The San Andreas fault is so sensitive that gravitational pull from the moon and sun is causing continual tremors deep underground.
"Tremors seem to be extremely sensitive to minute stress changes," said Roland Bürgmann, UC Berkeley professor of earth and planetary science. "Seismic waves from the other side of the planet triggered tremors on the Cascadia subduction zone off the coast of Washington state after the Sumatra earthquake last year, while the Denali earthquake in 2002 triggered tremors on a number of faults in California. Now we also see that tides – the daily lunar and solar tides – very strongly modulate tremors."
The team reckons that the fault's extreme sensitivity to stress – and specifically to shearing stress along the fault – means that the water deep underground it is under extreme pressure.
"The big finding is that there is very high fluid pressure down there, that is, lithostatic pressure, which means pressure equivalent to the load of all rock above it, 15 to 30 kilometers of rock," said seismologist Robert Nadeau of the Berkeley Seismological Laboratory. "Water under very high pressure essentially lubricates the rock, making the fault very weak."
"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. "But it seems like it must be very subtle, because we actually don't see a tidal signal in regular earthquakes. Even though the earthquake zone also sees the tidal stress and also feels the added periodic behavior of the tremor below, they don't seem to be very bothered."
Nevertheless, said Nadeau, "It is certainly in the realm of reasonable conjecture that tremors are stressing the fault zone above it. The deep San Andreas Fault is moving faster when tremors are more active, presumably stressing the seismogenic zone, loading the fault a little bit faster. And that may have a relationship to stimulating earthquake activity."
To learn more about the source of the tremors, the teamcorrelated eight years' worth of tremor activity with the effects of the sun and moon on the crust and with the effects of ocean tides.
They found the strongest effect when the pull on the Earth from the sun and moon sheared the fault in the direction it normally breaks.
"When shear stress on a plane parallel to the San Andreas Fault most encourages slipping in its normal slip direction is when we see the maximum tremor rate," Bürgmann said. "The stress is many, many orders of magnitude less than the pressure down there, which was really, really surprising. You essentially could push it with your hand and it would move."
It may be that tremors only occur on faults where fluid is trapped deep underground with no cracks or fractures allowing it to squirt away, Nadeau added. That may explain why tremors are not observed on other faults, despite intense searching.
"There is still all lot to learn about tremor and earthquakes in fault zones," he said. "The fact that we find tremors adjacent to a locked fault, like the one at Parkfield, makes you think there are some more important relationships going on here, and we need to study it more."
The paper appears in Nature.