Move over Poseidon. New research shows that human activity — mainly pumping large volumes of water into the earth near natural gas and oil wells — may make certain places more vulnerable to seismic activity.
In the latest issue of the journal Science, researchers from Columbia University propose that an increase in underground wastewater disposal at natural-gas and oil extraction sites, particularly in the southern and western U.S., has been accompanied by an increase in earthquakes that are triggered by distant seismic events.
Wastewater injection is one way to deal with the polluted byproducts of oil and gas extraction – both from hydraulic fracturing (also know as fracking) and other more traditional means. Water is often mixed with chemicals or contaminated as it is pumped into the earth to fracture rocks or force oil and gas to the surface. Once the oil and gas have been separated out, the water is then often injected back into the earth for safe keeping.
Research earlier this year made the connection between wastewater disposal methods at oil operations and local earthquakes. Now scientists think that some of the local earthquakes at injection sites are being triggered by distant tremblors – that some injection sites are more vulnerable to the global ripple effects of seismic activity.
Columbia University seismologist Nicholas van der Elst and his colleagues studied a set of injection wells near Prague, Oklahoma, among other sites. By monitoring changes in rock stress and sensitivity as small seismic waves from remote large earthquakes passed by, they concluded that sites with decades of injection history are more likely to succumb to quakes. They now believe it was a magnitude 8.8 quake in Chile in Feburary 2010 that triggered smaller quakes and tremors at the Oklahoma site in the days and months that followed. The same may be true of the 2011 Japanese and 2012 Sumatra earthquakes and tremors around injection wells in western Texas and southern Colorado, according to the researchers.
NOTE (9.12.13): Van der Elst has clarified, saying that it is not that the larger quakes abroad caused the smaller quakes at injection sites, rather the larger quakes sent smaller ripples that the researchers detected and took as a warning sign that these areas, near injection well sites, were under stress and vulnerable to quakes. He says the larger Oklahoma earthquake and the Chilean disaster were not related and that the Oklahoma quake would have happened regardless. It is also worth noting that none of the study sites in this most recent report were receiving wastewater from fracking operations. (But previous studies have looked at such sites.) "It has implications for fracking," Elst said. "If you dispose of fracking fluids on the timescale we studied [about a decade], you'd have to take that into account."
At the core of this earthquake phenomenon, according to researchers, is the pressure that fluid injection puts on rock in aquifers (pore pressure), which can lead to stress along fault lines. “The fluids are driving the faults to their tipping point,” explained study co-author van der Elst in a press release.
“We’ve known for at least 20 years that shaking from large, distant earthquakes can trigger seismicity in places with naturally high fluid pressure, like hydrothermal fields,” study co-author and Columbia University seismologist Geoffrey Abers said in a press release. “We’re now seeing earthquakes in places where humans are raising pore pressure.”
A complementary study, also published in this week’s Science, documented a correlation between earthquakes and groundwater extraction and wastewater injection for geothermal operations in southern California, near the Salton Sea. Seismic activity has increased with geothermal production and the required increase in groundwater pumping. Due to the super-heated nature of geothermal energy, some water is lost to evaporation, meaning a smaller volume is injected back into aquifers, resulting in a net loss of groundwater.
The authors of the study, from the University of California at Santa Cruz, question whether or not this correlation could extend to increased earthquake activity along the nearby San Andreas fault line.
“It’s hard to draw a direct line from the geothermal field to effects on the San Andreas fault, but it seems plausible that they could interact,” UC Santa Cruz geophysicist and paper author Emily Brodsky said in a press release.