Large-Scale Motion Detected Around San Andreas Fault

The findings may improve understanding of the world's most famous fault line.

For the first-time, scientists have confirmed the existence of a previously-hypothesized form of movement in Earth's crust surrounding the San Andreas Fault system.

That the crust around the fault -- an 800-mile stretch of California where the Pacific and North American tectonic plates rub up against each other -- is capable of movement is hardly a revelation. Just check out photographs of San Francisco on April 18, 1906. Or ask The Rock. The northern part of the fault, which ruptured along 300 miles and killed between 700 and 2,800 people in 1906, experienced its most recent notable temblor in the form of the 1989 Loma Prieta quake, which struck during Game 3 of the World Series and, more seriously, injured more than 3,000 people and took the lives of 63.

The southern segment of the fault, however, has not experienced a major release of energy since 1857, and the most southerly portion of that southern segment hasn't suffered a major quake since 1690. It is the thought of so much seismic energy building up over the course of centuries that feeds anxieties about the prospect of the imminent arrival of "The Big One." Such anxieties are shared by some experts, too; in May, Thomas Jordan of the Southern California Earthquake Center observed that, "The springs on the San Andreas system have been wound very, very tight. And the southern San Andreas fault, in particular, looks like it's locked, loaded and ready to go."

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Unfortunately, scientists are unable to predict earthquakes; furthermore, there is no consensus as to whether they ever will be able to. The best that researchers can do in the meantime is continue to gather as much information as they possibly can about the geology and seismology of fault lines; and several million San Franciscans and Los Angelenos are hanging anxiously on their every utterance. Hence the interest in a recent paper in the journal Nature Geoscience, which revealed a little bit more information about the way the earth is moving around the San Andreas Fault.

The fault is what's known as a strike slip, or transform, fault: the two plates are pushing horizontally against each other. But modelling of the plates' horizontal movements suggested that there should also be a small amount of ongoing vertical motion, as well. Such movements have been difficult to detect, but through careful analysis of data from GPS sensor arrays deployed around the fault, researchers from the University of Hawaii at Mānoa, University of Washington and Scripps Institution of Oceanography have now succeeded in doing so.

The movements are not large: around 2 millimeters a year. But they are spread out over a large area, in the form of 125-mile-wide "lobes" of uplift and subsidence. Notwithstanding some overly dramatic tabloid headlines, the new paper says nothing about the likelihood or likely scale of "The Big One." But it does add more knowledge and understanding of the way the crust around the fault moves and may help scientists calculate some of the likely impacts when and if The Big One eventually strikes.

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