WASHINGTON (Reuters) - Tidal forces parallel to a segment of the San Andreas Fault in central California may be causing non-volcanic tremors that could help predict earthquakes, researchers said on Wednesday.

An Uninhabited Aerial Vehicle Synthetic Aperture Radar image of the San Andreas fault in the San Francisco Bay area just west of San Mateo and Foster City is shown in this image released by NASA June 17, 2009. REUTERS/NASA/JPL/Handout

Low-level tremors have long been associated with volcanoes, because they often warn of impending eruptions.

A study published in the journal Nature says these tremors beneath the San Andreas Fault could provide similar clues about earthquakes.

The researchers say the faint tug of the sun and the moon on the fault causes tremors well below the level where earthquakes occur.

The finding suggests that rock far underground is lubricated with pressurized water, allowing the rock to slip easily and weakening the fault.

“If you could go down there and push the rock with your hand you would cause a tremor,” said researcher Amanda Thomas of the University of California, Berkeley.

Thomas and colleagues analyzed nearly 2,000 tremors detected over eight years in the Parkfield segment of the San Andreas Fault. They also looked at micro-earthquakes in the same area and calculated the stresses produced by the Earth and ocean tides.

The study says they found a strong correlation between non-volcanic tremor activity and extremely small, tidal stresses parallel to the San Andreas Fault.

Thomas says studying tremors could help seismologists better understand and, perhaps, predict earthquake activity.

“They’re looking at tremor as kind of an indicator of large earthquakes as tremors signal the buildup of strain below the fault zone,” Thomas said. “So we might be able to use tremor to tell us something about when the next large earthquake will happen.”