USGS Upgrades Pawnee County Earthquake To 5.8 Magnitude
by Matthew Nuttle, Sept 7, 2016, NEWS9
The United States Geological Survey (USGS) upgraded the magnitude of Saturday’s earthquake in Pawnee County from a 5.6 to a 5.8 on the Richter Scale.
The earthquake struck at 7:02 a.m. on Sep. 3, eight miles northwest of the town of Pawnee at a depth of about four miles.
The USGS initially rated the quake as a 5.6, but now say, after further analysis of seismic recordings. The quake damaged buildings in Pawnee and on the campus of Oklahoma State University in Stillwater. Some buildings in SE OKC also sustained damage.
In addition to upgrading the Pawnee earthquake, the USGS also retroactively upgraded the November 2011 Prague earthquake from a 5.6 to a 5.7.
Pawnee earthquake upgraded to magnitude 5.8 by Adam Wilmoth, September 7, 2016, The Oklahoman
Earthquake researchers on Wednesday upgraded Saturday’s earthquake near Pawnee to the strongest in recorded state history and warned of a potential for more strong tremors in the future.
The U.S. Geological Survey raised the earthquake near Pawnee to a magnitude 5.8, up from the preliminary listing of 5.6. The upgrade means the quake released about twice as much energy as previously believed.
“The magnitude revision is based on further in-depth analysis of seismic recordings,” the geological survey said in a statement on Wednesday. “Changes in estimated magnitude for an earthquake are common in the hours-to-days following the event, as more data are analyzed in greater detail than is possible in the first minutes after the earthquake occurs.”
Also on Wednesday, the geological survey upgraded the official magnitude of the Nov. 6, 2011, earthquake near Prague to magnitude 5.7, up from 5.6 previously.
More to come?
While researchers increased the ratings for the state’s strongest quakes, they also warned that more such tremors are likely over the next several years.
“I could envision an earthquake bigger than the one we experienced, but the kind of earthquake they had in New Madrid, I think that’s relatively unlikely scenario here,” Oklahoma Geological Survey Director Jeremy Boak said, referring to the magnitude 7.5 quake that struck in southeast Missouri in 1811. “Potentially we could have one as large as magnitude 6,” Boak said during a technical conference in Norman on Wednesday. “There is some probability of that, but it’s relatively small based on our estimates.”
Oklahoma researchers and regulators have focused their quake reduction efforts on saltwater disposal volumes in the Arbuckle formation, which in much of Oklahoma is the deepest sedimentary rock layer and is the oil industry’s favorite layer for produced water disposal.
Before Saturday, the Corporation Commission had directed operators to shut in 90 wastewater disposal wells and reduced disposal volumes on an additional 250 wells. The most recent orders shut in another 54 disposal wells near Pawnee.
The wells dispose of the large amounts of saltwater produced along with oil and natural gas. Sometimes called fossil water, the produced water is believed to be remnants of ancient oceans, containing many times the salt content of seawater, along with other chemicals and components.
Corporation Commission regulations over the past year have reduced the state’s disposal volume by about 1 million barrels a day, or more than 40 percent of the volume pumped into the Arbuckle formation in the earthquake-prone areas last year. While the volume reduction is helpful, it will not immediately solve the problem of increased pressure caused by large disposal volumes in recent years, Boak said.
“We’ve come down 40 percent in the number of magnitude 2.8 or larger earthquakes, but we’ve still had this large earthquake,” he said. “The injection pulse that’s out there will continue to influence the crust. That’s already in motion.”
Oklahoma had 579 magnitude 3.0 or greater quakes in 2014 and 907 last year. So far this year, the state has had 472, well less than the more than 600 at this point one year ago.
“As we move across the crest, you still see relatively high probabilities of earthquakes of magnitude 4.5 or even 5.0,” Boak said. “But we’ve seen hints of some decline of probability.”
U.S. Geological Survey research geophysicist Daniel McNamara agreed that strong earthquakes are unlikely to stop immediately. In the 1960s in Colorado, the strongest earthquake in an area of induced seismicity occurred a year and a half after injection operations stopped, he said.
“Even if everything were to shut down today, there’d still be a lot of energy persistent,” McNamara said. “Earthquakes will continue for some time. How long we don’t know. But they should taper off in magnitude and frequency over time.”
On the edge
While regulators and researchers have focused on disposal wells, it’s difficult to tie the Pawnee earthquake to nearby disposal activity, Boak said.
“There’s no obvious choice for what caused this thing,” he said. “To me, it looks like it’s part of the broader picture of seismicity.”
