In the United States, most earthquakes happen along the West Coast, especially in California, because of major fault lines in Earth's crust. A fault is a crack in the crust where blocks of rock can slip past each other. Oklahoma sits in the middle of the country, far from any major fault line. From the 1970s through 2008, the state averaged about two earthquakes per year of magnitude 3.0 or greater, a measure of an earthquake's size.
Then the numbers began to climb. Oklahoma recorded 20 earthquakes in 2009, 42 in 2010, and 109 in 2013. In 2014 the total jumped to 585. In 2015 it reached 903, more than California recorded that year. Magnitudes ranged from 3.0 to 5.8, and the shaking damaged homes and buildings in towns that had never needed earthquake insurance. The largest, a magnitude 5.8 quake near Pawnee on September 3, 2016, was felt across seven states.
Scientists plotted every earthquake on a map and noticed something. The quakes were not spread evenly across Oklahoma. They clustered near high-volume disposal wells, where oil and gas companies pump leftover wastewater from drilling deep underground.
A side view cutting straight down into the ground beneath Oklahoma. At the top is the flat land surface with a small pump and tank labeled 'Disposal well at the surface.' A narrow pipe runs straight down from the pump through several flat rock layers, labeled 'Well pipe carries wastewater down.' The pipe ends in a thick, deep rock layer with many tiny dots, labeled 'Deep rock with tiny holes that soak up the water.' Blue arrows spread sideways and downward out of the pipe's end, labeled 'Water pressure spreading through the rock.' Below and beside that layer, a slanted crack cuts through darker, harder rock, labeled 'Old fault that had not moved.' A blue arrow points from the spreading water to the crack, labeled 'Pressure reaches the fault.' A short jagged line at the fault is labeled 'Earthquake starts here.' A depth marker on the side shows the fault is much deeper than the well pipe, far below where people live.
Wastewater pumped into deep rock spreads sideways and raises the pressure inside old faults far below the surface.
The timing matched too. When companies injected more wastewater in an area, the number of earthquakes there rose. Researchers concluded the added water raised the fluid pressure inside old faults that had stayed still for a long time. Higher pressure reduced the friction holding the rock in place, and the faults slipped.
Try to push a heavy box across a floor. It barely moves, because friction between the box and the floor holds it back. Now put that same box on an air hockey table. Air blowing up from below lifts the box a tiny bit, so there is less friction, and a small push sends it sliding. Inside a fault, water works like that air. When wastewater is pumped into the rock, the water pressure pushes the two sides of the fault apart just a little. That lowers the friction. The rock was already being squeezed sideways by slow forces in Earth's crust, like a hand that was already pushing on the box. With less friction, the fault suddenly slips, and the ground shakes. Take away the extra water, and the friction comes back.
Extra fluid pressure in a fault acts like the air under an air hockey puck: it lowers friction so the rock can slip.
In 2015 and 2016, the Oklahoma Corporation Commission, the state agency that regulates these wells, ordered companies to cut injection volumes in the affected areas. The earthquake count fell to 623 in 2016, then 304 in 2017, then 81 in 2019. The decline roughly matched the drop in injected wastewater. Scientists are still watching the faults closely.
The faults were quiet before heavy injection, very active during it, and calmed down after the injection volumes were cut.