Seismic researchers at /*UC Berkeley*/ say they've recorded over 2,000 tremors on the fault's central section over an eight-year period.
Unlike small earthquakes, tremors occur deeper below the surface, and the shaking lasts longer.
"They're deeper earthquakes, and they also are slower, so the amount of energy that would be released in just a few seconds, or a few minutes, a regular quake would take a much longer time to be released than one of these slower longer type quakes," said Dr. Morgan Page from the U.S. Geological Survey.
They found that tremors increased twice during the eight-year-period, both before significant quakes: a 6.5-magnitude quake in 2003 and then again in 2004.
"There was increased tremor activity about three weeks before the earthquakes. Now what we don't know is if that's cause-related. Perhaps the tremor caused the powerful earthquake six weeks later, and if we had more instances where there was increased tremor activity right before an earthquake, it could help us predict when big earthquakes might happen," Page said.
Seismologists say the study is intriguing and could help warn Californians of what's ahead, but they say there's not enough data to know what the San Andreas is doing in the long term.
"It's certainly an interesting possibility," Page said. "We've only known about tremor since about 2002."