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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.
The pace of uplift in the Sierra is measured only in millimeters, but when California experienced bone-dry seasons between 2003 and 2010 and pumping increased up and down the Central Valley, the High Sierra rose by about 10 millimeters, the geophysicists say. That's nearly half an inch during those seven years alone.
"The periodic stress on earthquake faults would be very small, but in some circumstances even such small stress changes can be the straw that breaks the camel's back," Bürgmann said. "The stresses from the rebounding mountains would give just that extra force needed to unclamp the (San Andreas) fault and encourage, not only small earthquakes, but also larger ruptures to occur."