Oklahoma is well on pace for a fourth straight annual drop in seismicity, despite a May earthquake near Medford with a magnitude 4.5 — tied for the state’s 13th largest ever.
There have been 27 earthquakes of magnitude 3.0 or greater in the first half of 2019. That is 72% fewer than at this time in 2018 (97) and 433% fewer than 2017 (144). But the rate remains substantially higher than the historical activity of two or three a year at 3.0 magnitude before induced seismicity took hold.
The most in a year was 903 in 2015, of which 27 were at least magnitude 4.0.
State Seismologist Jake Walter said Oklahoma, particularly the north-central area, still has a high likelihood for a damaging quake in the next several years based on the observed seismicity.
So Oklahomans shouldn’t be complacent or think seismic activity has returned to more normal levels that have been seen in the past.
“If you take the last five years and compare the number of 3.0s in Oklahoma relative to the number of 3.0s in California, Oklahoma has had more earthquake activity than the state of California,” Walter said.
He said researchers are still unraveling the finer points of how man-made earthquakes work.
For example, there was a recent magnitude 2.7 quake near Cushing. The question becomes: Was it an aftershock from the 5.0 in November 2016, or is it from continued wastewater injection even though the disposal now is in a shallower formation?
State regulators have imposed volume caps on the deepest disposal wells in a 15,000-square-mile area of interest prone to induced seismicity. Some well operators have avoided the restrictions by plugging back wells into shallower formations or moving activity into other areas.
“There’s still a lot to be done to understand induced seismicity, and what we learn will better inform regulators and the public and enhance public safety in general,” Walter said.
A project the Oklahoma Geological Survey is conducting compares aftershock zones in the state.
Walter said large earthquakes near Pawnee, Cushing and Cherokee had fewer aftershocks than Fairview. He explained that the Oklahoma Corporation Commission reacted more strongly and swiftly to the others and not Fairview because the Fairview quakes were much farther from disposal wells.
“There’s still a load of research questions we’re trying to answer because I think some of these answers might assist the OCC in intelligently regulating induced seismicity and protecting the public,” Walter said.
The U.S. Geological Survey the past three years has produced a short-term hazard forecast for induced seismicity that put Oklahoma’s risk in some areas on the same level as the shakiest parts of California.
The federal agency didn’t do a forecast for 2019. A USGS spokesman said the agency is unlikely to do another one in the future.
“The reason is because that induced seismicity has been decreasing every year since we did our first forecast back in 2015-2016, and as such, we’re moving on to different priorities,” said Drew LaPointe, a USGS public affairs specialist. “The long-term forecast we’ll be releasing in the next few weeks for 2019 mentions this decrease in activity as well, but not in great detail.”
Walter said that viewpoint represents a lack of federal interest in long-term monitoring of induced seismicity in the central U.S.
He also called it a significant public relations matter for Oklahoma. Businesses looking to relocate here and insurance companies have contacted Walter to better understand the seismic hazard.
The USGS forecast for 2018 showed a broadening of the area considered to be at an elevated risk.
“So unless it gets updated, that document isn’t representative of the seismic hazard as it exists today,” Walter said.
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