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Slip-sliding away the rough edges.

Slip-sliding away the rough edges

Because the geologic clock moves so slowly, scientists studying faults such as the San Andreas can only witness a moment or two in the life of the fault. It's something like an ornithologist taking snapshots to glean truths about the growth of owls: One snapshot tells little, but a whole photo album featuring owls of all ages can reveal some trends. Using the geological equivalent of such a technique, a fault researcher reports evidence that certain types of faults actually evolve with time, becoming smoother -- and possibly more destructive -- as they mature.

In a study of seven quake-generating faults in California and Turkey, Steven G. Wesnousky from Memphis (Tenn.) State University focused on so-called strike-slip faults. Motion on these faults resembles two trains passing in opposite directions. The two sides of the fault slip past each other with little vertical motion. The offset of such faults is the total distance one side has moved in relation to the other during the lifetime of the fault. The classic strike-slip San Andreas runs about 1,000 kilometers long and has built an offset of about 250 km over the last 5 million years.

Wesnousky found that all seven faults in his study fit the same growth pattern, he reports in the Sept. 22 NATURE. The ones with the greatest offsets were the smoothest; the faults ran hundreds of kilometers without an appreciable break. Conversely, those with the smallest offsets were disjointed and broken into short segments. A fault segment might continue for 20 km, then stop and start again a few kilometers to the right or left.

This trend suggests that as strike-slip faults move, they smooth out the steps along their course. Laboratory models have suggested this intuitively logical relationship, and the concept has long circulated among geologists. Yet before now, no one had tested to see if real faults actually fit the theory.

"Steve has made some very simple observations that should have been made a long time ago," says Richard H. Sibson of the University of California at Santa Barbara. He adds that not all researchers will agree with Wesnousky's suggestions. Some believe faults can create steps as they evolve.

Steps on strike-slip faults play an important role in limiting the size of earthquakes. Quakes occur when a patch of the fault breaks and the rupture runs along the fault. These ruptures usually stop when they reach a sufficiently large step through which they cannot continue.

The relationship between smoothness and offset seems to suggest that as faults develop, they can sustain larger earthquakes. However, Wesnousky cautions that "not all earthquakes are going to stop at steps, so [the relationship] is not a panacea for predicting the strength of earthquakes." Other factors, such as the local subsurface stress and the amount of groundwater lubricating the fault, help determine how far a rupture will spread.
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Title Annotation:research on strike-slip faults
Author:Monastersky, Richard
Publication:Science News
Date:Oct 1, 1988
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