Sun and moon trigger deep tremors on San Andreas Fault

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Washington, December 24 (ANI): A new study by University of California, Berkeley, seismologists, has suggested that the faint tug of the sun and moon on the San Andreas Fault stimulates tremors deep underground, suggesting that the rock 15 miles below is lubricated with highly pressurized water that allows the rock to slip with little effort.

"Tremors seem to be extremely sensitive to minute stress changes," said Roland Burgmann, 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," he added.

UC Berkeley graduate student Amanda M. Thomas, seismologist Robert Nadeau of the Berkeley Seismological Laboratory and Burgmann argue that this extreme sensitivity to stress - and specifically to shearing stress along the fault - means that the water deep underground 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 (10 to 20 miles) of rock," Nadeau said.

"Water under very high pressure essentially lubricates the rock, making the fault very weak," he added.

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, according to the researchers.

The situation on the San Andreas Fault is not so clear, however.

"These tremors represent slip along the fault 25 kilometers (15 miles) underground, and this slip should push the fault zone above in a similar pattern," Burgmann 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," he added.

"Nevertheless, it is certainly in the realm of reasonable conjecture that tremors are stressing the fault zone above it," said Nadeau.

"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," he added. (ANI)

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