USGS Report Links Old Oklahoma Quakes to Injection, But It’s Not That Simple

This week, researchers with the U.S. Geological Survey (USGS) released a new study suggesting that some of the more significant earthquakes in Oklahoma throughout the 20th century may have been induced by underground wastewater injection wells. But the linkage is correlative at best, relying mostly on a spatial connection between the earthquakes epicenters and nearby wells. Notably, this runs counter to what other scientists have advised when examining potentially-induced earthquakes.

Indeed, regulators and scientists – including those at the USGS itself – have noted on numerous occasions that proximity alone is insufficient to determine whether a seismic event is induced.  Such an assessment should be based on a number of important factors, including downhole pressure, injected volumes, and location, including the orientation of any nearby faults. None of those other factors were considered in the new USGS report.

To be fair, the researchers were evaluating events for which there very little data are available. The epicenters of the larger seismic events in the early to mid-20th century, for example, have not exactly been located with precise measurements. In fact, much of what we do know about the earthquakes that occurred before modern instruments were installed in Oklahoma come from “postcard questionnaires or archival accounts,” according to the study.

As the researchers acknowledge:

“A total of seven independent earthquakes occurred in Oklahoma in the 1950s.  The locations of these earthquakes are quite uncertain […] Considering these large location uncertainties, events located within 35 Km of a well could, in fact, be nearly collocated with the well.  We find that six of the seven earthquakes are located within 35 km of a previously permitted wastewater disposal well.” (emphasis added)

In addition to the “uncertain” nature of where these earthquakes actually occurred, the study also asserts that if an estimated epicenter is within 35 kilometers of an injection well, it is enough to assume that they were induced. In fact, 35 kilometers is a far greater distance than is generally used to identify potentially induced earthquakes.  For instance Southern Methodist University (SMU) researchers looking at the Irving, Texas cluster are evaluating distances of about eight miles.  A recent peer-reviewed study by Stanford researchers on the Timpson, Texas seismic events looks at disposal wells within 10 kilometers.  In other words, the researchers of this new study are using more than twice the distance of previous papers to establish the connection.

With so much uncertainly and lack of data, the report explains,

“Given the limited duration of the time series and the lack of injection data, the observed correlation is suggestive but not conclusive.”

Interestingly, the researchers’ methods actually contradict USGS’ own benchmarks. The USGS has been very clear that proximity alone is not adequate for a scientific evaluation of induced seismicity:

“A combination of many factors is necessary for injection to induce felt earthquakes. These include: the injection rate and total volume injected; the presence of faults that are large enough to produce felt earthquakes; stresses that are large enough to produce earthquakes; and the presence of pathways for the fluid pressure to travel from the injection point to faults.” (emphasis added)

The Environmental Protection Agency (EPA) has come to a similar conclusion, pointing out in a recent report,

“[T]he three key components behind injection-induced seismicity are (1) sufficient pressure buildup from disposal activities, (2) a Fault of Concern, and (3) a pathway allowing the increased pressure to communicate from the disposal well to the fault. All three components must be present to induce seismicity.” (emphasis added)

The necessity of addressing a number of factors is something that the scientists at SMU emphasized in their recent report, which evaluated potentially induced seismic events in Azle, Texas by developing a subsurface model that assesses what actually happens underground, rather than making claims based on whether an injection well was within a few miles of the earthquake.

To their credit, the researchers of this latest USGS study do admit,

“For historical and early instrumental earthquakes, for which precise locations and depth estimates are unavailable, it is difficult to prove an association of events with oil production activities.”  (emphasis added)

As noted above, the researchers from USGS were working with extremely limited data sets. The fact that they could not find a “smoking gun” is unsurprising, given that fact. But that also raises questions as to how such a bold assertion – i.e. “A Century of Induced Earthquakes in Oklahoma?” as the title reads – could possibly be justified.

The researchers claim that the earthquakes were “likely induced,” but that claim is made almost entirely on the fact that an impossible-to-pinpoint earthquake may have been within 20 miles of an injection well. In a state like Oklahoma, where oil and natural gas production activities are currently found in 70 out of its 77 counties, that kind of “close-ology” could be used to suggest practically any earthquake is induced, even though we know that’s not true.

Predictably, the news headlines associated with the report had few caveats or conditional statements. Far more research is certainly needed before we can consider it “accurate” to link earthquakes in the 1950s to injection wells.

1 Comment

Post A Comment