Researchers Use Planes to Map Oklahoma Earthquake Faults

Researchers are turning to the sky to help stop earthquakes from rumbling deep beneath the Oklahoma soil.

The U.S. Geological Survey in coordination with the Oklahoma Geological Survey and other groups last week began using a modified small airplane to study the state’s geology in an attempt to map deep, existing faults.

The aircraft over the next two months will fly over portions of the state at an altitude of about 400 feet, using sensitive equipment to measure the naturally occurring magnetic fields created when different rock types are next to each other.

“The goal of this project is to better map the faults that are deep within the ground, and particularity to map the faults that are likely to create an earthquake,” USGS research geophysicist Anji Shah said Aug. 15.

“We have found many of the earthquakes have occurred in sequences that form lines, which suggests there are faults associated with those earthquakes. But when we compare it to our existing fault maps, they don’t seem to line up. This makes it difficult to tell what the earthquake risk is and suggest there are faults we are not aware of.”

The Oklahoman reports that researchers over the past two years have used data from oil and natural gas companies to improve their fault maps, gaining a much better picture of the ancient cracks that crisscross the state.

But most of the industry data covers only the first mile or so beneath the surface. Most of the state’s earthquakes have been much deeper.

“The deep faults are the ones that are moving,” said Jeremy Boak, director of the Oklahoma Geological Survey. “A better understanding of those faults will give us a better idea of how the earthquakes are working.”

The magnetic survey is designed to measure faults up to five miles below the surface.

Earthquake research and regulations have focused on saltwater disposal wells that take high volumes of water produced along with oil and pump them into the Arbuckle formation, which is the deepest sedimentary rock layer underlying much of Oklahoma.

Earthquake activity peaked in mid-2015 and has dropped steadily since then, although three of the largest earthquakes in state history occurred late last year.

“We’ve seen a 50-percent reduction, but that’s still more than the people of Oklahoma are comfortable with,” Boak said. “Every step of the way, we’re trying to get better knowledge and see how much we can localize what we need to do with specific actions.”

While researchers have a strong understanding about part of the problem, they are continuing to study many aspects of how the forces underground have interacted to cause the state’s ongoing earthquake swarm.

“We’re still learning a lot about how injection creates earthquakes,” Shah said. “We’re learning about how far a single injection well can reach. That’s an active area of research and one we would like to know more about and better answer the question.”

A better understanding of the faults and the underground forces will help researchers make better recommendations to regulators at the Oklahoma Corporation Commission, Boak said.

The study will take place over the next six to eight weeks as the modified aircraft flies back and forth in a tight grid pattern over parts of 18 counties. The onboard equipment is passive only, measuring existing magnetic forces on the ground, Shah said.

For the most sensitive view possible, the aircraft was modified to reduce its own magnetic noise by replacing steel bolts with brass and other similar modifications.

The project is a joint effort of different agencies and divisions. The first sweep is looking at southwest Oklahoma, roughly following the Meers Fault. The area is not part of the Oklahoma Corporation Commission’s Area of Interest and has not experienced earthquakes as part of the state’s swarm over the past six years. But there is evidence of earthquake activity up to magnitude 7.0 thousands of years ago, Boak said.

Besides faults, researchers also are studying ancient buried magma in the area.

The magnetic survey plane then will move further north to two areas that have experienced the strongest earthquakes over the past few years. The surveys will include an area in northwest Oklahoma near Fairview and a larger area in north-central Oklahoma stretching from Prague through Cushing and to Pawnee.

The Oklahoma Geological Survey has contributed $30,000 to the project, Boak said.

The last large-scale magnetic map using aircraft in Oklahoma was conducted in the 1970s, but that survey had a much lower resolution and was not as clear as what the researchers hope to collect with the current project, Shah said.

“We tried to work with that existing data, but they weren’t sufficient,” she said. “The surveys we’re doing now are much higher resolution. We’re hoping that will give us a much better image of the faults.”

Updated fault maps could be released a few months after the surveys are completed, Shah said.