The swarm of earthquakes that rattled Edmond this week reveal a previously unknown fault line and could signal additional shaking in the area, according to seismologist Daniel McNamara.
The research geophysicist at the U.S. Geological Survey’s Geologic Hazards Science Center in Golden, Colo., has been studying the state’s ongoing earthquake swarm.
Researchers have used oil and natural gas industry data and other maps to identify previously undocumented fault lines. But sometimes faults are revealed when a series of earthquakes fire off with epicenters in a linear pattern.
“These are all ancient faults that have not been active since the days of the dinosaurs,” McNamara said. “They’re being reactivated at depth. They don’t even come to the surface. They’re all 5 to 10 kilometers down.”
This week’s quakes indicate a fault that runs northeast at least two miles from near Interstate 35 and Second Street, McNamara said. Because the newly revealed fault is not on any existing fault map, it’s difficult to tell how far it extends or whether it underlies more of Edmond or connects to a larger fault, he said.
“The fault may continue on, but we don’t know,” he said.
Another concern is whether the fault connects to a larger, deeper fault, such as the Nemaha Ridge — which runs roughly parallel to I-35 from central Oklahoma to southern Kansas — or the Wizetta Fault, which produced the magnitude 5.6 Prague earthquake in 2011.
“We’re concerned these smaller faults could be connected at depth to the longer structures,” he said. “Once you get to those larger faults, you could produce larger quakes.”
This week’s quake was the 29th magnitude 4.0 or greater experienced in the state in 2015, according to the Oklahoma Geological Survey. So far this year, more than a dozen faults have produced multiple 4.0 or greater earthquakes.
McNamara said this week’s activity near Edmond fits with the induced earthquakes the state has experienced over the past few years. While Oklahoma has naturally occurring earthquakes, the rate of magnitude-4.0 or greater quakes has skyrocketed from one every decade or so to 29 this year.
Researchers and regulators have attributed the ongoing earthquake swarm to saltwater disposal wells, which pump underground millions of gallons of ancient water produced along with oil and natural gas. New regulations over the past year have focused on disposal wells that were drilled too deep and that dispose at high volumes and pressures.
The efforts are working, McNamara said. He pointed to restrictions on disposal wells near the oil hub at Cushing in October 2014 and again this fall.
“There were two magnitude 4s on a fault that went right under the storage tanks. They (The Oklahoma Corporation Commission) limited injections, and we could see a significant die off of seismicity within a week or so,” McNamara said.
Edmond is different from some of the state’s recent 4.0 or greater quakes in that there are no high-volume disposal wells or large horizontal producing wells nearby, although there are several older wastewater disposal wells in the area.
This week’s quakes were centered just outside the area the Corporation Commission identified in August as the “Logan County Trend.” At the time, regulators asked operators of 23 disposal wells in northern Oklahoma, Logan, Lincoln and Payne counties to cut volumes back to levels below the area’s total from 2012.
Original Article:http://newsok.com/article/5470002
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