California Central Valley groundwater depletion slowly raises Sierra Nevada mountains
Changes may trigger small earthquakes, scientists find
Winter rains and summer groundwater pumping in California's Central Valley make the Sierra Nevada and Coast Mountain Ranges sink and rise by a few millimeters each year, creating stress on the state's faults that could increase the risk of an earthquake.
The scientists report their results in this week's issue of the journal Nature.
If these subtle seasonal load changes are capable of influencing the occurrence of microquakes, it's possible that they can sometimes also trigger a larger event, said Roland Bürgmann, a geoscientist at the University of California, Berkeley and co-author of the Nature paper.
The study, based on GPS measurements from California and Nevada between 2007 and 2010, was led by scientists Colin Amos at Western Washington University and Pascal Audet of the University of Ottawa.
The detailed GPS analyses were performed by William Hammond and Geoffrey Blewitt of the University of Nevada, Reno, as part of a National Science Foundation (NSF) grant. Hammond and Blewitt, along with Amos and Audet, are also co-authors of this week's paper.
"Other studies have shown that the San Andreas Fault is sensitive to small-scale changes in stress," said Amos.
"These appear to control the timing of small earthquakes on portions of the fault, leading to more small earthquakes during drier periods of the year. Previously, such changes were thought to be driven by rainfall and other hydrologic causes."
This work ties overuse of groundwater by humans in the San Joaquin Valley to increases in the height of nearby mountain ranges and possible increases in the number of earthquakes on the San Andreas Fault, said Maggie Benoit, program director in NSF's Division of Earth Sciences, which funded the research.
"When humans deplete groundwater," said Benoit, "the amount of mass or material in Earth's crust is reduced. That disrupts Earth's force balances, causing uplift of nearby mountains and reducing a force that helps keep the San Andreas fault from slipping."
Draining of the Central Valley
In that time, about 160 cubic kilometers (40 cubic miles) of water was removed--the capacity of Lake Tahoe--dropping the water table in some areas more than 120 meters (400 feet) and the ground surface 5 meters (16 feet) or more.
Torrential winter storms drop water and snow across the state, which eventually flow into Central Valley streams, reservoirs and underground aquifers, pushing down the crust and lowering the Sierra 1-3 millimeters.
In the summer, water flow into the Pacific Ocean, evaporation and ground water pumping for irrigation, which has accelerated because of drought, allows the crust and surrounding mountains to rise again.
The measurements revealed a steady yearly rise of the Sierra of 1-2 millimeters per year, which was initially ascribed to tectonic activity deep underground, even though the rate was unusually high.
The new study provides an alternative and more reasonable explanation for the rise of the Sierra in historic times.
"Both ranges have uplifted over the last few years and both exhibit the same seasonal up and down movement in phase. This tells us that something has to be driving the system at a seasonal and long-term sense, and that has to be groundwater recharging and depletion."
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