Thousands of Little Quakes Jiggling the Owens Valley : Environment: Swarm of tiny temblors--most below magnitude 1.0--has lasted for two weeks in a remote area. Scientists do not think they are related to volcanic activity.

For the last two weeks, a swarm of more than 10,000 small earthquakes, most of them tiny with with a magnitude of less than 1.0 but some reaching magnitude 3 or 4, have struck a remote desert area along U.S. 395, about 135 miles north of Los Angeles.

The quakes near Coso Junction, east of the Southern Sierra in the Owens Valley, are in located in what is one of the two volcanic fields closest to Southland urban areas. But scientists with the U.S. Geological Survey say they do not believe the swarms are a precursor to volcanic activity, but rather are linked to faults bordering the mountains.

Most earthquake swarms in California last less than 72 hours. But this one has been going on since Feb. 15, and the latest temblor noticeable to those living in the area, magnitude 3.7, occurred Thursday night. Scientists are not sure why they have lasted so long.

Four geothermal power plants are in the immediate area, and several hilltops formed by volcanic cones and ancient lava flows are readily visible east of the highway. Although some of the cones may look fresh to the layman, a Geological Survey scientist said Friday that it is probably “a few ten thousands of years” since any of them erupted.

“The earthquake swarm appears related to regional tectonics rather than foreshadowing an eruption,” said Wendell Duffield, a research geologist who has done a detailed study of the area. “It sits there right at the east face of the Sierra Nevada in a zone of young and historic faulting. It’s not unexpected to have earthquakes there.”

Peter Lipman, the Geological Survey chief of volcanic hazards in the West, said the area “has been intermittently active volcanically over the last 10 million years.” But, he noted, “The character of the earthquakes to date are not like those that characteristically occur during the initial stages of volcanic unrest.”

That type of harmonic tremor was observed in 1980 at Mt. St. Helens in Washington before the top of the mountain blew off. A harmonic tremor reflects the rhythm of molten rock moving beneath the surface, possibly building toward an eruption.

No harmonic tremors have been detected in the last two weeks near Coso Junction.

Volcanic activity has occurred within the past several hundred years on the eastern side of the Sierra, but more than 100 miles north of the present swarm, in the Mammoth Lakes area.

Through Friday, scientists said, there have been more than 750 quakes registering higher than magnitude 1.0 near Coso Junction. A 1.0 quake is not detectable except by instruments. Thousands of other quakes were less than 1.0.