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Irrigation Near San Andreas Fault Could Trigger Earthquakes

By Laura Dattaro
Published: May 15, 2014

Aerial view of the San Andreas fault in California. A new study shows that tapping groundwater may be increasing earthquakes along the fault (Cultura Travel/Chris Sattlberger/Getty)

Some earthquakes along California’s San Andreas Fault may in part be caused by pumping groundwater for human consumption, a new study reports.

Researchers used GPS data to measure the growth of mountains in California, according to The Los Angeles Times. They found that mountains closest to the state’s Central Valley, which has a groundwater reserve that has been supplying farms and cities since the mid-19th century, are growing at a rate of 1 to 3 millimeters per year, faster than ranges further way, the Times reports.  At the same time, according to The Associated Press, parts of the San Joaquin Valley have been sinking for decades.

All of that tapped water is enough to fill Lake Tahoe, LiveScience reports. Without all that heavy water weighing on the Earth’s crust, the ground itself springs back, increasing mountain height and triggering small earthquakes.

“It reduces the forces that are keeping the fault clamped together — leading to more small earthquakes during dry periods of time,” Colin Amos, a geologist at Western Washington University and lead author of the research, told the Times.

This effect has been documented on a large scale in a phenomenon known as glacial rebound. Freed of the weight of miles of ice after the last ice age, land began to spring upward and is still doing so, at a rate of nearly a millimeter per year.

California’s ongoing drought is expected to exacerbate the problem, the AP reports, as communities tap groundwater faster than it can be replenished. The Central Valley is the second most-pumped aquifer system in the United States, according to the U.S. Geological Survey, supplying about 20 percent of the nation’s groundwater demand. The Valley produces 25 percent of the country’s food.

"This study shows that human-induced changes are significant and must be considered in earthquake hazard analyses," sPaul Lundgren, a geophysicist at NASA's Jet Propulsion Laboratory, who was not involved in the study, told LiveScience.

The study researchers point out that earthquakes along the fault are going to happen regardless of human activity. Seasonal variation in rain and snow also affects the number of earthquakes. But according to the Times, small earthquakes roughly doubled between 1984 and 2005, and human use of groundwater could be a big part of this.

“Large earthquakes are going to occur on the San Andreas fault no matter what we do,” Amos told the Times. “[The study is] really opening up a possibility that humans are changing stresses on faults. It’s a simple realization that human use of groundwater is having small but perhaps measurable impacts on the San Andreas fault.”

The study is published in the journal Nature.

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