WASHINGTON - While most scientists assume that both sides of a geologic fault move equal distances during an earthquake, 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 Dr. Kevin P. Furlong, professor of geosciences at Penn State. "These faults have always been modeled as if both sides were equal by definition."
Furlong; doctoral student Rocco Malservisi, and Prof. Timothy H. 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. The 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.
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, which the researchers believe 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 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]. They found a difference of several millimeters [a fraction of an inch] a year on the rigid side out of a total movement along the fault of 12 millimeters [0.5 inch]. Their findings provide a more accurate method for modeling this earthquake data, one that allows computer models to better fit the ground reality in the Eastern California Shear Zone.
At 19 kilometers [12 miles] beneath the surface, the temperature on the Sierra Nevada side is 191 degrees Celsius [375 degrees Fahrenheit], while the Basin and Range side is 600 degrees Celsius [1,112 degrees Fahrenheit]. According to the researchers, the colder Sierra Nevada side acts like a solid block, recovering fairly quickly from an earthquake, while the warmer, more viscous Basin and Range side deforms more like rubber.
When an earthquake occurs, the Sierra Nevada side only needs to snap back a small distance, while the Basin and Range side rebounds much more and then continues to recover for a much longer time. In between earthquakes, the softer Basin and Range side accumulates strain faster than the more rigid Sierra Nevada side.
One reason the Eastern California Shear Zone is a good place to study an unevenly deforming fault is that a very large earthquake of magnitude 8 or more occurred in this area in 1872. The Owens Valley earthquake was far enough in the past so that its effects can be well accounted for, making the differences in movement on each side of the fault observable.
"Before the accuracy of G.P.S. became so good, it was impossible to do this kind of research," says Furlong. "We could not have seen the difference before." Beside the accuracy issue, the researchers had another problem: "We cannot just go to the literature and check out old data sets, because the assumption was symmetry and, in the past, the data were forced to fit that assumption," says Furlong.
If the researchers' results hold true, their approach could be applicable in many places. While local geography can cloud the existence of true contrasts across sides of a fault because of local areas of hard rocks, gravels or sands, there are hints of asymmetry occurring in other places. Satellite images of a 1997 earthquake in Tibet show that the earthquake occurred more on one side of the fault than the other.
The area is so remote, however, that it is not currently possible to determine if subsurface differences are the cause. Near Papua-New Guinea, in the Bismark Sea, measurements of islands using G.P.S. are showing asymmetric patterns as well. Furlong and Malservisi caution that these are only hints that this phenomenon occurs in other places and that nothing has been proven.
The National Science Foundation funded this research and has funded the researchers to continue their work and obtain additional G.P.S. data for the Eastern California Shear Zone.
Notes for Journalists:
The paper, Rocco Malservisi, Kevin P. Furlong, Timothy H. Dixon, "Influence of the earthquake cycle and lithospheric rheology on the dynamics of the Eastern California shear zone," will appear in Geophysical Research Letters (GRL), Vol. 28, no. 14 (15 July 2001), pages 2,731-2,734. Reporters may obtain a copy by writing to Dawn McGee at firstname.lastname@example.org. Specify pdf or fax version, and include your name, name of publication, and phone and fax numbers.