of oil and gas production wastewater, known as induced seismicity. This activity is distinct from hydraulic fracturing. The primary culprit is the disposal of “produced water,” which is hypersaline water naturally brought to the surface along with oil and gas.
This produced water is injected into deep disposal wells, commonly targeting the Arbuckle Group, a porous sedimentary layer that lies directly above the crystalline basement. The crystalline basement is the hard rock layer where the state’s dormant faults are located. High-volume injection increases the fluid pressure, or pore pressure, within the Arbuckle and the underlying basement rock.
The increased pore pressure acts like a lubricant, reducing the effective stress clamping the existing faults together. When the fluid pressure is high enough, it overcomes the frictional resistance on these pre-stressed faults, allowing them to slip and release seismic energy. This process can trigger earthquakes kilometers away from the injection site and at depths of up to 12 kilometers, where the faults are most susceptible to movement. The volume and depth of the injected wastewater are the two most significant factors determining the location and magnitude of the induced earthquakes.
State-Level Regulatory Action and Mitigation Efforts
In response to the escalating seismic hazard, the Oklahoma Corporation Commission (OCC), which regulates the state’s oil and gas industry, began implementing a series of mitigation policies. Early regulatory efforts focused on a “traffic light” system, which required operators to reduce or halt injection based on seismicity levels near their wells.
A more comprehensive strategy involved widespread mandatory volume reductions for disposal wells injecting into the Arbuckle formation. Furthermore, the OCC issued directives requiring operators to “plug back” their wells, meaning they had to restrict the depth of their injection to a level significantly above the crystalline basement. The goal of these depth restrictions was to prevent the pressure front from reaching the deepest, most seismogenic faults.
These regulatory actions, combined with a decline in oil and gas production, have been effective in reducing the rate of induced seismicity. Injection volumes into the Arbuckle formation peaked in 2015, and the subsequent regulatory-mandated reductions correlate with a notable decline in the number of earthquakes in the following years. Studies confirm that the combination of volume limits and well plug-backs was an effective strategy for reducing the hazard associated with the induced earthquakes.
Personal Safety and Preparedness
Although regulatory action has reduced the frequency of large events, the risk of earthquakes remains elevated compared to the state’s historical norm, requiring residents to adopt new safety practices. The most important action during shaking is to follow the “Drop, Cover, and Hold On” procedure. This involves immediately dropping to the ground, taking cover under a sturdy piece of furniture like a desk or table, and holding on until the shaking stops.
Securing heavy items within the home is a practical step to prevent injury from falling objects. Residents should also prepare an emergency kit and establish a communication plan.
Preparedness Measures
- Bookcases, water heaters, and large wall hangings should be anchored to the wall studs to prevent them from toppling.
- Prepare an emergency kit with non-perishable food, water, a first-aid kit, and a battery-powered radio for communication in case of power loss.
- If an earthquake occurs while driving, pull over to a clear location away from overpasses, bridges, and power lines, set the parking brake, and wait inside the vehicle until the shaking subsides.
- Have a family communication plan, including an out-of-state contact, to ensure family members can reconnect if they are separated during the emergency.