Oklahoma, a state not situated on a tectonic plate boundary, has experienced a significant and complex rise in earthquake activity over the last decade. The presence of natural geological fault lines within the state has long been known, but these ancient structures were historically responsible for only infrequent, low-magnitude seismic events. The recent surge in shaking, however, is a phenomenon largely attributed to human industrial activity interacting with this pre-existing, deep-seated geology. Understanding the seismic landscape of Oklahoma requires looking at both the state’s deep, natural structures and the recent changes caused by modern industrial practices.
The Natural Geological Structures of Oklahoma
The bedrock beneath Oklahoma contains extensive systems of faults formed hundreds of millions of years ago during ancient mountain-building events, such as the Ouachita and Arbuckle orogenies. One of the most notable features is the Nemaha Uplift, a buried ridge of Precambrian crystalline rock that extends from Nebraska down into Oklahoma City. This uplift is associated with a complex fault zone that was active during the Pennsylvanian period.
Another significant natural structure is the Meers Fault, located in the southwestern part of the state, which is one of the largest faults visible at the surface. Historically, Oklahoma averaged only about two earthquakes of magnitude 3.0 or greater per year between 1961 and 2008, establishing a very low baseline of natural seismicity. These ancient fault systems have been mostly dormant for millennia, only occasionally releasing stress. These ancient fault planes represent planes of weakness within the Earth’s crust that can be reactivated if stress conditions change.
The Phenomenon of Induced Seismicity
The dramatic increase in earthquakes beginning around 2009 is directly linked to induced seismicity, which refers to earthquakes triggered by human activities. In Oklahoma, this is overwhelmingly caused by the deep underground disposal of wastewater generated during oil and gas production. This wastewater, often called “produced water,” is a highly saline fluid that naturally coexists with oil and gas deposits.
The process involves injecting massive volumes of this produced water into deep underground formations, primarily into the Arbuckle Group, a porous rock layer above the crystalline basement rock. The disposed fluids migrate downward, eventually reaching the crystalline basement where numerous ancient, dormant faults are located. The injection volume into the Arbuckle formation increased dramatically, from approximately 20 million barrels per year in 1997 to about 400 million barrels per year by 2013.
This substantial increase in fluid volume raises the pore pressure within the rock surrounding the faults. Elevated pore pressure reduces the effective stress holding the fault planes together, essentially lubricating the fault. This reduction in friction allows the pre-stressed, ancient faults to slip much sooner than they would have naturally, releasing seismic energy. This mechanism is distinct from hydraulic fracturing, or “fracking,” which is not the primary driver of the state’s major seismic events.
Current Seismic Activity and Risk Assessment
The peak of seismic activity occurred in 2015, recording over 900 earthquakes of magnitude 3.0 or greater. This figure represented a nearly 450-fold increase above the state’s long-term average. The elevated risk included the state’s largest recorded earthquake, a magnitude 5.8 event near Pawnee in 2016.
In response, the Oklahoma Corporation Commission (OCC) created an Induced Seismicity Department and a Coordinating Council to manage the hazard. Regulators delineated a large Area of Interest (AOI) in central and northern Oklahoma centered near high-volume disposal wells. Mitigation efforts focused on mandating significant reductions in injection volumes, particularly in the Arbuckle formation, and restricting injection depths relative to the basement rock.
The regulatory actions have been effective, leading to a marked decline in the annual rate of earthquakes. The number of M \(\ge\)3.0 earthquakes dropped from the 2015 peak to 29 events recorded in 2021. The state now monitors activity using a robust seismic network and pressure monitoring wells in the Arbuckle formation, allowing for a more targeted response. While activity has decreased significantly, the ongoing presence of deep wastewater injection means that the potential for induced earthquakes remains a permanent feature of the state’s seismic landscape.