The federal government has issued a rare earthquake alert that predicts as much as a 1-in-3 chance of a significant quake this week in a hamlet bisected by the San Andreas Fault.

The alert, issued Tuesday, was for an earthquake of magnitude 6.0 or below on the Richter scale by Friday in the Parkfield area.However, scientists cautioned the prediction could be wrong because it's based on ground movements that may have been caused by recent rain.

The warning was issued "for informational purposes" by the Office of Emergency Services in Sacramento and the U.S. Geological Survey's office here.

"This statement does not require action by government agencies or the public," it said.

Austa Hamann, one of about 34 people who live in Parkfield, was unfazed by the warning.

"We are used to it," she said Wednesday. "We have some kind of an alert all the time."

The tiny community, situated halfway between Los Angeles and San Francisco in grassy, oak-dotted ranchland, has long been studied by scientists because of the unusual cyclical nature of its quakes, which occur an average of once every 22 years.

Moderate to strong quakes measuring 5.5 to 6.3 on the Richter scale have ruptured a 20-mile-long section of the fault near Parkfield roughly every 22 years since 1857.