Recent rumblings in two cities likely are aftershocks, not signals of stronger earthquakes to come, said state and federal scientists.
However, researchers and regulators said they keep a close eye on Cushing and the surrounding area, in part because the nation’s largest crude storage hub is there. They’re also concerned about potential cumulative damage to structures from previous quakes.
A magnitude 5 quake struck Cushing in November, and there have been 27 aftershocks since then, according to the Oklahoma Geological Survey. Six of those aftershocks were magnitude 3 or stronger, three of which have happened in the last two weeks. A magnitude 4.2 hit Stroud July 14, about 20 miles south of Cushing. Six aftershocks have hit Stroud since then, including a magnitude 3.9 recorded Saturday.
State seismologist and OGS researcher Jake Walter said scientists home in any time there are clusters or swarms of earthquakes.
“There is a heightened awareness when we observe this, especially in sensitive areas (like Cushing) and in areas where we’ve seen seismicity in the past,” Walter said.
Both recent swarms are most likely aftershocks because those quakes were not as strong as the main shock, he said.
Aftershocks generally subside to the baseline rate, but how soon that happens isn’t cut and dry, he said. Unlike natural earthquakes observed in California, Oklahoma’s aftershocks continue for a longer time. In Cushing, the aftershocks haven’t been very strong, relative to what’s expected for natural quakes.
There were nearly no earthquakes near Cushing and Stroud in 2012 and 2013. Stronger quakes hit Cushing in 2014 and in 2015.
Oklahoma’s seismic activity is interesting in part because, in that area, there appears to be a long lead time between the smaller-magnitude temblors, or foreshocks, that preceded the magnitude 5 last fall, Walter said. If OGS researchers could examine smaller-scale seismic swarms in greater detail, they could develop short-term forecasts for potentially larger quakes to come.
“Most of our effort is spent producing the (earthquake database) catalog,” Walter said. “We’re limited in our staffing.”
OGS scientists have asked for more funding to hire postdoctoral researchers to dedicate their time to studying and producing short-term forecasts.
Many of Oklahoma’s earthquakes have been linked to wastewater disposal from oil and gas activity. But proving causation is difficult, in part because the state has so many wastewater wells and so many temblors. Proving causation is also difficult because there’s a lag between injection time and measured earthquakes.
Oil and gas regulators have placed restrictions on disposal wells, which researchers said likely contributed to the declining earthquake rate statewide in 2016 and in 2017. Reduced oil and gas production also likely contributed.
U.S. Geological Survey research geophysicist Daniel McNamara agreed with Walter’s analysis. The aftershocks have been on the same fault as last year’s magnitude 5.0. McNamara has done research on Cushing’s medium-magnitude quakes in 2014 and in 2015.
“Those (recent quakes) are probably aftershocks,” McNamara said. “But it is hard to know since Oklahoma quakes behave differently than regular, tectonic quakes that we see in places like California.”
Many of the larger Oklahoma quakes have had foreshocks.
Corporation Commission spokesman Matt Skinner said there isn’t much more regulatory staff members can do to further mitigate earthquakes near Cushing, particularly if the recent temblors are aftershocks. The Stroud swarm is still under investigation, he said. Skinner said field staff members reviewed nearby wastewater disposal well operations to ensure companies are following the rules.
“We’re never going to be turning loose of it, but there are no immediate further action plans,” he said.
Walter said it’s unclear what more he can recommend to regulators. Reduced injection appears to have helped.
“That is the only clear evidence that has been working to reduce the frequency of smaller quakes,” he said.