The number of “felt” earthquakes in seismically active Harper and Sumner counties has decreased since Kansas regulators set limits on wastewater disposal from oil and natural gas drilling, according to preliminary data released Tuesday.
Early findings indicate that as the amount of allowable wastewater disposal was gradually decreased, there was a corresponding reduction in the strength of earthquakes. But seismic monitoring also shows the number of “unfelt” earthquakes — those registering below the magnitude of 2.5 — have increased at the same time.
The finding suggests seismic energy is being released in smaller events that may mitigate the risk of larger, more destructive earthquakes, the Kansas Corporation Commission said in a news release.
More than 200 earthquakes have been recorded in Kansas since Jan. 1, 2013, mostly centered in Sumner and Harper counties. Only five were detected in the previous 10 years. The Kansas Geological Survey said last month that the increase in the two counties was linked to saltwater injection after oil and natural gas drilling.
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Kansas Corporation Commission staff told the Harper County Commission on Tuesday that they are recommending that KCC continue the existing limits on wastewater disposal and extend the observation and monitoring period, the agency said. The staff is also recommending to continue to study seismic activity and its relationship to wastewater disposal but says the early data are encouraging.
The KCC is expected to review those recommendations later this month.
Last March, the agency issued an order setting wastewater disposal limits in Harper and Sumner counties. The order also gradually reduced the amount of wastewater disposal allowed at individual wells in five areas with the largest clustering of seismic activity and required operators there to verify the vertical depth of disposal.
The agency said that 120 days after the order became effective, there were 87 fewer earthquakes compared with the same time frame before issuance of the order. As of Sept. 2 — 157 days after the order — there were 22 earthquakes of magnitude 3.0 or larger.
Although the strength of earthquakes in Kansas is trending downward, the agency cautioned that the results are not conclusive. It noted the data did not include such things as crude oil production levels, reduced drilling activities and other factors tied to the low price of crude oil.
From Jan. 1 to Sept. 2 of this year, drilling in the state declined to 1,168 wells, compared with 3,797 wells for the same period in 2014, the KCC said.
Gov. Sam Brownback last year appointed a task force led by the Kansas Geological Survey to study the issue. The task force called for enhanced seismic monitoring and introduced a scoring system for earthquakes that defined “felt” earthquakes at magnitude 2.5 or above.
Among the latest Kansas quakes is one that struck Saturday night near the small Sumner County town of Caldwell. It had a preliminary magnitude of 2.5.