It might come as cold comfort for Oklahomans still shaking after Sunday night’s 5.0 earthquake in Cushing, but the fault line that produced what might be the state’s most devastating quake in history could continue to be active in the near future.
Oklahoma Geological Survey director Jeremy Boak said on Monday that a “pulse of activity” happened south of Cushing in 2014, and that the same fault zone “appears to have moved in 2015.”
The earthquakes from previous years were nowhere near the strength of Sunday night’s — the fifth strongest in recorded state history. Boak said it will take a while to unravel exactly what caused the fault line to ignite again.
“It’s a puzzle right now as to why we’re getting an earthquake in an area like that,” he said.
When Boak describes a “pulse,” he’s talking about a massive increase in wastewater injection that began in 2013 and peaked in 2014. The wastewater, created during horizontal drilling techniques, is pumped back into the Arbuckle Formation, a 7,000 foot deep, sedimentary formation.
Water has been pumped into the Arbuckle since the 1930s, Boak said, but the fact that the recent ramp up of injection occurred just before the number of earthquakes in the state shot up is good evidence there’s a cause and effect. As is, he said, the realization that as injection wells have been plugged back and the amount of wastewater being pumped into the earth has been scaled back, the earthquakes have slowed.
Sept. 3, 2016: 5.8 — Pawnee
Nov. 6, 2011: 5.7 — Prague
April 9, 1952: 5.5 — El Reno
Feb. 13, 2016: 5.1 — Fairview
Nov. 6, 2016: 5.0 — Cushing
Oct. 22, 1882: 4.9 — Bennington
Nov. 8, 2011: 4.8 — Prague
Nov. 5, 2011: 4.8 — Prague
Jan. 6, 2016: 4.8 — Fairview
Nov. 30, 2015: 4.7 — Nash
Source: U.S. Geological Survey
“The proof (the surge in quakes) was caused is because injection ramped up and quakes ramped up,” Boak said. “It’s even stronger proof that dropped as injection dropped.”
When 2016 ends, it will likely have seen about two-thirds as many earthquakes of 3.0 magnitude or greater as last year.
“We had 539 earthquakes of a 3.0 magnitude or greater in 2014, then 903 in 2015,” Boak said. “This year we’re at 558 in November with about two months left. So we’ll be higher than 2014 but well lower than last year.
“It will be interesting to see what the rates look like leaving this year and going into next.”
The good news, Boak said, is that Sunday night’s earthquake, as well as the 5.8 magnitude earthquake in Pawnee in September, don’t seem to represent a shift in strong earthquakes occurring in the eastern part of the state.
Those two quakes (the 5.8 magnitude in Pawnee on Sept. 3 was the strongest in state history) are the furthest east any of the 10-strongest temblors have occurred. But the Cushing quake happened on a fault line that’s already been triggered in the past, and Pawnee has seen many earthquakes in recent years, just not any that strong.
“I haven’t seen any movement like (the pressure) is jumping faults,” Boak said. “It’s strange to have the earthquake rate drop, but find the right faults so that there are these big ones. So while we see a distinct decline in the rate, there’s lot more energy being released.”
Perhaps more disconcerting was the amount of damage caused by the Cushing earthquake. The city announced Tuesday morning that while no serious injuries occurred there, at least 40-50 buildings received substantial damage.
A structural engineer will be checking out the damaged buildings to make sure there’s no future safety hazards from falling debris, Cushing City Manager Steve Spears said during a media conference late Sunday.
Boak said the Cushing quake is unique in that it happened near such a relatively populated area (Pawnee’s population is only about 2,000, while Cushing has a population of more than 7,000.) He said he hopes to see more detailed damage information and estimates from Cushing than have been gathered from past large earthquakes.
“It would be nice to have better damage estimates,” Boak said. “If you live through a 4.0 that causes a crack, and then a bunch of 3.0s accentuate that, it would be nice to know. We don’t know much about the cumulative effect, and that seems to be the crux of the matter.”
The Oklahoma Corporation Commission, which regulates the oil and gas industry and oversees injection wells, announced only that they were “working on an action plan” to modify disposal well operations in the area of Sunday’s earthquake, but that nothing had been finalized. The Cushing oil storage terminal resumed “normal operations” as well, according to the OCC.
The Oklahoma Department of Transportation announced Tuesday that no structural damage had been done to any bridges in the area.
Quake could be world’s largest
William Leith, senior science advisor to the USGS, said pinpointing whether injection wells caused an earthquake and especially which injection wells is tricky.
“It’s very difficult to do the forensics on an individual earthquake …. Most of these cases are made by sort of the weight of evidence argument.”
If scientists are able to confirm that the Pawnee earthquake was triggered by injection wells, as it appears it was, the quake would be the world’s largest caused by fluid injection, Leith said.
To make a conclusion about the Pawnee earthquake’s trigger, USGS plans to review injection well data from OCC. However, Leith said there’s little disagreement anymore about the cause of Oklahoma’s dramatic increase in earthquakes.
“It has come to be accepted that most of the earthquakes in Oklahoma are triggered by fluid injection,” he said.
In an area with high fluid injection, earthquakes can also travel far from the central point.
“We’ve seen earthquakes that appear to be triggered by injections that were more than 10 miles away. … That has been measured.”
Currently, the world’s largest fluid-induced earthquake was Oklahoma’s 2011 quake in Prague. Before that, an earthquake in Colorado held the record.
Leith said the recent decrease in seismicity “has much to do with the oil production being down this year.” He expects when the USGS recalculates its forecast of earthquake hazard for Oklahoma in January, the hazard is expected to be decreased.
Even with the OCC’s measures to reduce injection, the state will probably keep shaking for awhile, he said.
“These earthquakes are going to still occur because over a number of years there have been very large disturbances in the pressure pattern.”