Geothermal’s potential as a clean energy source has raised huge hopes, and its advocates believe it could put a significant dent in American dependence on fossil fuels  potentially supplying roughly 15 percent of the nation’s electricity by 2030, according to one estimate by Google. The earth’s heat is always there waiting to be tapped, unlike wind and solar power, which are intermittent and thus more fickle. According to a 2007 geothermal report financed by the Energy Department, advanced geothermal power could in theory produce as much as 60,000 times the nation’s annual energy usage. President Obama, in a news conference Tuesday, cited geothermal power as part of the “clean energy transformation” that a climate bill now before Congress could bring about.

Image An earthquake halted Markus O. Häring's geothermal project in Basel, Switzerland. Credit... Christian Flieri for The New York Times

Dan W. Reicher, an assistant energy secretary in the Clinton administration who is now director of climate change and energy at Google’s investment and philanthropic arm, said geothermal energy had “the potential to deliver vast amounts of power almost anywhere in the world, 24/7.”

Power companies have long produced limited amounts of geothermal energy by tapping shallow steam beds, often beneath geysers or vents called fumaroles. Even those projects can induce earthquakes, although most are small. But for geothermal energy to be used more widely, engineers need to find a way to draw on the heat at deeper levels percolating in the earth’s core.

Some geothermal advocates believe the method used in Basel, and to be tried in California, could be that breakthrough. But because large earthquakes tend to originate at great depths, breaking rock that far down carries more serious risk, seismologists say. Seismologists have long known that human activities can trigger quakes, but they say the science is not developed enough to say for certain what will or will not set off a major temblor.

Even so, there is no shortage of money for testing the idea. Mr. Reicher has overseen a $6.25 million investment by Google in AltaRock, and with more than $200 million in new federal money for geothermal, the Energy Department has already approved financing for related projects in Idaho by the University of Utah; in Nevada by Ormat Technologies; and in California by Calpine, just a few miles from AltaRock’s project.

Steven E. Koonin, the under secretary for science at the Energy Department, said the earthquake issue was new to him, but added, “We’re committed to doing things in a factual and rigorous way, and if there is a problem, we will attend to it.”

The tone is more urgent in Europe. “This was my main question to the experts: Can you exclude that there is a major earthquake triggered by this man-made activity?” said Rudolf Braun, chairman of the project team that the City of Basel created to study the risks of resuming the project.