LOS ANGELES – An increase in tremors deep under California's San Andreas fault may be the harbinger of a major earthquake, according to a study out Friday in the journal Science.

Seismologist Robert Nadeau of the University of California at Berkeley reached this conclusion after analyzing tremors along a segment of the San Andreas Fault near Parkfield, California.

Nadeau found that after the 2003 6.5-magnitude San Simeon quake and the 2004 6.0-magnitude Parkfield quake -- both located mid-way between San Francisco and Los Angeles -- tremors became more frequent and underground stress increased at the end of a "locked segment" of the San Andreas fault.

A "locked segment" is a portion of a fault that has not moved in years and is at high risk of a major earthquake.

The researchers believe that the increase in tremors could mean that stress is accumulating faster than in the past along that segment of the fault, "which ruptured in the moment magnitude 7.8 Fort Tejon earthquake of 1857," read the article in Science.

"We've shown that earthquakes can stimulate tremors next to a locked zone, but we don't yet have evidence that this tells us anything about future quakes," Nadeau said.

"But if earthquakes trigger tremors, the pressure that stimulates tremors may also stimulate earthquakes."

Seismologists believe there is a 70 percent probability that a devastating earthquake will strike California in the next 30 years.

The San Andreas fault runs through much of the western state of California, the most populous in the United States.