California Fault Found to be Extra Slippery

The same stuff that makes baby powder silky soft may also be making one of the most dangerous faults on earth extra slippery. Scientists say they have found talc, as in talcum powder, inside the mighty San Andreas fault, which cuts through California.

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The same stuff that makes baby powder silky soft may also be making one of the most one of the most dangerous faults on Earth extra slippery. Scientists say they have found talc, as in talcum powder, inside the mighty San Andreas Fault, which cuts through California.

Here's NPR's Richard Harris.

RICHAD HARRIS: It's bad enough that scientists can't predict when earthquakes will occur on the San Andreas Fault. Mark Zoback, strolling the halls of the American Geophysical Union meeting in San Francisco last week, said the whole fault is a bit of an enigma.

Mr. MARK ZOBACK (Stanford University): For about the past 35 years seismologists have been trying to answer a fundamental question about how the San Andreas Fault works.

HARRIS: Zoback is from Stanford University. He says the fault is 10 times more slippery than you'd expect just from looking at the rock there. One idea is that the fault gets lubricated by water deep underground. A second hypothesis is once the rocks starts to slip, the friction generates heat, which in turn changes the properties of the rock and makes it more slippery.

Mr. ZOBACK: The third class of ideas is that the San Andreas Fault has within it unusual minerals.

HARRIS: Evidence for that idea came out of the meeting in San Francisco last week. Dianne Moore from the U.S. Geological Survey has been looking closely at California's state rock, Serpentine, which makes up much of the fault. She says the Serpentine itself isn't slippery enough to explain why the fault moves so easily.

Ms. DIANNE MOORE (U.S. Geological Survey): But another mineral that can form in Serpentine, which is talc, could.

HARRIS: So Moore went to see if she could find talc in the actual San Andreas Fault. To do that, she picked through rock fragments that had been brought up from two miles underground last summer, a jumbled sample of rock from the San Andreas Fault itself.

Ms. MOORE: And I started looking in the grains and have found at least small amounts of talc.

HARRIS: You heard right - talc. The stuff you can sprinkle lovingly on a baby's bottom may also be playing a role in treacherous California earthquakes.

Ms. MOORE: Why, it makes it - kind of lubricate your skin, makes it feel soft and smooth; that same thing is making it easy to shear. And if you can just get a narrow shear zone with talc in it, it would be very weak.

HARRIS: And that weakness could help explain why the fault slips so easily. So where's the talc coming from?

Ms. MOORE: Steve Hickman, one of the PIs for the drilling project accused me of standing over the fault zone with a jar of talcum powder. (Unintelligible)

HARRIS: Actually, she says, the mineral talc can from Serpentine right in the fault zone.

Ms. MOORE: The Serpentine reacts to form talc if you flow fluid through it that are heated and have silica. It's just a chemical thing. It's a reaction that occurs.

HARRIS: The fault itself actually might be forming the talc, is that the idea?

Ms. MOORE: It would be helping it come along, because it would be a place where fluid will be coming up. So that would help the reaction occur.

HARRIS: Mark Zoback from Stanford says this is a very exciting discovery, though it remains to be seen whether there's enough talc in the fault to explain the slippery nature of the San Andreas.

Mr. ZOBACK: We really won't know the answer until the summer of 2007, when we actually core through the San Andreas Fault and bring back intact samples. And if we get a lot of talc back then, that's truly going to be exciting and may put to rest a question that plagued us for over 35 years.

HARRIS: That drilling project is supposed to produce large orderly samples of rock from deep underground, right across the San Andreas Fault. And you can be sure Dianne Moore will be at hand, looking for more talc.

Richard Harris, NPR News.

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