Earth Institute News Archive
posted 1999
New Interpretation of San Andreas Defies 30 Years of Conventional Wisdom
by Kurt Sternlof
Over the past 30 years an entire geological subspecialty has evolved around studying the San Andreas as a unique case among the world's major fault zones. But according to Lamont-Doherty Earth Observatory geophysicist Chris Scholz, this has been a mistake that continues to hold back the field of earthquake mechanics
In a paper presented on December 15th at the fall 1999 meeting of the American Geophysical Union in San Francisco, Scholz argued that the world's most scrutinized fault is not so special after all.
In fact, the behavior of the San Andreas, and indeed all earthquake-producing faults, can now be described by a general law of earthquake mechanics based on the physics of friction. This new interpretation has broad implications for our understanding of California's defining geologic feature, and invalidates a large body of research that has been devoted to interpreting the fault as a special case, Scholz said.
Since the 1960s, the San Andreas has been considered unusual because it appears to be relatively cool. An active fault of its size - along which a large section of California's southern coast grinds northwest relative to the rest of the state - should produce a substantial amount of frictional heat that warms the surrounding ground. But because no such heating around the fault zone has been detected, the San Andreas has long been interpreted as unusually weak - that is, of low friction such that little heat is produced.
Researchers in favor of this interpretation also contend that, because the direction of maximum crustal compression is at nearly right angles to the fault in some locations, the San Andreas must be unusually weak and slippery. Otherwise there would be no movement on the fault at all, they argue.
In order to account for its apparently unique behavior, proponents of the "weak-fault" theory have hypothesized that the San Andreas is lubricated by a plastic-like core that allows it to slide without heating up. If a typical fault is like two dry bricks grinding against each other, a weak fault consists of two bricks with a thick layer of axle grease between them.
The low-strength hypothesis remains dominant today, but Scholz expressed skepticism for any complicated scenario that depends on special circumstances while flying in the face of basic physics. "As scientists we're just not free to construct theories that aren't physically plausible, whatever the apparent evidence," he said.
In reviewing all the available San Andreas data for this paper, Scholz found conclusively that the weak fault hypothesis does not hold up, and that the fault's behavior falls well within that predicted by what is known as the Constitutive Law of Rock Friction - a law that explains the full range of observed earthquake phenomena very well, Scholz said.
"This is a pretty bullet-proof finding that's really an example of the triumph of scientific method," Scholz said. "The stress measurements that seemed to support a weak fault are clearly due to other geologic features in the area. There is no reason to think of the San Andreas as a special case. Thus the question everyone should be asking is not 'why is no heat being produced,' but rather 'where is all the heat going?'"
The problem may well lie in the central interpretation of heat-flow measurements that all heat produced by a fault dissipates by solid conduction, like a brick heated on one side that becomes warm on the other. If, however, a fluid is involved, the heat could be carried away much more quickly and efficiently by convection, leaving the mistaken impression that the fault is unusually cool.
This is an example of an erroneous assumption that took on a life of its own to the point that it now holds back the progress of the science, Scholz said. "People have based their entire careers on the notion that this type of weak-fault movement is possible in the upper crust. "It's a story about a bandwagon with the wheels finally coming off."
Luckily, there are also interesting implications for the future of San Andreas research. If the fault works in accordance with the law of rock friction, it may produce related geological phenomena that could be used to better predict how and when earth quakes will occur, Scholz said.
No one has even looked for any of these related and potentially predictive phenomena yet, because the assumption was, since the fault was weak, they wouldn't be there. The thing to do now is go look for them Scholz said. "The coming decade should be a very interesting one for research along the San Andreas."
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