The answer could matter far beyond East Texas. Induced earthquakes have become increasingly common in the United States over the last decade, a kind of geological gurgle tailing the surge in natural-gas extraction. Between 1973 and 2008, the central United States saw only about 20 earthquakes per year with a magnitude of 3.0 or higher (which is roughly the threshold for when people start reporting shaking). But then, starting in 2009, the numbers began to surge. By 2013, there were 99 observable quakes per year. Then, in just 2014, there were 659, according to the U.S. Geological Survey. Somehow, in less than a decade, the once-solid seeming North American continent had been destabilized.
The quakes clustered around fracking wells, but seismologists came to believe the quakes aren’t actually caused by the removal of natural gas itself. Rather, they’re caused by injecting waste water left over from the fracking process back into the earth. Often this water oozes with salt, heavy metals, and toxic chemicals, so drilling companies try to place it beneath the water table.
After fracking began nearby around 2008, four wells of this type were built around Timpson—two to the east and two to the west. Some of these wells were enormous, pumping more than 2 million gallons of waste water underground every month. And in 2011 and early 2012, tiny earthquakes started to rattle the town.
According to new data from Shirzaei and Ellsworth, the quakes weren’t the only thing that was changing about outlying Timpson. According to observations they obtained from a Japanese satellite radar sensor called ALOS, all those gallons of water had started to nudge the ground. Land between the injection wells lifted up at a rate of about 3 millimeters per year. This is not an enormous distance, as far as the changing Earth goes—California slides into the Pacific every year at about 11 times that rate—but it is not negligible. In the months before the quake, the ground silently rose as tall as a sesame seed is long.
This change, though, didn’t happen where the earthquake eventually struck, on a basement fault running beneath the town. Rather, the ground lifted mostly around the wells to the east. The earth beneath the western wells didn’t seem to move.
And this is the key of Shirzaei’s hypothesis. One of the few differences between the wells was their depth: Eastern wells deposited water about half a mile below the surface into hard rock, where water eventually caused the ground to rise. To the west, the wells forced water much deeper, more than a mile underground, where, Shirzaei believes, it leaked down and irrigated the basement fault. He reached this conclusion by feeding the InSAR data into a model of the earth beneath Timpson and calculating the water pressure at certain depths.
“Wells to the west are deep and not sealed from the bottom by harder rock, so fluid injection could reach downward, lubricate faults and trigger the earthquake,” he told me. “The shallow system was sealed from bottom, so fluid had no opportunity to reach faults.”