On Tuesday, seven earthquakes rocked Texas in less than seven hours.
While there is not yet direct evidence that they were related to the state's fracking boom, there have been "scores" of Lonestar-State quakes since at least 2013. The state only had 20 quakes between 1974 and 2003.
So far, oil and gas boom-linked earthquakes examined by scientists have not found to have been caused by hydraulic fracturing itself. That process involves shooting huge volumes of fluid into the ground to free up hydrocarbons.
Instead, the quakes have been attributed to a different process called wastewater injection, which involves forcing the fracking fluid into the ground, which can weaken nearby faults.
But Monday, researchers from Miami University in Ohio said they now have evidence tying multiple eastern Ohio quakes, include one measuring 3.0 in magnitude, to the fracking process itself.
"It appears the hydraulic fracturing induced slip along a pre-existing fault/fracture zone," Robert J. Skoumal, Michael R. Brudzinski, and Brian S. Currie write.
The quakes did not cause damage or injuries.
The trio's results hinge on correlations between the fracking stage of well drilling and spatial calculations of the underlying rock formations' faults.
The map below shows the link between where the quakes (orange and yellow circles) occurred and when in the drilling process wells were being stimulated (stars). The thin lines trace sections of the well holes, and the colors show time before and after March 1, 2014, which is right around when activity began to be measured.
"The earthquakes occurred within 1 km of a group of recently drilled oil and gas wells in the area, one of which was undergoing active hydraulic fracture stimulation at the time of the ML 3.0 seismic event," they write.
How exactly did it happen? The researchers aren't sure, but their main guess is that the natural fluidity of the underlying rock may have been increased by frac fluids.
They also found that the earth stopped moving once extraction activities ended.
"Following termination of completion operations on the afternoon of 10 March 2014, there was a marked decline in seismicity, with only six events in the following 12 hours and only a single event (13:58 12 March) over approximately the next two months," they write.
Brudzinski told Fusion no wastewater wells were activated when the quakes occurred.
"The earthquakes are only happening during very particular timeframes," he said.
Seismic detectors were installed in the area in November 2010.
The Ohio Department of Natural Resources ended up preventing the well under construction, by Texas-based Hilcorp Energy, from being completed.
Hilcorp said in a statement that it was still reviewing the study.
"Hilcorp is committed to conducting all of its operations in a safe and responsible manner and will continue to participate with various state and federal regulatory bodies and other stakeholders in the ongoing efforts to further study and understand the issues related to induced seismicity," the company said.
After a 3.9 quake hit Youngstown, Ohio, city council members proposed multiple bills to ban fracking there, but the measures have failed.
The state drilled its 1,000th gas well this summer, but gas production growth there has proven slower than anticipated because of lower prices and pipeline bottlenecks.
Rob covers business, economics and the environment for Fusion. He previously worked at Business Insider. He grew up in Chicago.