July 3, 2001 While most scientists assume that both sides of a geologic fault move equal distances during an earthquake, National Science Foundation (NSF)-funded researchers at Pennsylvania State University and the University of Miami have discovered that not all strike slip faults act that way.
"In the past, no one looked at the contrast between the two sides of a strike slip fault," says geologist Kevin Furlong at Penn State. "These faults have always been modeled as if both sides were equal by definition." Furlong, doctoral student Rocco Malservisi, and geologist Timothy Dixon of the University of Miami investigated the Eastern California Shear Zone, a strike-slip fault system running parallel to the San Andreas fault about 240 kilometers [150 miles] east of San Francisco. This area, on the Nevada/California border, is the eastern edge of the interface of the Pacific and North American plate boundaries and is linked to the San Andreas fault.
"By considering variations in mechanical properties of the crust, these scientists have put a new twist on how we look at ground motion associated with large earthquakes along strike-slip faults," says David Fountain, program director in NSF's division of earth sciences, which supported the research. "The results suggest that ground motion could be much larger on one side of a fault than the other. These differences in motion persist long after an earthquake occurs and, using modern instrumentation, such as GPS and satellite interferometry, should be detectable for decades."
In a strike-slip fault, the ground on each side of the fault moves along the fault line, but in opposite directions. The western side of this fault (consisting of the Sierra Nevada Mountains) and the eastern side of the fault (known as the Basin and Range) have very different heat flow properties. Researchers believe this difference causes the contrast between the two sides.
"The Sierra Nevada to Basin and Range is an abrupt transition, thermally and mechanically," says Furlong. The heat flow on the Sierra Nevada side is much lower than on the Basin and Range side, making the Sierra Nevada side colder as well. These temperature differences can be dramatic.
Malservisi, Furlong, and Dixon report on their on-site study of this fault in the July 15 issue of the journal Geophysical Research Letters, published by the American Geophysical Union. Using permanent location markers and Geographic Positioning System (GPS) equipment, they were able to record the difference in movement on each side to about one millimeter [0.04 inches].
"Before the accuracy of GPS became so good, it was impossible to do this kind of research," says Furlong. "We could not have seen the difference before."
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