July 12, 2013
Are Humans Causing Seismic Activity?
Brett Smith for redOrbit.com - Your Universe Online
Two studies published in the journal Science have both found human geologic activities could be causing nearby seismic activity. One study found that massive earthquakes from around the world have been triggering localized seismic events surrounding waste-fluid injection wells in the central United States.
Large amounts of water are used in both hydrofracking and the more conventional mining of oil and gas from underground wells. After the resources have been extracted, the waste water is often disposed of by pumping it back underground.
"The fluids are driving the faults to their tipping point," said the study's lead author Nicholas van der Elst, a postdoctoral researcher at Columbia University. "The remote triggering by big earthquakes is an indication the area is critically stressed."
Using data on previous earthquakes, the research team discovered faults near wastewater-injection sites in Texas and Colorado were approaching a critical state when sizeable earthquakes far away triggered local tremors. The researchers said passing surface waves from the big events could have placed additional pressure on faults, triggering small bouts of seismic activity.
"These passing seismic waves are like a stress test," said co-author Heather Savage, a geophysicist at Columbia. "If the number of small earthquakes increases, it could indicate that faults are becoming critically stressed and might soon host a larger earthquake."
Another study found a geothermal power facility in southern California could be causing seismic activity near the southern end of the San Andreas Fault. The power plant extracts hot water out of the ground, turns it to steam to run turbines, recaptures as much water as possible, and injects it back into the ground. Because some of the water is lost through evaporation, the net result is fluid extraction from the Salton Sea Geothermal Field that surrounds the plant.
"We show that the earthquake rate in the Salton Sea tracks a combination of the volume of fluid removed from the ground for power generation and the volume of wastewater injected," said co-author Emily Brodsky, a geophysicist at the University of California, Santa Cruz.
"The findings show that we might be able to predict the earthquakes generated by human activities. To do this, we need to take a large view of the system and consider both the water coming in and out of the ground," Brodsky added.
To reach their findings, the research team tracked the rate of fluid extraction over time and matched it against seismic activity. Because local human-induced seismic activity could sometimes be confused with aftershocks from the San Andreas, the researchers created a statistical method to eliminate aftershocks, allowing them to measure the localized activity over time.
"We found a good correlation between seismicity and net extraction," Brodsky said. "The correlation was even better when we used a combination of all the information we had on fluid injection and net extraction. The seismicity is clearly tracking the changes in fluid volume in the ground."
The researchers noted the Salton Sea Geothermal Field sits relatively close to the San Andreas fault, and human geothermal activity could be setting the region up for a major seismic event.
"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," Brodsky said.