Published by the American Geological Institute
Newsmagazine of the Earth Sciences
the Quake, Into the Mantle
by Kristina Bartlett
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For at least the past two years, Mount Lewis in the Shoshone Range of northern Nevada has been moving toward Mineral Hill, an old mining district in the Sulfur Springs Range. Between these two points lies the Crescent Valley fault. A Global Positioning System (GPS) receiver planted in each spot shows the peaks are moving toward each other, and toward the fault, at 2 millimeters per year.
The geologists studying the northern Basin and Range with GPS thought their data would show a slow extension around the Crescent Valley fault, as expected of any good Basin and Range fault. But instead they found rapid compression. This modern behavior, they say, could be the decades-old aftereffect of the magnitude-7 Pleasant Valley earthquake that hit northern Nevada 85 years ago. The geodetic data might show how the 1915 quake set the sub-crust mantle material in motion.
“The geodetic picture is measuring something a lot more complex besides
some kind of steady-state buildup of strain,” says Brian Wernicke of the
California Institute of Technology. Wernicke and his colleagues at CalTech
and at the Harvard-Smithsonian Center for Astrophysics published work in
the November 2000 GSA Today of GPS movements they measured over two years
with the Basin and Range Geodetic Network, which covers an area bounded
by Flagstaff, Salt Lake City, Reno and Bakersfield. Wernicke
says the unexpected contraction around the Crescent Valley fault might
reveal the viscoelastic effect. The viscoelastic effect suggests that the
slow deformation of the mantle after an earthquake redistributes earthquake
stress. When an earthquake ruptures a fault, the crust springs back instantly.
But below the crust, from tens to hundreds of kilometers deep, material
is viscous and takes its time absorbing and redistributing earthquake stress.
Because motion in the mantle and crust go hand-in-hand, understanding the viscoelastic effect can be significant for determining earthquake hazard. “If the viscoelastic effects occur, then they redistribute the stress that causes earthquakes throughout the brittle crust,” says Wayne Thatcher, a scientist with the U.S. Geological Survey who has, since the 1970s, been looking for evidence of the viscoelastic effect.
Researchers mobilize quickly to measure the immediate effects of an earthquake, some occurring within minutes. Understanding a fault’s activity beyond that has meant inferring an average strain rate from the geologic record. But comparing the long-term geologic rate and the short-term geodetic rate can offer new insight. “Just ten years ago, if you said we’d see the viscoelastic response of a magnitude-6.8 quake 82 years after it happened, people would look at you like you had three heads,” Wernicke says.
The geodetic strain rate of 2 millimeters per year around the Crescent Valley fault is much higher than the tenths of millimeters per year the geologic record would imply, Wernicke says. But that doesn’t mean an earthquake is imminent, he adds. The modern velocities around the Crescent Valley fault are actually lower than the modern velocities around neighboring faults, some approaching 5 millimeters per year. The faster moving receivers are probably riding atop mantle extending rapidly from the Nevada seismic belt in the wake of the 1915 quake, while the receivers near Crescent Valley are riding slower mantle that is compressing. “In Crescent Valley, the good folks can go to bed knowing the stress on that fault is actually decreasing with time,” Wernicke says.
This same viscoelastic relaxation may be happening at the Owens Valley fault in California. There, a team led by Meghan Miller of Central Washington University’s (CWU) Geodesy Laboratory found a geodetic slip rate of 7 millimeters per year. That far exceeds the geologic rate of 1.7 to 4.7 millimeters per year, which is probably closer to the lower end, according to CWU researcher Jeffrey Lee.
Miller says the geodetic data, a combination of GPS data from the 1990s and trilateration data from the 1970s and 1980s, is probably still recording activity from the 1872 quake that destroyed the town of Lone Pine. Models her team has designed suggest that the lower mantle is relaxing after the quake, and the fault is rapidly slipping over it. The high GPS rate shows the fault is probably in motion from an earlier quake, rather than building up for the next, Miller says. “We need to understand decade-long intervals rather than earthquake-long time scales.”
In those decades, the mantle absorbs earthquake stress slower than crust would. In the process, the mantle might carry the stress below the crust until the stress shows itself again later as another earthquake on another fault. Fred Pollitz of the U.S. Geological Survey presented work during the fall meeting of the American Geophysical Union of evidence for mantle motion after the 1992 magnitude-7.3 Landers earthquake and 1999 magnitude-7.1 Hector Mine earthquake. Using GPS and Interferometric Synthetic Aperture Radar data his USGS colleagues collected in the months after the Hector Mine quake, Pollitz could model widely spread mantle motion around the culprit faults. Evidence hints at similar motion after the Landers quake. “Viscoelastic relaxation likely helped trigger the Hector Mine earthquake,” Pollitz reports.
The best way to understand how the viscoelastic effect might play into earthquake hazard is to monitor crustal motion continuously over the years following a quake, Thatcher says. He first began noticing possible mantle movement when he studied aftereffects of the 1946 Nankaido earthquake off the coast of southwest Japan. Japanese scientists made geodetic surveys 50 years before and 40 years after the quake, showing changes in vertical crust motion. “The long haul matters a lot with these types of measurements,” Thatcher says.
U.S. researchers hoped to begin the long haul by deploying a dense network of GPS receivers in the Basin and Range and along the West Coast as part of the proposed Earthscope project. Congress didn’t fund the project for fiscal year 2001, but the proposal is expected to be reconsidered as part of the National Science Foundation budget for fiscal year 2002.