Geologists have found what they believe is a fault running the full length of a 10-year-old dam in California that furnishes half the state's residents with water.

"If the dam ever broke because of fresh movement of the fault it would not represent a serious flood danger," said Dr. Darrell Herd of the U.S. Geological Survey in Menlo Park, Calif. "The danger lies in the fact that it could seriously disrupt the water supply to a large area of the state that relies on this dam for water."

The newly discovered fault runs beneath a reservoir known as O'Neill Dam, which is located in central California's Merced County about 110 miles southeast of San Francisco.

First filled with water in 1968, O'Neill Dam acts as a "switchyard" for larger dams in northern California that feed water to the croplands and residential centers of southern California.

It is the distribution point for irrigation water moving south to 500,000 acres of cropland in the San Joaquin Valley, and a funnel for drinking water on its way south to about half the state's population.

USGS geologists discovered the fault's existence during recent investigations in the San Francisco Bay region. They found that the fault under O'Neill Dam is connected to a larger zone of newly found north-south faults that run about 100 miles along the eastern foothills of the Coast Range between Tracy and Los Banos.

Evidence for the faulting under the dam lies in the fact that land east of the fault and south of the dam has moved upward. Further evidence lies in aerial photographs taken as long as 15 years ago that show an exposed fault in the region of the dam's pumping plant.