|Science Advances maps of quakes of magnitude 3 or higher; click on image to enlarge it.|
Previous studies have suggested a correlation between injection wells and earthquakes in Oklahoma, which adopted guidelines last year to force drilling companies to pause and evaluate the effects of their operations when an earthquake of sufficient strength occurs. However, it's almost impossible to directly prove the cause of a particular quake by looking at its characteristics or plotting quakes on a map. "A cluster of earthquakes around a drilling project can, at best, suggest a relationship," Ben Guarino reports for The Washington Post.
So researchers from Southern Methodist University in Dallas and the U.S. Geological Survey took a different tack: they performed seismic reflection imaging, a sort of ultrasound, on Texas bedrock to see if active faults could be causing a recent spate of earthquakes. They compared Texas with the Mississippi Embayment, a region with similar seismic activity and geologic placement but far less fracking. They found that the Mississippi Embayment's earthquakes were caused by continuous fault activity for the past 65 million years, but faults in the area of Texas they studied hadn't been active for 300 million years, so there is no known geologic process that could explain them.
Cliff Frohlich, a geophysicist at the University of Texas in Austin who was not involved in the study, told Guarino that the study provides a powerful argument that "These earthquakes are something new and different."