Sun and moon 'trigger deep tremors on San Andreas Fault'

"Tremors seem to be extremely sensitive to minute stress changes.  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," said Professor Roland Bürgmann, who led a team at California University which carried out the study.

Added co-researchers Robert Nadeau: "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 (10 to 20 miles) of rock. Water under very high pressure essentially lubricates the rock, making the fault very weak."

Though tides raised in the Earth by the sun and moon are not known to trigger earthquakes directly, they can trigger swarms of deep tremors, which could increase the likelihood of quakes on the fault above the tremor zone, the researchers said.

To learn more about the source of these tremors, the researchers began looking for tremors some five years ago in seismic recordings from the Parkfield segment of the San Andreas Fault obtained from sensitive borehole seismometers placed underground.
Using eight years of tremor data, Thomas, Bürgmann and Nadeau correlated tremor activity with the effects of the sun and moon on the crust and with the effects of ocean tides, which are driven by the moon.

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. Because the San Andreas Fault is a right-lateral strike-slip fault, the west side of the fault tends to break north-northwestward, dragging Los Angeles closer to San Francisco.

"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. 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," Prof Bürgmann wrote in the latest issue of the 'Nature' journal.