Magazine article Science News

Slip-Sliding Away the Rough Edges

Magazine article Science News

Slip-Sliding Away the Rough Edges

Article excerpt

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. …

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