Oklahoma has been the site of thousands of earthquakes associated with the deep injection of wastewater from hydraulic fracturing or more commonly fracking. This induced seismicity, has been puzzling to seismologists because most of the earthquakes have not occurred on known faults, making seismic hazards difficult to estimate, predict or prevent by choosing a "safer" geology. Now, in a recently published study the U.S. Geological Survey and the Oklahoma Geological Survey believe that they have identified a potential cause of these earthquakes.
In hydraulic fracking on average 2.5-5 million gallons of chemicals and water is pumped into the shale formation at 9,000 pounds per square inch and literally cracks the shale or breaks open existing cracks and allows the trapped natural gas to flow. While geologists and engineers believe that there is little risk that the fracking itself will cause an earthquake in areas not associated with known faults, now concern is focused in on the disposal of the flowback water that has be found to induce earthquakes in ways we do not yet fully understand.
Airborne magnetic surveys were conducted in Oklahoma from August 11th, 2017-October 28th, 2017. The U.S. Geological Survey (USGS) and the Oklahoma Geological Survey (OGS) used the airborne magnetic data to image rocks where the earthquakes are occurring miles beneath the surface. The magnetic maps created reveal boundaries or contacts between different rock types, some of which are linear, similar to faults. A number of these types of contacts, are aligned with sequences of earthquakes. This suggests that some of them represent ancient faults that have been reactivated due to wastewater injection, which generates, or “induces” earthquakes.
According to USGS scientist Anji Shah, lead author for the study, the data show that there is a dominant “grain” direction to the magnetic contacts (like wood grain) in the deep rocks where the earthquakes are occurring. This “grain” was formed hundreds of millions of years ago and may be composed in part by faults that are oriented favorably to move in response to natural background stresses within the earth. This alignment of deep features may contribute to the high levels of seismicity occurring in response to the fracking wastewater injection.
According to Dr. Jeremy Boak of the OGS the scientists are hoping to be able to use this data to ultimately find answers to “some of the mysteries of induced seismicity in Oklahoma,” They are hoping to be able “ ... to bring these data to bear on addressing the persistent seismic activity and sharing our interpretations with Oklahomans and other stakeholders regarding this challenging issue.”
Many of the possible deep faults highlighted by the magnetic data are different from those on previous fault maps. According to the USGS this discrepancy is probably because the previous maps reflect relatively young faults in the shallow rocks, whereas the magnetic data image the deeper, older rocks. The USGS attributes the differences in the fault directions between these rock types to the different histories of ancient tectonic and magmatic events that shaped the rocks.
The citing for the research is:
Shah, A.K., and Finn, C.A., 2018, Airborne Magnetic Surveys over Oklahoma, 2017: U.S. Geological Survey data release, https://doi.org/10.5066/F7ZG6RJP
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