LOS ANGELES May 14, 2014 (AP)
Excessive groundwater pumping for irrigation in California’s agricultural belt can stress the San Andreas Fault, potentially increasing the risk of future small earthquakes, a new study suggests.
GPS readings found parts of the San Joaquin Valley floor have been sinking for decades through gradual depletion of the aquifer while the surrounding mountains are being uplifted. This motion produces slight stress changes on the San Andreas and neighboring faults.
“The magnitude of these stress changes is exceedingly small compared to the stresses relieved during a large earthquake,” lead researcher Colin Amos, a geologist at Western Washington University, said in an email.
The findings were released Wednesday by the journal Nature.
The study suggests that human activities “can cause significant unclamping of the nearby San Andreas Fault system” through flexing of the Earth’s crust and upper mantle, Paul Lundgren of the NASA Jet Propulsion Laboratory wrote in an accompanying editorial. Lundgren had no role in the research.
In the past century, the amount of groundwater drawn from the Central Valley for crop irrigation is equal to the volume of Lake Tahoe. The ongoing drought is expected to exacerbate the problem as communities tap groundwater faster than it can be replenished. As the valley subsides, this change in load causes the Sierra Nevada and Coast ranges to rise, according to GSP measurements taken between 2007 and 2010.
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For years, scientists have wondered about the forces that keep pushing up California’s mighty Sierra Nevada and Coast Ranges, causing an increase in the number of earthquakes in one part of Central California.
On Wednesday, a group of scientists offered a new, intriguing theory: The quakes are triggered in part by the pumping of groundwater in the Central Valley, which produces crops that feed the nation.
“These results suggest that human activity may give rise to a gradual increase in the rate of earthquake occurrence,” said the study published in the journal Nature.
During wet periods of time when the fault is loaded down, the forces that are keeping the fault clamped down are greater. It inhibits the sliding of the fault. – Colin B. Amos, assistant professor of geology at Western Washington University
Using new GPS data, the scientists found that mountains closest to California’s thirsty Central Valley were growing at a faster-than-expected rate compared to nearby ranges. The growth spurt — about 1 to 3 millimeters a year — was enough to lift them by half a foot over the last 150 years.
Groundwater has been slowly depleted in the Central Valley to quench the thirst of farms and cities since the mid-1800s.
That irrigation has already caused dramatic changes. In the 1930s, water diversions prompted the disappearance of Tulare Lake, once the largest body of fresh water west of the Mississippi, which was 60 miles across.
Over the last hundred and fifty years, the Central Valley’s groundwater reserve has lost about 38 cubic miles of water — enough to drain Lake Tahoe.
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Ohio regulators did something last month that had never been done before: they drew a tentative link between shale gas fracking and an increase in local earthquakes. As fracking has grown in the U.S., so have the number of earthquakes—there were more than 100 recorded quakes of magnitude 3.0 or larger each year between 2010 and 2013, compared to an average of 21 per year over the preceding three decades. That includes a sudden increase in seismic activity in usually calm states like Kansas, Oklahoma and Ohio—states that have also seen a rapid increase in oil and gas development. Shale gas and oil development is still growing rapidly—more than eightfold between 2007 and 2o12—but if fracking and drilling can lead to dangerous quakes, America’s homegrown energy revolution might be in for an early end.
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