The Geysers, located in the Mayacamas Mountains about 30 miles north of Santa Rosa, is the biggest geothermal energy plant in the world. It’s capable of producing 725 megawatts, enough electricity to power 725,000 homes, and emits no greenhouse gases. Owned by Calpine, a Houston-based energy company, the Geysers produces 20% of California’s renewable energy.

Some consider geothermal energy ideal in the climate change era: cheap, renewable, and predictable.

Unfortunately, there are few places so well-suited for geothermal energy as the site of the Geysers, which was built over an area with naturally occurring steam and a reservoir of hot water. That is why some experts advocate for enhanced geothermal energy systems (EGS), which pump water into the ground to tap natural heat sources, creating conditions for geothermal energy in areas where it would otherwise be impossible. However, other experts warn that the techniques used by enhanced geothermal systems could lead to earthquakes.

Might pace of earthquakes increase with EGS?

“These faults are going to slip eventually,” said Joern Ole Kaven, a research geophysicist with the U.S. Geological Survey. “But EGS can certainly speed up the clock.”

EGS pumps water in a way similar to hydraulic fracking techniques used to drill for oil and natural gas, but at lower pressures. The water creates a heated underground reservoir which can then be pumped back up to the surface to generate electricity. Enhanced geothermal systems would dramatically expand the number of places capable of producing geothermal energy.

“You can effectively put a power plant anywhere,” said Will Pettit, the director of the Geothermal Resources Council. “All you have to do is drill deep enough and you will find hot rock.”

Even a place as well suited for geothermal energy as the Geysers could benefit from EGS. Certain areas in the field had hot dry rock, but no natural reservoir. The Geysers became one of a handful of projects funded by the U.S. Department of Energy to test enhanced geothermal systems, in part to access those areas and increase power production.

Prospects for ‘induced seismicity’?

Until recently, experts said that EGS could cause only small tremors. New research has challenged that assumption.

Craig Hartline, Calpine’s senior geophysicist, said the Geysers follows procedures such as keeping the underground reservoir at a steady state and evenly injecting water throughout the geothermal field to limit seismic risk.

Steve Glaser, a systems engineering professor at UC Berkeley, has studied what scientists called “induced seismicity” in enhanced geothermal systems. In his view, there should be no concerns at the Geysers.

“There are thousands of very small events a month. But they’re extremely small,” Glaser said. “They’re not going to set off a big earthquake.”

That doesn’t mean EGS is safe everywhere, however. New research in Seismological Research Letters linked enhanced geothermal systems and a 5.5 magnitude earthquake in Pohang, South Korea, that caused $75 million (U.S.) in damages. Previously, experts had believed EGS could not cause earthquakes of that magnitude.

William Ellsworth, a researcher professor at Stanford University and the lead author of the study, said that in that case, there were “multiple missed opportunities” to address the risks of EGS and take action accordingly.

Research by Evelina Trutnevyte, the head of the Renewable Energy Systems group at the University of Geneva, also pushed back against the idea that EGS cannot cause a major earthquake. She said that there are varying viewpoints within the scientific community and a wide range of possibilities within seismic probability models.

One certainty: inevitable unknowns persist

“There certainly are unknowns,” Kaven said.

Emily Brodsky, a professor of Earth sciences at the University of California, Santa Cruz, says EGS certainly has the potential to cause large earthquakes.

“There’s pretty clear evidence that you can have magnitude 5. Whether or not you can make 7s or 8s … people tend to be a little unsure,” Brodsky said. A 5.0 earthquake that hit near Napa, California, in 2000 caused between $10 and $50 million in damages. A 7 or 8 magnitude earthquake, meanwhile, would be even larger than the 1989 Loma Prieta quake in the San Francisco Bay area that killed 63 people, injured thousands more, and caused $6 billion in damages.

According to Kaven, there’s no “one-size-fits-all” way to approach earthquake risk with EGS. Geologic conditions vary from site to site. Earthquakes happen regardless of industrial activity. And no one can confidently indicate how much EGS might be speeding up the timeline of a major earthquake … or how bad that earthquake could end up being.

“We know that when human activity initiates an earthquake it grows in magnitude,” Ellsworth said. “As with natural earthquakes, most end up small, but a few grow large.”

Brodsky says it’s all about risk analysis. The closer a plant is to a fault, the higher the risk of earthquakes.

“If we reduce global warming by making a few earthquakes, so be it. Everything has its cost,” Brodsky said. “But if we have a geothermal plant at the southern terminus of the San Andreas fault, which we do, that does not seem a particularly clever thing to do.”

The plant Brodsky is referring to is the Salton Sea Geothermal field in Southern California. She said that energy companies ideally would make that cost-benefit analysis in conjunction with regulators and the community.

The International Energy Agency predicts 5% annual growth in the geothermal sector through 2023. A large part of that growth is expected to be enhanced geothermal systems. The Department of Energy is contributing funding to four other EGS plants across the country in addition to providing funding for the Geysers.

Will McCarthy is a freelance audio/print reporter and master’s candidate at the UC-Berkeley Graduate School of Journalism.