Sunday, January 8, 2017

Seismic hazard remains high for state

While earthquake rates were down in 2016, and a state official anticipates further declines, a U.S. Geological Survey official says the seismic hazard remains very high compared to historical times.
“The sheer number of earthquakes is not the whole story,” USGS Geophysicist George Choy said. “As magnitude increases, the energy released increases exponentially. The energy of the (magnitude) 5.8 Pawnee earthquake, for instance, is equivalent to that of approximately 16,000 magnitude 3 earthquakes. So, in 2016, the energy release exceeded the energy in 2015.”
Despite the relative decrease in earthquakes in 2016, it still is too early to declare that a long-term trend has been established, he said.
“It is of great concern that three magnitude greater than 5.0 earthquakes occurred in this same year (2016),” Choy said. “In the previous century, Oklahoma had only one earthquake of (greater than magnitude) 5.0. Scientists still are trying to evaluate the long-term effects of the presence of the injected water ... and how​ the dynamics of regional stress might have been changed. We have to remember that billions of gallons of water have been newly introduced into the Arbuckle formation.”
Oklahoma Geological Survey Director Jeremy Boak said he feels positive about the decrease in earthquakes.
“In part because it is additional confirmation of the OGS’ conclusion that produced water from oil and gas operations were a primary causative factor, as injection rates have fallen by more than one million barrels per day over the last two and one half years,” he said. “The drop was initially driven by the drop in oil prices, but much of the reduction is now locked in by reductions mandated by the (Oklahoma) Corporation (Commission). This strongly suggests that the reduction in earthquake frequency will continue to decline.”
Boak anticipates further de­clines in the rates.
“The wild card in this will be the question of whether there will be any more large earthquakes. The USGS has pointed out, and we concur, that the size of the earthquake is not related to the amount of injection, but rather reflects the properties of the fault that is triggered by injection, and these cannot be known ahead of time. So, the main way to reduce the probability is to continue to keep injection at its lower level as the earthquake frequency declines,” he said. “The reduction this year was about 280 (magnitude 3.0 or greater) earthquakes, about 30 percent. Another 30 percent reduction would put us around 400-425, whereas a reduction of 280 would put us around 340-350.
“These numbers would be lower than 2014, but still above 2013. I expect something like that amount of reduction, but we and the Corporation Commission will be watching closely to see whether the decline rate slows, stays the same, or increases.”
In 2016, the USGS recorded 639 magnitude 3.0 or greater earthquakes in comparison to 889 in 2015. Choy said totals for 2016 are still preliminary and will likely change over the next few weeks as analysis is finalized.
OGS recorded 623 magnitude 3.0 or greater earthquakes, State Seismologist Jacob Walter said. The count for magnitude 2.5 and greater quakes stands at 2,121, though it is expected to rise before the final count.
Preliminary numbers show there were 15 magnitude 4.0 or greater quakes recorded by OGS in 2016, down from 27 in 2015. There were three magnitude 5.0 or greater earthquakes in 2016, compared to none that strong in 2015.
The USGS recorded a preliminary 21 magnitude 4.0 or greater quakes in 2016, down from 30 in 2015. The 2016 total included the three magnitude 5.0 or greater temblors.
There have been five magnitude 3.0 or greater earthquakes recorded in the state so far this year, USGS records show.
This year, officials at OGS anticipate some results of investigations funded by the secretary of energy, the governor and the industry, Boak said.
“We are optimistic that these will provide better understanding of the relationships among earthquakes, injection and the properties of the Arbuckle Group sedimentary unit which receives most of the injection. Our new seismologist, Jacob Walter, has plans to enhance both our seismic network and the analytical software that helps us catalog these events, so that we can record more of the smaller earthquakes that help us better locate and define the deep faults on which they occur,” Boak said. “And we will continue to seek funding from a wide variety of sources as well as other collaborations to enhance our characterization of the geologic, seismologic and geophysical characteristics of Oklahoma.”
With new oil and gas operations expected to be launched in the South Central Oklahoma Oil Province (SCOOP) and the Sooner Trend Anadarko Basin Canadian and Kingfisher Counties (STACK) plays, the possibility of some earthquakes in a broader area than the existing Area of Interest defined by OCC are anticipated, Boak said.
“These will occur primarily in the operational area of the SCOOP and STACK plays, possibly in other plays where large scale, multi-stage hydraulic fracturing is occurring. These are expected to be smaller earthquakes, relatively closely related to the completion process of a fraction of the new wells. Once a well is completed, we have seen no continuing earthquakes of this type. These events are unlikely to increase overall earthquake rates in the state,” he said.
Choy said any injection into or extraction of fluid from the ground causes stress changes with the potential to cause earthquakes.
“However, the geology in the SCOOP and STACK plays is different from that in central and northern Oklahoma. Until we have additional statistics about the extent of frac(k)ing operations and the proportion of produced water that will come with the gas and oil extraction, it is not feasible to predict the extent of future behavior,” Choy said.
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