So much for urban legend. But could there be more to it? According to Rockwell, if the geothermal station ever causes an earthquake, it will be on the Cerro Prieto or Imperial faults that go through, rather than around, the Mexicali Valley. The two faults run parallel to each other in a northwesterly direction from the tip of the Gulf of California, and the Cerro Prieto geothermal fields lie in a “spreading center” between them. They “take most of the motion between the Pacific and North American plates and they feed it up into the San Andreas system,” says Rockwell. “These faults are by far the most active in the region.… The Imperial fault ruptured end to end in 1940. That was preceded in 1934 by the Cerro Prieto fault rupture, and we expect similar earthquakes every 100 years or so.”
What the wells at the geothermal power station are doing is to make the ground in between the faults sink. Seismologists call it “subsidence.” The loss of elevation occurs naturally whenever there is an earthquake on one of the two faults. “But the Cerro Prieto plant, because of the production wells, has been causing additional subsidence,” Rockwell says. “They would probably deny it, but the ground has been going down there pretty fast. I remember going down to Saltillo and seeing scarps along the fault cutting through churches and in schoolyards and aqueducts being offset so much they won’t flow. This was back in the 1990s. I don’t know if it’s continued because I haven’t been down there for a while. But if the ground is still subsiding, the Cerro Prieto plant, by pulling water out, is reducing the fluid pressure below the ground, causing it to sink.
“Now, theoretically, reducing the fluid pressure takes the faults further from failure,” says Rockwell, who goes on to imagine an opposite effect. The latest technique being used at geothermal facilities is called “enhanced geothermal,” because it involves forcing surface water into cracks between hot rocks below ground and then removing it as steam. “If the Cerro Prieto plant is putting so much water in the ground that it’s increasing the fluid pressure, it would be bringing the faults closer to failure and you might trigger an earthquake.” Rockwell notes, however, that the epicenters of the most recent quakes on the Imperial fault have been too far away from the Cerro Prieto plant for its production processes to have influenced the shaking one way or the other.
I ask Rockwell if the Cerro Prieto geothermal station could say its activities have been benign after all. “If a crack opens up beneath your house, you’re not going to be too happy,” says Rockwell, referring to the effects of subsidence and small earthquakes he thinks the plant regularly causes in the Mexicali Valley. The shaking from the April 4 quake exacerbated the ground cracking and caused a common effect of earthquakes called liquefaction. Especially hard hit were rural areas, where sulfurous water rose to the surface, ruining agriculture.
Radio Formula’s Alejandro Rivera says such water flooded many small communities. He cites Sakamoto, founded by Japanese settlers, as one of the hardest hit by the flooding. According to Rivera, people are either camping in a baseball field or coping with their flooded homes to protect their belongings from thieves.
What about the effects of geothermal power plants on the U.S. side of the border? “I am not aware of subsidence occurring at the Heber geothermal field or at the one near Salton View,” says Tom Rockwell. “Perhaps because they’re able to keep the fluid pressures close to equal. In an ideal case, you’re putting as much fluid in the ground as you’re taking out. That’s what they’re supposed to do.”
As for the enhanced geothermal technique, alarms seem to have gone off. In 2006, at a plant in Basel, Switzerland, the technique set off swarms of small earthquakes. In January, according to the New York Times, the U.S. Department of Energy put restrictions on future geothermal power production that supposedly will reduce earthquake triggering. The restrictions require the use of “ground-motion sensors,” plans to close operations if earthquakes become a problem, and checking plant operation plans with seismologists prior to receiving approval.
Meanwhile, a magnitude 5.7 earthquake struck last week a few miles east-southeast of Ocotillo, in Imperial County. Seismologists are calling it an aftershock of the El Mayor-Cucapah temblor. One can hardly keep from wondering whether the ground motion moving north is a sign that the southern San Andreas fault is about to slip. And could geothermal plants ever trigger it? “Probably not,” says Tom Rockwell. “If you just happen to be in the wrong place with a well and you jack the fluid pressure up too high, you might start a slip that would cascade into a bigger earthquake. But the southern San Andreas is going to fail whether there are geothermal plants nearby or not. It’s the result of stress building up in the crust of the earth due to plate motion.”
I ask how close we are to the big one. In the last 1200 years, Rockwell tells me, “the southern San Andreas has failed with a return period of 150 to 200 years in the Salton Trough region. And now it’s been 300 years. So you make the calculation. The southern San Andreas is ten months pregnant.”