https://phys.org/news/2018-06-deep-faults-earthquake-source.html

"This study really gave us some new interesting information about the orientation of faults," Boak said. "You can see something in the grain and structure (of the rock) that continues that fault," it's deeper.

The Oklahoma agency and the U.S. Geological Survey studied areas where four quakes of magnitude 5.0 or stronger have struck since 2011. A report by lead author Anji Shah of the U.S. Geological Survey was published in Geophysical Research Letters .