A recent U.S. Geological Survey study “strongly suggests” the magnitude-5.8 earthquake last September near Pawnee may have been triggered by short-term variations in well injection volumes, triggering a fault primed to slip after years of saltwater disposal loaded it to the point of failure.
The study, published last week in Seismological Research Letters, focused on two types of deep injection wells in Osage County near the epicenter — ones with relatively steady volumes and others whose injection rates drastically changed in recent years. Both types had similar overall volumes. The model simulations of the variable-rate injection wells “exhibits remarkable agreement” with the Pawnee area’s seismicity.
Research geophysicist Andrew Barbour, lead author of the study, explained in a telephone interview that the research is “highly suggestive” that changing injection rates can have a “profound effect” on quakes occurring.
“It’s not that the variable rate injectors are the predominant source of the seismicity; their effect is more pronounced in terms of fault destabilizing effects,” Barbour said.
Scientists pin the cause of Oklahoma’s induced seismicity on the vast volume of saltwater from oil and gas production injected into the state’s deepest geologic formation — the Arbuckle — in a 15,000-square-mile region in central and northwestern Oklahoma.
Barbour described the short-term variations in disposal volumes as a “pulse or kick to the system” that likely acted as a tipping point to Oklahoma’s record 5.8 quake.
Oklahoma experienced a notable version of a similar situation playing out in January 2016. About 30 earthquakes of at least magnitude-2.5 struck near Fairview in a 24-hour period, including a 4.7 — tied for ninth-largest in state history.
State regulators wrote a letter to well operators stating that the seismicity appeared to have a “very close correlation” to storms in the area that led to widespread power outages. Regulators noted that many disposal wells simultaneously turned back on when power was restored, creating a scenario in which a “tremendous volume” of saltwater went underground at once.
Barbour said the USGS study of the Pawnee quake is indicative that details are critically important in trying to assess causation and mitigation as scientists obtain a more nuanced view of induced seismicity.
“It certainly does point to the situation being a little more complex than just reducing volumes, for an example,” Barbour said.
The study also is notable because data on injection volumes in Osage County hasn’t been as well publicized. Osage County is regulated by the EPA, whereas the rest of the state’s counties are regulated by the Oklahoma Corporation Commission.
Seismological Research Letters also published another study researching the Pawnee quake. Jake Walter, Oklahoma’s state seismologist, is the lead author of it.
The study notes that there is nothing to distinguish the aftershock rate from that of a natural earthquake’s rate except for a lack of large aftershocks.
Walter said the expectations based on scaling would indicate about 10 quakes of 4.0 or larger. However, there has been none that size.
“We don’t necessarily have an answer,” Walter said.
Aftershocks are still occurring, with 20 that have been at least magnitude-3.0, he said.
Walter said the concentration of quakes in that area “has tapered off significantly” and isn’t the most seismically active part of the state. However, he cautioned, the seismic hazard remains elevated there and in other areas of the state.
“It’s still across a broad reach of north-central Oklahoma,” Walter said.