It won't send Los Angeles plummeting into the Pacific Ocean, but the gravitational pull of the sun and the moon do stimulate small tremors in the San Andreas Fault, researchers say.
The tidal forces of the sun and moon don't directly trigger earthquakes, but they appear to initiate deep tremors 25 kilometres below the surface. These small tremors can increase the likelihood of future earthquakes above.
"Tremors seem to be extremely sensitive to minute stress changes," said Roland Burgmann, a professor of Earth and planetary science at the University of California, Berkeley.
Tremors on the U.S. West Coast have been triggered by seismic waves from earthquakes on the other side of the world, said Burgmann.
"Now we also see that tides — the daily lunar and solar tides — very strongly modulate tremors," he said.
Water under pressure
In their study published this week in Nature, Burgmann and his colleagues say the tremors' extreme sensitivity to small stresses suggests that water in the fault is under extremely high pressure.
The researchers used eight years of data from the university's network of high-resolution seismometers placed in bore holes. They correlated the tremor data with sun and moon tides, as well as ocean tides, which are driven by the moon.
"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 kilometres of rock," said Robert Nadeau of Berkeley.
"Water under very high pressure essentially lubricates the rock, making the fault very weak," said Nadeau.
The pressure 25 kilometres underground is around 600 million Pascals, the researchers said, while the stress from the gravitational force of the moon and sun is about 100 Pascals. (Normal atmospheric air pressure is slightly more than 100,000 Pascals, or 100 kPa.)
"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," said Burgmann.
Tremors at this depth were first discovered just seven years ago. Seismologists call them non-volcanic tremors to distinguish them from tremors associated with the flow of magma under volcanoes.