Thursday, December 8, 2016

Oklahoma 'almost certain' for another damaging earthquake; risks to be elevated for next decade but on decline

Oklahoma is “almost certain” to have at least one damaging earthquake in the next five years, with heightened risks of a large quake probable to endure for a decade even with declining the frequency, according to an author of research modeling the state’s man-made quakes.
Stanford geophysicist Cornelius Langenbruch points out his and a colleague’s recent study also further solidifies the "clear relation" between wastewater injection volumes and earthquake rates.
For example, the study highlights that the seismic energy released in the past eight years of record saltwater disposal volumes from oil and natural gas production is equivalent to "more than 1,900 years of naturally occurring energy.”
In another finding, the number of quakes outside two large areas of interest that feature the high disposal volumes is "generally consistent" with "tectonic background activity" Oklahoma historically experiences. Combined, the study says, those two areas of interest “contain almost all recent earthquakes.”
The peer-reviewed study published online recently in Science Advances offers welcome news: The state's seismicity should return to its typical background levels with the regulatory cap on injection volumes. But effects from years of putting so much saltwater into deep disposal wells will be felt by Oklahomans for several more years.
Langenbruch, who authored the research paper with fellow Stanford geophysicist Mark Zoback, spoke with the Tulsa World by telephone. He said elevated risks will remain in the earthquake zone in the next five to 10 years for a quake capable of inflicting damage to buildings, not ruling out the possibility for multiple large temblors.
The state's 180-day moving average of magnitude-2.8 or greater quakes peaked at approximately 4.5 per day in summer 2015, tailing off to about 2.3 a day this fall.
"It is very important to note that the earthquake rates are not low, they are really high compared to what has been observed prior to 2009," Langenbruch said. "So it means that the probability of potentially damaging earthquakes in the next five to 10 years is still high compared to what has been observed in the past."
Notably, Langenbruch said the study's parameters allow for the prediction or quantification of earthquake probabilities based on planned injection volumes. Or in other words, the study offers a model to regulate injection levels based on whatever is deemed to be an acceptable level of induced-seismicity risk.
The model predicts a 37 percent chance in 2017 for Oklahoma to experience a quake that exceeds magnitude-5.0. The Pawnee area is at a much higher risk after a state-record 5.8 in September, with a 58 percent probability to exceed a 5.0 and 7 percent chance to surpass a 5.8.
Regulatory cap 'should be enough'
Two Oklahoma Corporation Commission directives in the spring imposed a cap on injection volumes in a 15,000-square-mile region of interest at 40 percent below 2014’s output. A combination of the depressed energy market and the regulatory restrictions have achieved that reduction.
The mandated disposal rate in the region of interest after May “remains almost twice as high” as 2009 levels, which the study pegged as the start of the sequence of induced seismicity. But because those volumes now are spread throughout a region twice as large as in 2009, “the average injection volume per unit area” is similar to that of 2009.
“On a large scale, we think this 40 percent reduction should be enough,” Langenbruch said. “But on a local scale, things might look different.”
He explained that the pressure increase in the underground from years of disposing vast saltwater volumes into deep wells is slowing but remains high. On a local level, fluid injection especially close to a critically stressed and optimally aligned fault might present a greater risk than the quake zone as a whole.
For that reason, Langenbruch said, it’s key to gather better data on underground pressures to begin effectively addressing seismicity from a localized standpoint — perhaps even by individual well. The state’s crystalline basement is laced with pre-existing faults poised to slip when pressures generated by the wastewater touch them. The majority are small but some are quite large, he said, and many are unknown or haven’t been mapped.
“Maybe these faults can be mapped, but right now we’re not sure,” Langenbruch said. “These faults could be everywhere.”
A prime example is demonstrated by the record Pawnee quake on Sept. 3. The magnitude-5.8’s aftershocks revealed an unknown fault and prompted amended disposal well restrictions in the area.
Langenbruch said he believes the pressure is still spreading outward. However, the pressure far away from the disposal wells is “very low,” meaning it’s not as likely to trigger fault slips, he said.
“I don’t think (the earthquake zone) will get significantly bigger unless there are other places in Oklahoma where the injection rates are now significantly increased,” Langenbruch said.
The study notes that before 2009 the state averaged about one magnitude-3.0 or greater quake each year. In 2015 there were about 900 of those size quakes — by far Oklahoma's record.
The authors offered a caveat that forecasting the earthquake hazard is difficult, and their study isn't a "final and reliable seismic hazard model" for the state. But the model yields promise as a scientifically valid tool to manage man-made quakes.
“The good thing is that I think finally everybody understands that seismic hazards have to be mitigated in Oklahoma, and everyone is working together,” Langenbruch said.
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