University Park, Pa - While most scientists assume that both sides of a geologic fault move equal distances during an earthquake, Penn State researchers 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. "These faults have always been modeled as if both sides were equal by definition."
Furlong; Rocco Malservisi, Ph.D. student in geosciences; and Timothy H. Dixon of University of Miami, investigated the Eastern California Shear Zone, a strike slip fault system running parallel to the San Andreas fault about 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 the fault, consisting of the Sierra Nevada Mountains, and the eastern side of the fault, that of the Basin and Range, have very different heat flow properties, which the researchers believe is the cause of 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.
At 12 miles beneath the surface, the temperature on the Sierra Nevada side is 375 degrees Fahrenheit, while the Basin and Range side is 1112 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. This Owen's Valley earthquake is far enough in the past so that the effects of the actual earthquake can be well accounted for, making the differences in movement on each side of the fault observable.
Furlong, Malservisi and Dixon report in the July 15 issue of the journal Geophysical Research Letters, on their on-site study of this fault. Using permanent location markers and Geographic Positioning System equipment, they were able to record the difference in movement on each side down to about 1 millimeter. They found a difference of a fraction of an inch a year on the rigid side out of a total movement along the fault of 0.5 inch. Their findings provide a more accurate method for modeling this earthquake data, one that allows the computer models to better fit the ground reality in the Eastern California Shear Zone.
"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 was 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 this 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 has funded the researchers to continue their work and obtain additional G.P.S. data for the Eastern California Shear Zone.