An unnatural number of earthquakes hit Texas in the past decade, and the region's seismic activity is increasing. In 2008, two earthquakes stronger than magnitude 3 struck the state. Eight years later, 12 did.
Natural forces trigger most earthquakes. But humans are causing earthquakes, too, with mining and dam construction the most frequent suspects. There has been a recent increase in natural gas extraction - including fracking, or hydraulic fracturing, but other techniques as well - which produces a lot of wastewater.
To get rid of it, the water is injected deep into the ground. When wastewater works its way into dormant faults, the thinking goes, the water's pressure nudges the ancient cracks. Pent-up tectonic stress releases and the ground shakes.
But for any given earthquake, it is virtually impossible to tell whether humans or nature triggered the quake. There are no known characteristics of a quake, not in magnitude nor in the shape of its seismic waves, that provide hints to its origins.
"It's been a head-scratching period for scientists," said Maria Beatrice Magnani, who studies earthquakes at Southern Methodist University in Dallas. Along with a team of researchers at the U.S. Geological Survey, Magnani, an author of a new report published Friday in the journal Science Advances, attempted to better identify what has been causing the rash of Texas quakes.
A cluster of earthquakes around a drilling project can, at best, suggest a relationship. "The main approach has been to correlate the location to where there has been human activity," said Michael Blanpied, a USGS geophysicist and co-author of the new study.
The study authors took a different approach in the new work - they hunted for deformed faults below Texas. "This technique is called high-resolution seismic reflection imaging," Magnani said. Seismic reflection is the same tool that allows extractors to find oil and gas deposits in underground structures.
To collect seismic reflection data, an artificially generated wave ripples through the ground and reflects back to the surface, like light off a mirror. The result is "a little bit like an ultrasound," Magnani said, revealing not baby toes but twisted rock.
The scientists compared the Texas earth with Mississippi, another seismically active region that, like Texas, is not close to a turbulent edge of a tectonic plate. Unlike Texas, though, north Mississippi has a much longer history of recorded earthquakes, going back to the early 1800s.
An underground ultrasound revealed that, beneath Texas, the most recent signs of active faults were in a geologic layer 300 million years old - 70 million years before the first dinosaur took its first step. All the younger layers above it were stable.
"All the displacement was stopping at a layer that is 300 million years old," Magnani said. "The fault did not move after that layer was deposited."
In the Mississippi region, in contrast, the rock told a story of continuous fault activity, unbroken for the last 65 million years.
Given the lack of faults in Texas's most recent 300 million years of history, there is no known geologic process that could explain its sudden quake outbreak. "There is no other explanation" except that these earthquakes are caused by human activity, Magnani said.
The study is in line with what other earthquake experts had surmised using different analyses. "We don't expect a lot of pushback" from the scientific community, Blanpied said.
"This is a landmark contribution in the question of whether the Fort Worth basin earthquakes are man-made," said Cliff Frohlich, a geophysicist at the University of Texas, Austin who was not involved with the study. Frohlich said this research eliminates the possibility, sometimes raised by the oil and gas industry, that the Texas quakes are part of a natural cycle of faults that awaken every few thousand or million years.
The seismic reflection data provide a powerful argument "that these earthquakes are something new and different," he said - activity stemming from the injection of wastewater deep into basement rock. ("Most of the time it's the large volume injection," he said, "not the little frack jobs.")
In March 2016, the USGS published a map of the likely areas where human-made earthquakes will strike in the United States. About 7 million people are at risk of these events, with Texas and nearby Oklahoma among the most susceptible states.
"We hope that the response from our colleagues will be to deploy this [technique] elsewhere," Blanpied said. Magnani said she would like to apply seismic reflection to Oklahoma, which has also experienced an unusual uptick in earthquakes.
But seismic reflection data are difficult and expensive to collect, though the oil and gas industry will sell the information. As for why no one else had attempted to do this before, the authors said Magnani was simply the first person to have the idea to use seismic reflection to look for old faults.
There is still more work to do. Any successful regulatory process, Magnani said, will require an understanding of the physical processes that trigger human-made earthquakes.
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