Depletion of Central Valley’s groundwater may be causing earthquakes
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.
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 is very heavy, and its weight depresses the Earth’s upper crust. Remove the weight, and the crust springs upward — and that change in pressure can trigger more small earthquakes, the researchers said.
“It reduces the forces that are keeping the fault clamped together — leading to more small earthquakes during dry periods of time,” said Colin B. Amos, assistant professor of geology at Western Washington University, the study’s lead author.
Other scientists studying a seismically active area of southern Monterey County near Parkfield observed that there tend to be more earthquakes during dry months than during wet months. The number of earthquakes there every year has roughly doubled between 1984 and 2005.
“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,” Amos said.
But the question as to why earthquakes have been increasing in Parkfield over time has been a mystery. The groundwater theory introduced by Amos and his colleagues gives one possible answer.
“Over the long term, because we’re losing more groundwater, it could give rise to more seismicity by reducing these overall forces,” Amos said.
Groundwater has been slowly depleted in the Central Valley to quench the thirst of farms and cities since the mid-1800s.
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