4.2 Quake Rocks Area Around Geothermal Plant

A magnitude 4.2 earthquake centered at a geothermal field near Clear Lake in Northern California on Tuesday highlights the increased seismic activity that has occurred since officials began generating electric power from steam heated by magma underneath the area.

The quake at 9:50 a.m. at the Geysers was the strongest at the geothermal field this year. It was the eighth quake of magnitude 4 or higher since power generation began at the site north of Santa Rosa in the 1960s.

Before the generating facility was built, there was negligible seismic activity in the area.

The facility currently produces 836 megawatts of power, about equal to the power needs of the city of San Francisco, according to Calpine Corp., its main operator.

Scientists said they do not think it possible that a larger quake would occur, because there are no major faults in the area. But they have studied the increased seismic activity with great interest.

There were no damage reports after Tuesday’s quake.

Colin Williams of the U.S. Geological Survey in Menlo Park said a surge of hundreds of quakes, most of them small, followed the 1997 completion of a project in which large quantities of waste water are injected thousands of feet under the surface to assist generation of steam used in power production.

Calpine officials said Tuesday’s quake occurred four miles from the waste water injection site, which sends water 8,000 feet below the surface.

“We do not think the waste water injection had anything to do with today’s quake,” said Mitch Stark, who monitors quake activity for Calpine. “But we acknowledge that seismicity has increased since geothermal production began.”

At Caltech, seismologist Egill Hauksson, a native of Iceland, where geothermal power is common, said most experts believe the quakes may actually be beneficial, in terms of facilitating a looser subterranean environment in which the underlying magma can generate more steam used to produce electrical power.

Hauksson noted that development of geothermal facilities at Obsidian Butte, near the Salton Sea, and in the Coso volcanic field near Little Lake on U.S. Highway 395, have also seen an increase in the number of earthquakes.

At the Geysers, scientists believe the magma lies about five miles below the surface and may extend about nine miles horizontally, although they emphasize that there has been no surface volcanic eruption in the area for 500,000 years.

Since quakes increased beginning in 1971, Williams said, scientists have carefully monitored their number, dates and precise epicenters.

Virtually all have been beneath the geothermal facility, many, like Tuesday’s temblor, at shallow depths.

The quake Tuesday was centered about 1,000 feet below the surface.

The quake was felt widely in Lake, Mendocino, Sonoma and Napa counties north of San Francisco Bay.