An earthquake swarm that rattled an area of Northern California early Thursday was another round of recurring temblors in a seismically active region that is home to the world's largest geothermal field. 

The Geysers Geothermal Field, located about 72 miles north of San Francisco, covers some 45 square miles among Lake, Mendocino and Sonoma counties. According to the U.S. Geological Survey, the activities associated with harnessing the steam to generate electricity cause frequent, minor earthquakes in the region. The quakes can be felt by those who work among the 18 geothermal power plants and by nearby residents.

Seismologists offer several explanations for why earthquakes are induced at The Geysers. The withdrawal of steam and heat from the field causes the surrounding rock to contract, which can trigger quakes from the contractional stresses. Pumping reclaimed water back into the steam reservoirs is also a factor because of the thermal contrast between the cold returned water and the hot surrounding rock. 

In addition, the hydraulic pressure of the injected water may be finding its way into faults, facilitating fracturing due to increased fluid pressures, according to the USGS. No large continuous fault line is known to exist at The Geysers, but there are numerous small fractures in the rock located near the many steam and injection wells, the agency says. The activities cause local stress changes that, combined with regional tectonic stresses, induce the quakes on these small faults.

Most of the major seismic hazards in Northern California are from large earthquakes that strike along regional faults miles away from The Geysers, such as the San Andreas and Healdsburg-Rodgers Creek faults. The largest quake recorded at The Geysers is approximately M4.5, and while it's possible that a magnitude 5 could occur, larger earthquakes are thought to be unlikely, the USGS said.

The Calpine Corporation, which owns and operates 13 plants at The Geysers, says the field's current capacity is 725 megawatts of electricity around the clock, enough to power 725,000 homes. Energy generated at The Geysers totals about 25% of California's renewable energy and about 40% of all geothermal electricity generated in the U.S., according to the National Association of Regulatory Utility Commissioners.

Despite the name, no natural geysers exist anywhere in the geothermal field. According to the USGS, the name was suggested after early explorers discovered the fumarolic activity and steam rising from hot springs in the area.