Ten Oklahoma counties had more water disposal than Pawnee County last year, and the area had about one-sixth of the disposal volume of Alfalfa County.
Boak also pointed out that the biggest earthquakes of the past five years — near Prague, Fairview and Pawnee — are along the outer edges of the area of interest researchers and regulators have focused on.
“There may be some geological reason for that,” he said. “I’m not clear on that, but it’s a fascinating relationship.”
The distribution also is somewhat unexpected, Boak said.
“That it was as far out on the fringe took me a bit by surprise,” Boak said. “The idea that we might have another earthquake the size of Prague or even larger was something I hoped wouldn’t happen, but I knew there was some probability out there.”
The Pawnee earthquake also surprised researchers by occurring in the wrong direction.
“The orientation of the fault is not optimal for large earthquakes,” McNamara said. “It tends to be that the strongest earthquakes like Fairview and Prague are on faults oriented from northeast to southwest. Pawnee is more east-west, which is not where we would expect it.”
The quake might force researchers to change their models and could indicate that more stronger earthquakes are likely in Oklahoma.
“That means more faults can rupture,” McNamara said. “That would mean a lot of faults running east-west and maybe north-south that we didn’t think would rupture could produce a large earthquake.”
Industry reaction
While researchers praised the Oklahoma Corporation Commission’s decision to shut down wells near Pawnee, some in the oil and natural gas industry are not as pleased.
“I thought the latest directive was possibly a little bit aggressive, but I certainly understand that given the situation they wanted to err on the side of caution rather than doing too little and regretting it later,” said Kim Hatfield, chairman of the Oklahoma Independent Petroleum Association’s induced seismicity working group.
Saturday’s order to shut in 37 disposal wells in Pawnee, Payne and Noble counties affects a total of 60,000 barrels of produced water per day. The order likely also shut in about 6,000 barrels of oil production a day, Hatfield said.
“That’s about $300,000 a day that’s not going into the state’s economy,” Hatfield said of the oil production. “They’ve gone out 10 miles from a fault line. There’s no research I’m aware of that indicates wells that far away would have that impact.”
Hatfield said the industry generally has supported the Corporation Commission’s actions.
“I think that in general, given the scope of the problem, they have taken a pretty measured approach,” he said. “Part of their problem is they don’t have a lot of tools. There isn’t science to know whether this well is a problem or that one is not.”
Oil companies are working with Oklahoma Geological Survey researchers to gain a better understanding of the seismic dynamic underlying the state.
“If your only tool is a hammer, then every problem is a nail,” Hatfield said. “We want to give them a full toolbox to work in a measured way.” [Emphasis added]
Pawnee Residents Continue To Recover After Earthquake
by Tiffany Liou, September 7, 2016 NEWS 9
The USGS upgraded Saturday’s earthquake in Pawnee County from a 5.6 magnitude to 5.8.
For those in Pawnee, they are still concerned about aftershocks that could be coming.
City leaders said they are preparing for possibly big aftershocks.
Pawnee police Chief Wesley Clymer said the city, county and state emergency management coordinators have put together a good plan for mutual aid, a lot of which was implemented on Saturday. [Who’s paying for all this? The impacted communities?]
“From an emergency personnel standpoint, we got a plan in place not only for rescue, but for other agencies to come in and assist with that. God forbid, that it’s worse,” Clymer said.
There have been so many aftershocks already. He said a 3.5-magnitude quake, like residents felt on Tuesday, is now normal. While it’s a lot to think about, city leaders are preparing for the worst.
Besides mutual aid coordination, the City is also making sure areas that could be a hazard are taped off. Some of the buildings that were damaged on Saturday are still unstable.
“If we get another quake, the chances of it coming down are pretty good,” Clymer said.
He said until they get it fixed, it will remain marked off.
A USGS research geophysicist told News 9 on Monday that a large magnitude aftershock is very possible based on the patterns of big earthquakes in Oklahoma’s recent history.
USGS crews have been out in Pawnee County installing seismometers.
Todd Halihan, a geology professor at Oklahoma State University, confirmed 15 to 20 normal seismometers are being deployed by USGS, OGS, Cornell University, and OSU.
Halihan said an additional 400 nodal arrays are being installed by Cornell. Nodal arrays, in layman’s terms, can be described as short-term mini seismometers.
Clymer is leaving the studies and research of earthquakes up to the experts. He said his focus is to protect the residents of Pawnee in case another large earthquake strikes. [Emphasis added]
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