In the wake of a major, record-tying 5.6 magnitude Oklahoma earthquake that struck last Saturday morning, Governor Mary Fallin did not wait around to hear any more evidence about the link between seismic activity and the practice of disposing oil and gas wastewater into wells. She just went right ahead and ordered all disposal wells to shut down in Pawnee County, within a 725-square mile area in the Arbuckle shale formation.
The order was included in Fallin’s state of emergency declaration. She also committed state agencies to work with EPA to develop a course of action for disposal wells in an additional area of 211 square miles in Osage County, over which EPA has jurisdiction.
Oklahoma Earthquake, By The Numbers
The US Geological Survey provides a rundown of the latest temblor to strike Oklahoma:
The September 3, 2016 M 5.6 Oklahoma earthquake occurred as the result of shallow strike-slip faulting about 15 km northwest of the town of Pawnee. The earthquake occurred within the interior of the North America plate, far from any plate boundaries.
That’s about all the agency can definitively determine, for now. The specific location is still up in the air:
The preliminary focal mechanism solution for the earthquake indicates rupture occurred on either a left-lateral fault striking east-southeast, or on a right lateral fault striking north-northeast.
USGS also notes that correlating the Oklahoma earthquake to a specific fault will have to await further study.
The agency also draws attention to several smaller-magnitude earthquakes that occurred before and after the main one:
In the immediate vicinity of the September 3 event, a M 3.2 earthquake occurred on September 1, 2016, just to the southwest. Within an hour of the September 3 M 5.6 earthquake, 4 aftershocks have been located, the largest being a M3.6 event 56 minutes after the mainshock.
Fracking Earthquakes Really Are A Thing, Sort Of
If your impulse is to link the latest Oklahoma earthquake to natural gas fracking, you’re only part right. Fracking — short for hydrofracturing, an oil and gas drilling method — has been definitively linked to seismic activity in one instance, a series of temblors that struck Ohio in 2014.
However, a much more pervasive linkage has been demonstrated for the contribution that fracking makes to the wastewater stream resulting from all oil and gas operations.
That includes wastewater from conventional drilling as well as fracking. It’s also worth noting that there is a lot of regional variation in the proportion of used fracking fluid that enters the oil and gas wastewater stream.
For example, most of the wastewater currently disposed in Oklahoma wells consists of saltwater, which comes up during oil operations. Spent fracking fluid accounts for less than 10 percent of the total.
So, as the latest Oklahoma earthquake demonstrates, the problem of induced seismicity is a broad issue for the oil and gas industry overall, not just for natural gas fracking.
Oklahoma had a relatively quiet seismic history, right up until the recent boom in oil and gas operations. Last year, the Oklahoma Geological Survey issued a report that definitively nailed oil and gas wastewater disposal for an increase in the rate of earthquakes, from 1.5 yearly in 2008 to a current rate of 2.5 daily — yes, daily.
Last year, seismologists at Stanford University also published a study linking the disposal well problem to peculiarities of the Arbuckle formation. The study offered this explanation:
…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.
Here’s a visual interpretation of the linkage:
Oklahoma Earthquake Response: Too Little, Too Late?
The latest Oklahoma earthquake caps off a busy year of regulatory activity for the state’s Corporation Commission, but it may be a case of locking that barn door at the wrong time.
Just last March, the agency issued a new regional plan aimed at reducing the amount of wastewater disposed in the Arbuckle formation. The final amount was to be phased in over a two-month period in order to avoid a sudden change in pressure.
The March plan represented an acknowledgement that the problem is regional in scope. Earlier attempts at solving the earthquake problem were targeted more narrowly.
Last Saturday’s emergency order steps up the state’s game considerably, by shutting everything down within the perimeter affected by the quake. The order only affects 37 wells, but that could just be the beginning.
The Arbuckle formation currently hosts hundreds of disposal wells, and many of them are located in “areas of interest” established by the Corporation Commission.
Back in March 2015, the Commission issued a directive covering 300 such disposal wells. The directive enabled the wells to keep operating if they reduced their volume.
Apparently that wasn’t enough. In July 2015, the Commission expanded the areas of interest to add another 211 wells. The Commission also tacked on an additional requirement governing well depth, with this observation:
There is broad agreement among seismologists that disposal below the Arbuckle poses a potential risk of causing earthquakes, as it puts the well in communication with the “basement” rock.
Beginning last fall, the Commission launched a series of directives targeted at specific wells, but it looks like the state still has an induced seismicity problem.
Coincidentally (or not), this year marks the first time that the US Geological Survey has included human activity in its annual seismic hazards forecast, partly in response to the amount of activity in Oklahoma.
It looks like Governor Fallin and the Corporation Commission will not be pussyfooting around the issue any more, so stay tuned.
Images: top (screenshot) via US Geological Survey, bottom via Stanford University.