The recent spike of earthquakes in parts of Oklahoma in the US coincides with dramatic increase in the disposal of salty wastewater into the Arbuckle formation - a 7,000-foot-deep, sedimentary formation under Oklahoma - a study says.
The researchers showed that the primary source of the quake-triggering wastewater is not so-called "flow back water" generated after hydraulic fracturing operations.
Rather, the culprit is "produced water" - brackish water that naturally coexists with oil and gas within the Earth. Companies separate produced water from extracted oil and gas and typically re-inject it into deeper disposal wells.
"What we have learned in this study is that the fluid injection responsible for most of the recent quakes in Oklahoma is due to production and subsequent injection of massive amounts of wastewater and is unrelated to hydraulic fracturing," said study leader professor Mark Zoback from Stanford University.
Before 2008, Oklahoma experienced one or two magnitude four earthquakes per decade, but in 2014 alone, the state experienced 24 such seismic events.
"We know that some of the produced water came from wells that were hydraulically fractured, but in the three areas of most seismicity, over 95 percent of the wastewater disposal is produced water, not hydraulic fracturing flowback water."
Some of the quakes occurred months or even years after injection rates peaked and in locations that were sometimes located miles away from any wells.
These discrepancies had previously puzzled scientists and had even been used by some to argue against a link between wastewater disposal and triggered earthquakes, but Zoback offered an explanation.
According to him, wastewater disposal is increasing the pore pressure in the Arbuckle formation, the disposal zone that sits directly above the crystalline basement, the rock layer where earthquake faults lie.
Pore pressure is the pressure of the fluids within the fractures and pore spaces of rocks at depth.
The Earth's crust contains many pre-existing faults, some of which are geologically active today. Shear stress builds up slowly on these faults over the course of geologic time, until it finally overcomes the frictional strength that keeps the two sides of a fault clamped together.
When this happens, the fault slips and energy is released as an earthquake, the researchers said.
Even if companies opt to use producing formations to store wastewater, however, the quakes would not cease immediately.
"They have already injected so much water that the pressure is still spreading throughout the Arbuckle formation. The earthquakes won't stop overnight, but they should subside over time."
The results were published in the journal Science Advances.