As if the California drought wasn’t worrisome enough. Scientists now have shown there are groundbreaking affects of water use that lay just below the surface.
A new study published this week in Nature says that groundwater pumping from California’s Central Valley may spur an earthquake, due to shifting weight loads. With agriculture comprising about 80 percent of the water use in California, this might be another wake-up call for many California farmers to find more efficient water practices.
“The stress is very small, much less than you need to build up stress on a fault toward an earthquake,” said the study’s co-author Roland Bürgmann, a UC Berkeley professor of earth and planetary science. “But in some circumstances such small stress changes can be the straw that broke the camel’s back; it could just give that extra push to get a fault to fail.”
In just over 150 years, California’s Central Valley had about 160 cubic kilometers (or 40 cubic miles) of water pumped. To put that in perspective, that’s about the size of Lake Tahoe. As a result of that water removal, some areas have experienced the water table dropping by as much as 120 meters, and the ground surface water by 5 meters or more.
These surface changes ultimately affect what’s below. The pumping puts stress on the Sierra Nevada and Coast Ranges sink, causing the underlying crust or “lithosphere” to shift about the thickness of a dime each year. Since 1860, the lithosphere has raised by as much as half a foot, according to Bürgmann.
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But it’s not only the groundwater pumping that is a factor. Seasonal precipitation also affects the stress on the lower-lying lithosphere. Bürgmann said that winter rains flowing into the Central Valley streams, reservoirs and aquifer actually pushes down the crust by about 1-3 millimeters, clamping the San Andreas Fault tighter and slightly reducing the risk of a quake. During the summer, water flow to the Pacific, evaporation, and groundwater pumping allows the crust to rise.
This latter part has been more extreme in the last few years, thanks to the extreme drought California now faces. About 30 cubic kilometers (or 7.5 cubic miles) of water were removed from Central Valley aquifers between 2003 and 2010, causing a rise of about 10 millimeters (2/5 inch) in the Sierra.
“The hazard is ever so slightly higher in the summer than in the wintertime,” Bürgmann said. “This suggests that climate and tectonics interact; that water changes ultimately affect the deeper Earth too.”
Using millimeter-precision measurements of elevation only made possible in the last few years, the study provides an alternative and more reasonable explanation for the 1-2mm annual rise seen in the Sierra in historic times. 1-2mm may not sound like a lot, but in tectonic terms, the slightest movement could lead to a ‘mountainous’ change.
“The Coast Range is doing the same thing as the Sierra Nevada, which is part of the evidence that this can’t be explained by tectonics,” Bürgmann said. “Both ranges have uplifted over the last few years and they 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.”
Science has shown yet again that the results of anthropogenic actions aren’t always visible at the surface. Sometimes, we have to dig a little deeper to figure out the full range of the cause and effect.
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