A study published Thursday in the journal Science has linked geothermal energy production in the Salton Sea area with an increase in local earthquakes. According to the study, conducted by researchers from the University of California, Santa Cruz (UCSC), an increase in groundwater pumping by geothermal energy producers since 2001 is strongly correlated with an increase in earthquakes in the region since that same year.
The study's authors, UCSC seismologist Emily Brodsky and graduate student Lia Lajoie, pored over the seismic record of the Salton Basin spanning the 30-year period from 1981-2012, then compared seismic activity in the tectonically active region with groundwater pumping activities in the Salton Sea Geothermal Field. Brodsky and Lajoie report that even after correcting for the statistical noise introduced into the seismic record by aftershocks, there's a significant statistical link between local temblors and the rate at which geothermal plants are pumping groundwater in and out of geothermal reservoirs.
Geothermal plants use steam from very hot geological formations to turn turbines, thus generating electrical power. "Flash-steam" facilities in the Salton Sea area recapture the water that condenses from that steam and reinject it into the geothermal reservoirs. A link between quakes and groundwater pumping has long been theorized, and groundwater pumping has been fiingered as a potential cause of the 2011 5.1 quake in Lorca, Spain that killed nine people.
The aftershock issue complicated things because Brodsky and Lajoie were trying to determine whether human activity directly caused earthquakes, and aftershocks are by definition triggered by previous quakes. As a good-sized quake might spawn dozens of aftershocks, quakes tend to cluster when graphed along a timeline. The prevalence of such "secondary triggering" tends to mask the "primary quakes" that might be triggered directly by human activity.
The two found away to filter out the aftershocks statistically, revealing that seismic activity and geothermal groundwater pumping rose and fell pretty much in tandem.
"We found a good correlation between seismicity and net extraction," Brodsky said in a UC Santa Cruz press release. "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."
Quakes aren't uncommon in the Salton Basin, as witness the 7.2 April 4, 2010 shaker that killed at least two people in Mexicali. The Brawley Seismic Zone, running from El Centro north to Bombay Beach, underlies the geothermal area in the Salton Sink. In fact, one wouldn't be there without the other: the magma rising to the surface that creates the Brawley Seismic Zone also provides the heat to run Imperial County's geothermal turbines.
The north end of the Brawley Seismic Zone is the south end of the San Andreas Fault, the southern section of which hasn't ruptured violently for more than a century. Some geologists suspect that alll the Southern San Andreas from the Salton Sea to Monterey County could let go in a 8.1 quake. Could quakes spurred by geothermal pumping trigger the San Andreas?
Brodsky isn't sure, but the prospect has occurred to her. "It's hard to draw a direct line from the geothermal field to effects on the San Andreas fault," she said, "but it seems plausible that they could interact."