Geologists report new findings about Kansas, Oklahoma earthquakes

KU notes that the study was prompted by a startling increase in earthquakes in what had previously been the seismically quiet midcontinent. In the more than three decades between 1977 and 2012, only 15 earthquakes with a magnitude of 3.0 or greater were recorded in the entire state of Kansas. A magnitude 3.0 earthquake is typically felt by humans. Since 2012 more than 100 earthquakes of 3.0 or greater have been recorded in only two counties in the state, Sumner and Harper. These include the largest earthquake ever monitored in Kansas in November 2014, a magnitude 4.9 event near the Sumner County town of Milan. The frequency of earthquakes has continued to increase. Between May 2015 and July 2017, the KU array of sensors detected more than 2,400 earthquakes in Sumner County alone, ranging in magnitude from 0.4 to 3.6.

A number of researchers have already linked the increasing occurrence of earthquakes in the area with human activity, specifically an oil boom that has produced ever increasing amounts of wastewater. Every oil well produces wastewater. This is true of conventional wells and of wells that employ hydraulic fracturing (known popularly as fracking). The oil boom led to more oil wells being drilled in the area, which led to a sharp increase in the volume of wastewater. Researchers now believe the increased injection of wastewater into the salty aquifer in the subsurface, the Arbuckle, caused the increase in earthquakes.

Because conditions in the subsurface vary and a perfect storm of problems must be present for a wastewater injection well to induce an earthquake, only a relatively small fraction of injection wells cause tremors. This makes it difficult for regulators and the energy industry to determine where they can place wells.

Like sponges made of rock, aquifers store fluid in their pores. Injected wastewater increases the pressure of fluid in the aquifer’s pores and in fractures (cracks) in the rock. An earthquake is triggered when the fluid pushing against the rock affects an existing fault that is already close to slipping. If researchers can detect regions of elevated fluid pressure in the aquifer’s pores, they might be able to predict where induced earthquakes would likely occur.  

The geologists tracked the increase in pore pressure by noticing a difference in the way seismic waves from recent earthquakes, presumably injection-induced, act compared with the way waves from older, naturally occurring earthquakes act. The group studied what geologists call shear-waves, or S-waves, looking closely at their anisotropy, a phenomenon where waves split in two with one component of the wave traveling along the fractures in the rock and the second component traveling perpendicular or nearly perpendicular to the orientation of the fracture.

In the naturally occurring earthquakes that do not involve high pressure, the wave component traveling along the fractures moves faster than the wave component moving perpendicularly to the fractures. In the induced earthquakes in Kansas, the geologists found the opposite.

“Such changes, or ‘flips’, in fast S-wave orientation had been previously documented in natural earthquakes and volcanic settings where there exist zones critically stressed by pore fluid pressure,” Tsoflias said. “Our observation of S-wave flips in recent southern Kansas earthquakes provides for the first time evidence of increasing pore pressure in the region from seismological data.”

— Read more in Keith A. Nolte et al., “Shear-wave anisotropy reveals pore fluid pressure–induced seismicity in the U.S. midcontinent,” Science Advances 3, no. 12 (13 Dec 2017): e1700443 (DOI: 10.1126/sciadv.1700443)