Study: Groundwater withdrawals uplifting Sierra Nevada
May 19, 2014
Maybe man can move mountains.
Researchers at University of Nevada, Reno, think rapid, ongoing uplift in the Sierra Nevada mountain range might be caused by extensive groundwater withdrawals in California's Central Valley.
The withdrawals have long been known to cause ground subsidence, with the valley floor falling as groundwater is pumped out. But the loss of the groundwater and its weight is actually causing the hard rock crust underneath to rise up, and lift the mountains with it, according to research published Wednesday in the science journal, Nature.
"We first wrote two years ago about the rapid rise of the Sierra, with its 14,000-foot peaks in the south and 10,000-foot peaks at Lake Tahoe moving as much as 1 to 3 millimeters per year," said Geoff Blewitt, at the Nevada Bureau of Mines and Geology.
"The puzzling results of our earlier research cannot be explained easily by geology alone. We've now found that a reason for the rapid uplift may be linked to human activity."
Over the last 150 years, about 40 trillion gallons of groundwater in the Central Valley have been lost to pumping, irrigation and evaporation. That's about equal to the amount of the water in Lake Tahoe and enough water to cover an area the size of California in 14 inches of water, according to UNR.
"This massive withdrawal of water has relieved pressure on the Earth's crust, which is now rebounding upwards in response," Blewitt said. "This is counter-intuitive to most people, even geologists, who tend to only think that water withdrawal causes subsidence, which is only true in the sediments of the valley from which the water is withdrawn. With the weight of the groundwater missing, the hard rock crust under the valley is actually rising, too."
Uplift of several millimeters per year in a large mountain range like the Sierra Nevada might not sound like much. It's about the thickness of a dime. But it is fast in geological timeframes that span tens of millions of years, with a total rise of up to six inches over the last 150 years, according to calculations by geophysicists who helped in the research.
Similar uplift has been documented in the California Coastal Range on the other side of the Central Valley, Blewitt said. He uses the analogy of a stone bathtub on a scale, with the scales and the tub rising or falling as water is added or taken out.
"We didn't expect human beings could actually create mountain growth. It's mind-boggling, but our calculations show that's what is indeed happening."
Groundwater subsidence was also found to correlate with seismic activity on the San Andreas Fault, according to UNR.
Blewitt and Bill Hammond, who run the Nevada Geodetic Laboratory at UNR, partnered with Western Washington University, University of California, Berkeley and University of Ottawa in the research.
Their study is based on GPS measurements from California and Nevada between 2007 and 2010 and data from the Nevada Geodetic Lab, which has the largest GPS data-processing center in the world. The lab can process information from about 12,000 stations around the globe continuously, and measures the shape of the Earth each day with data drawn from a global network of measurement stations and stations in space.
"The real importance of this research is that we are demonstrating a potential link between human activity and deformation of the solid Earth, which explains current mountain uplift and the yearly variation in seismicity," said Colin Amos, assistant professor of geology at Western Washington University and lead author of the Nature article. "These are questions that lots of geologists have been puzzling over and it's a real eye-opener to think that humans are the ultimate cause."
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