Wastewater did not significantly change the trend of seismic stress in southern Kansas

Although sewage disposal has been the primary driving force behind increased seismic activity in southern Kansas since 2013, a new study concludes that disposal did not significantly alter the direction of stress in the Earth’s crust in the region.

Activities such as sewage disposal can alter pore pressure, shape, and size within rock layers, in ways that cause nearby faults to fail during an earthquake. These effects are believed to be the cause of the most recent induced earthquakes in the central and eastern United States.

However, human activity could also trigger earthquakes by changing the direction of stresses acting on faults in the region, said USGS seismologist Robert Scomall, who co-authored the study in Seismology Research Letters with the US Geological Survey seismologist. USGS Elizabeth. Cochran.

“Since we see no evidence of significant pressure alternation ضغط [in the region]We believe that most earthquakes in southern Kansas are caused by changes in pore pressures or pore effects and not by stress circulation.”

One way researchers can learn more about the direction of the pressure field in rock layers as fluid fills fractures in the rock is through a seismic wave effect called shear wave splitting. Some shear waves traveling through the rock move parallel to the open fractures, and therefore are faster, while others move perpendicular to the fractures and have a lower speed. Estimating the direction of the fast waves can help determine the direction of the pressure field.

A previous study of shear wave splitting in southern Kansas estimated a 90-degree rotation in the fast direction starting in 2015, which the study authors attributed to higher pore fluid pressure from sewage disposal. However, the rotation has coincided with a change in the stations used to monitor the shear waves.

Scomal and Cochran decided to take another look at stress changes in southern Kansas as part of a larger effort to describe stress in reservoir rocks without expensive well drilling. When they analyzed shear wave splitting using high-quality data collected from a stable local seismic network, they found that the regional stress trend remained relatively constant between 2014 and 2017.

Scomal noted that the geological conditions of a wastewater reservoir may influence whether the injection can change the direction of pressure. Most of the wastewater injected in southern Kansas went into layers of rock called the Arbuckle group, “which is of low pressure — the liquid can be ‘poured’ without the need for a pump,” Scomal said, noting that pore pressures can spread rapidly into highly permeable rocks.

The authors note that there are no reports of significant pressure alternations due to wastewater disposal, suggesting that they may not be common, or that stress cycles are smaller than can be detected with current methods, or that this phenomenon has not been sufficiently studied. Until recently, seismic devices were in short supply in many places that have seen a significant increase in sewage disposal over the past decade.

“Documenting stress trends is really challenging in these regions, and describing changes in those stresses over time is even more challenging,” said Scomal. “In the areas we investigated, we did not see convincing evidence of significant stress alternation due to wastewater disposal.”

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