Parts of the San Andreas Fault are so sensitive to stress that the faint gravitational tug of the sun and the moon may be enough to cause tiny tremors 15 miles underground, a team of UC Berkeley seismologists has found.

Water under extremely high pressure apparently acts as a lubricant for the rock, allowing even the smallest stresses to cause a measurable slippage.

"For the first time, we're getting a picture of what's going on beneath where earthquakes are happening," said Robert Nadeau of the Berkeley Seismological Laboratory, one of the authors of a report appearing today in the journal Nature. "This is information from a region we've been virtually blind to in the past."

Unlike earthquakes, which can be large and generally short-lived jolts, the nonvolcanic tremors deep underground may last for many tens of minutes at the level of a magnitude 1 earthquake, making them detectable only with sensitive instruments.

The tremors were first discovered in Japan about seven years ago, and seismologists began looking for them along the San Andreas fault in 2005. Working at Parkfield, a tiny community in a geologically active part of southern Monterey County, scientists placed sensitive seismometers more than 600 feet underground, shielded from the clutter of movement at the surface.

Using years of readings from Parkfield and other sites, Nadeau, along with Roland Bürgmann, a UC Berkeley professor of earth and planetary science, and Amanda Thomas, a UC Berkeley graduate student, found that tremor activity varied with the effects of the sun, the moon and the ocean tides, which are driven by the moon.

When sun, moon align

Since the strongest effects were seen when the pull of the moon and the sun was aligned with the direction of the fault's break (Los Angeles toward San Francisco in the case of the San Andreas Fault), the researchers reasoned that water trapped deep underground was the likely explanation for the tremors, lubricating the rock to make it move easier.

The tremors so far have been found in only a relatively small number of fault zones, suggesting that underground water isn't found everywhere.

If the tremors have an effect on the earthquake zone closer to the surface, it's hard to find, Bürgmann said.

"These tremors represent slip along the fault 25 kilometers underground, and this slip should push the fault zone above in a similar pattern," Bürgmann said in a statement released by UC Berkeley. "But it seems to be very subtle, because we actually don't see a tidal signal in regular earthquakes."

But it's a reasonable conjecture that the tremors are affecting the fault zones above them, Nadeau said, because the deep San Andreas fault is moving faster when the tremors are more active, presumably placing more stress on the seismogenic zone, where earthquakes happen.

Looking for a link

If scientists can find a link between these almost undetectable tremors and the destructive quakes that are geologically inevitable along the huge fault zones that riddle California and other parts of the world, it could help them understand the processes taking place deep below Earth's surface.

"Clearly they are connected, since it's the same fault zone," Bürgmann said in an interview Tuesday. "But how they relate is a question that still has to be answered."

The next step is to expand the research to find other places, particularly in California, where these tremors occur, Nadeau said. There's also a need for more sensitive equipment to take a higher-resolution look at what's happening deep underground.

"We need to understand the mechanism," he said. "As we get a better picture of how each part of the Earth works together, we get closer to understanding what's going on with earthquakes."