Did removal of groundwater at the Cerro Prieto trigger Mexicali earthquake?

"It was a sequential unzipping of faults"

On April 4, Easter Sunday, Blanca Quintero was driving in Mexicali to visit her mother’s crypt when the magnitude 7.2 El Mayor-Cucapah earthquake began rocking her SUV. Cracks opened in the street before her, and plate glass windows fell out of car dealerships alongside the road. Dodging the cracks, Quintero kept driving. At the crypt, she tells me tearfully, she saw the shattered remains of a statue of Our Lady of Guadalupe. Quintero had placed the statue there after her mother died five years ago.

Later in the afternoon — the temblor struck at 3:40 — she went to her brother’s house nearby, where the water and electricity were off and large sections of drywall had fallen from the interior walls. In the backyard, a wooden fence now leaned slightly but was still standing. On the opposite side of the lot, a brick wall had cracked from end to end and the top half was on the ground.

As it was getting dark, Quintero went to check on her father, who lives in a mobile home close to the U.S. border. All the residents in the surrounding neighborhood were now camping in a huge parking lot next to the old Mexicali headquarters of Mattel.

Quintero, who was born in Mexicali, had to be at work in San Diego the next day. Late in the night, she drove to Mexicali’s central port of entry into the United States, where she joined a long line of cars. While waiting, she tried to use her cell phone to call her father and brother. But there was no service. There was also no movement in the line.

At 4:00 a.m., Quintero made a U-turn and escaped. She drove to Mexicali’s eastern port of entry and was able to cross into the United States quickly. “After I drove a short ways into the Imperial Valley,” says Quintero, “the lights of El Centro were visible ahead. But when I looked in the rearview mirror, there was nothing but darkness.”

Since Quintero’s night flight, many local residents on both sides of the border have wondered whether an electricity plant that normally keeps lights on in Mexicali could have caused the earthquake. The Cerro Prieto geothermal power station, the largest plant of its kind in the world, is located on the southern edge of the city. The plant’s turbines are run on boiling water and steam brought to the surface from the Cerro Prieto geothermal fields deep in the earth. Many Mexicali citizens believe that removing so much water destabilizes the earth above it. Yet the technology is gaining popularity across the border too, as the State of California puts pressure on utilities, such as the San Diego Gas and Electric Company, to ratchet up their use of green energy sources.

Ray Askins is one of the locals most suspicious of the Cerro Prieto power station. I met Askins in Calexico, Mexicali’s immediate California neighbor, where he showed me numerous buildings damaged by the quake. Perhaps the most striking is the De Anza Hotel, a local historical landmark that had become a long-term residential facility. The building was evacuated after the quake and is being reconstructed to ensure safety.

Although an American citizen, Askins lives in Mexicali and acts as an energy gadfly on both sides of the border. On the basis of interviews with geologists and doctors, he has written reports on dangerous hydrogen sulfide and bad water the Cerro Prieto plant’s production processes release into the environment. He is now preparing a report on the plant’s role in the many thousands of earthquakes that occur in the Mexicali Valley every year.

Mexico’s Comisión Federal de Electricidad, which owns and operates the Cerro Prieto plant, is Askins’s primary target. The agency stonewalls any attempt to criticize the lucrative facility, he told me, in a way typical of the Mexican government generally. To further make the point about the Mexican government, Askins told me of a friend who works in a government office. “During the earthquake,” he said, “a crack in the ceiling opened above the guy’s desk. When he reported the problem, his superiors told him to just move his desk to the other side of the room.”

Alejandro Rivera, a news and sports reporter for Radio Formula in Mexicali, relates similar anecdotes by phone about Mexicali’s Hospital General, which has been known as a seismic hazard since the last big local quake 18 years ago. He claims the Mexican government never did anything to retrofit the hospital. Blanca Quintero, who lives in San Ysidro, confirms the hospital’s reputation. She received her nurse’s training there seven years ago.

According to Ray Askins, “The earthquake is a by-product of corruption, deceit, and deception…the custom and culture of Mexico.” But is the suspicion that the geothermal plant caused the quake more urban legend than reality?

I put the question to Tom Rockwell, a geologist at SDSU and specialist in seismology. Was the removal of so much groundwater at the Cerro Prieto facility the trigger? “Not for this quake,” he tells me, which started 29 miles south of Mexicali in the Cucapah mountain range and ran for close to 75 miles in a northwesterly direction, skirting an urban area of approximately a million inhabitants. The remote location of the epicenter and fault system was one reason only two people died from earthquake damage.

According to the data, Rockwell continues, “It’s clear that this was not just a simple earthquake. There were probably three separate quakes embedded in what we felt as a single earthquake, which is part of the reason it seemed so long. Normally, a magnitude 7.2 is over within 15 or 20 seconds. That’s about how long it takes to unzip that much length of fault. In this case, it was more than 40 seconds.… It was a sequential unzipping of faults, and in each case, we can see in the field where the ruptures start and stop and then jump to another fault. So a slip goes to zero, and in the Sierra Cucapah, the second shock was presumably on the Pescadores fault, and then it ruptured up to the north, stopped, and started again on the Borrego fault.… The third one went all the way to the border.”

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.”

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Don, let's have your worst-case scenario. I ain't kiddin.

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