| Birth of a Fault |
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An embryonic fault is found in southern California
The writer John McPhee once sat in Lovelock, Nevada, as geologist Kenneth Deffeyes told him that tectonic movements in the American West are opening a new arm of the Pacific Ocean. He said it will reach, over the next few tens of thousands of years, all the way into Nevada. Deffeyes sketched the future sea on the map, extending from the Gulf of California through Death Valley and beyond to Lovelock.
McPhee asked the townspeople what they thought of that idea, and their answers form the delicious conclusion of "Basin and Range," part 1 of his great "Annals of the Former World." The flavor comes from the way ordinary people respond to geologic concepts. ("I've got a boat," said one.) The new seaway will be born so slowly that no one will see it happen. We live on an active planet, but it looks static on our personal time scales. The activity must be imagined from the evidence.
So it's quite a mental achievement that three seismologists have detected an earthquake fault in southern California in the process of being born—something that won't finish for many centuries. Gerald Bawden, Andy Michael, and Louise Kellogg published their report in the July 1999 Geology (it's now online).
The embryonic fault still hasn't broken the ground yet over much of its length; that is, it's a blind fault. And the part that is visible has a rough, jagged shape, not like the smooth curves of the big old faults like the San Andreas. The evidence for its existence is a line of earthquakes over the last 50 years, reaching across the rugged Sierra Nevada.
Map from Gerald Bawden's former UC Davis site showing seismic activity 1986–1996 (gray dots) and epicenter of 1952 Kern County quake. New fault is a continuation of the White Wolf fault.
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Its southwestern end touches the end of the White Wolf fault, a young jagged tear in the Earth's crust where the magnitude 7.5 Kern County earthquake struck in 1952. Its other end is in Indian Wells Valley, where the magnitude 6.0 Walker Pass earthquake of 1946 occurred. The many aftershocks of these two events, plus dozens of other small events running between them, trace a pretty convincing line on the map.
The many quakes on this newborn fault not only line up, but they also share the same orientation, as documented by their fault plane solutions. All of them are strike-slip events with a left-lateral sense, which simply means that the motion on the fault is strictly horizontal, with no uplift or subsidence, and that as you look across the fault the far side moves leftward. (See The Three Types of Faults.)
One telling detail is that most of the events had orientations that are turned slightly counterclockwise to the trend of the fault itself. This is exactly what lab experiments show in rocks that are just beginning to fracture—sets of cracks called Riedel shears. These are commonly seen within real faults, too, such as those in Death Valley. But the newborn fault appears to show Riedel shears at a scale never before documented and, for the first time, deep in the crustal zone where earthquakes themselves are born.
The new fault runs parallel to the Garlock fault, another left-lateral strike-slip fault that separates the Sierra Nevada range from the Mojave Desert. On the map, it looks as if the new fault leads to the "big bend" of the San Andreas fault. The authors speculate that the bend is moving northward from the Garlock fault and causing the new fault to form as the Garlock's replacement. We'll have quite a few centuries to see if they're right.
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Andy Michael is one of my ideals as a government scientist. Just being an earthquake researcher for the U.S. Geological Survey is a very public-minded specialty. And when I got on Usenet in 1992, he was already there posting information for the public and participating in the online discussions of those early days. Now he's got a great Web site, too, where he presents tidbits like this filtering trick he used to uncover the famous triggered events after the 1992 Landers quake.
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