The largest recorded earthquake in Kansas history likely came from waste water injected into the ground by just one or two nearby wells, according to a new study by scientists at the U.S. Geological Survey.
One of those two wells, operated by SandRidge Energy, is still injecting water at the same level as when the earthquake occurred two years ago.
The 4.9 magnitude quake struck about 40 miles southwest of Wichita in November of 2014. It is the largest recorded earthquake in state history, felt across 150,000 square miles of land, with reports as far away as Memphis.
By January of 2014 the amount of waste water injected near the fault had more than tripled. And the report, which will be published in Seismological Research Letters in November, indicates a small earthquake struck nearby in January. But at the time there weren’t enough seismic stations to identify its location precisely.
By July of 2014, the closest injection wells were pumping five times as much waste water into the ground as January, and there were two more small earthquakes. By September the earthquakes had grown in size – one was a 2.9 and one was a 3.4, both large enough to be captured with the existing measuring tools.
If the well is in the right place next to a fault and the fault is oriented the right way, a little change in stress could cause (an earthquake) to occur.
George Choy, U.S. Geological Survey
But it wasn’t until the 4.9 earthquake in November that the USGS realized that the area was more active than they had known. In Milan, the nearest town to the epicenter, the shaking was so intense it broke windows, emptied shelves and damaged the town’s old post office and school house.
“Within hours we had teams going out there setting up temporary stations so we could locate the earthquakes and its aftershocks better,” according to George Choy, one of the USGS geologists who conducted the study.
It’s hard for scientists to predict these earthquakes ahead of time, according to Choy, because there isn’t much historical data on earthquakes smaller than 3.5. So scientists couldn’t tell if the small earthquakes are natural and predictable or are being induced and might lead to an even larger quake.
Plus induced earthquakes in Kansas were so new so the risks weren’t clear: Until 2014 there had been fewer than one earthquake per year but in 2014 there were more than 125 earthquakes with a magnitude of 2.5 or larger.
So even though the study’s authors could look back and see that the smaller earthquakes were connected to the larger earthquake that followed, at the time there was little evidence of what was to come. “Some things you can’t recognize until hindsight,” Choy said.
The USGS increased the number of seismic stations in the area from five to eight within a week of the earthquake striking and the state of Kansas purchased six more. The energy from small aftershocks doesn’t travel as far, so they needed stations nearby to measure their force and location.
Until 2014 there had been fewer than one earthquake per year.
The aftershocks typically occur in adjacent rock that has been dislodged by the main earthquake. So by mapping the aftershocks, the scientists figured out the size, shape and location of the large earthquake. They identified a fault zone of around 5 square miles.
Two other earthquakes of a similar size are believed to have struck Kansas more than 100 years ago but they are estimated based on descriptions of them at the time.
The new report explains why scientists believe the earthquake was caused by waste-water injection:
▪ There had not previously been similar earthquakes in the area.
▪ There were waste-water injection wells nearby.
▪ The earthquake activity started after the amount of water injected in the wells increased.
▪ There’s a piece of earth that could be activated by changes in pressure.
Even as the local oil industry has fallen on hard times, the study shows that the potential for man-made earthquakes exists with even one injection well.
“If the well is in the right place next to a fault and the fault is oriented the right way, a little change in stress could cause (an earthquake) to occur,” Choy said.
Part of the risk is that scientists don’t know where all the previously inactive fault lines are. Both this quake and the 5.8 earthquake that struck Oklahoma last month, occurred on faults that scientists did not previously know existed. It was only after the earthquake caused damage that scientists could do a study to map the fault line.