Salton Sea Geothermal Plant Salton Sea, California

Geothermal Plant

A magnitude-5.5 Pohang South Korea earthquake, the second largest in the country’s modern history, struck the densely populated region of Pohang on 15 November 2017, injuring 90 people and causing $52 million in damage. It crumbled walls, cracked roads, and collapsed old buildings. And, according to two studies published in Science Magazine, it is likely the largest earthquake ever to be triggered by enhanced geothermal power. In the United States it is not unusual for geothermal plants to set off quakes in the M 3 and M4 range.

How does geothermal drilling work? The traditional geothermal drilling process has been to drill into the sandstone that has water in its pore spaces. When they drill into the rock, there's a surge of steam—just like popping a lid off a soda bottle—then the steam rises and spins a turbine.

What are the chances this deep geothermal drilling near The Geysers could set off one of the larger faults, like the San Andreas? Here's what we know: You can think about The Geysers—the upper three miles (4.8 kilometers) of crust—as a sponge, and the sponge is wet. Now we're taking fluid out of the sponge, and we're taking heat out of the sponge. When you dry out a sponge, it contracts. The Geysers is contracting. From the data, we can see it pulling in, which means that it's changing the stress field around it.

Does deeper drilling mean stronger earthquakes? The more important issue is how big a fracture is—how big an earthquake are they generating. If they intersect an existing fracture, and it's ready to go, they can trigger a bigger earthquake. For the residents of Anderson Springs, [the lack of] depth is a problem. The reason they feel so many earthquakes is because they're so close to the fractures—about 1.2 miles (two kilometers) above [the fracture]. The farther away or deeper the drilling [is], the less likely they are to feel them.