The water use that helped produce California’s agricultural bounty may be increasing the chances of earthquakes along the San Andreas fault, researchers said on Wednesday. A new study, published in Nature on Wednesday, said groundwater depletion in California’s Central Valley – the heart of its agricultural industry – is putting additional pressures on the fault, and promoting the chances of an earthquake.
The study did not predict how and when that earthquake might occur. The paper is among the first to attribute a human component to earthquakes along the San Andreas fault. Other researchers have established a connection between small earthquakes in Ohio and underground disposal of waste water from fracking.
The researchers, led by Colin Amos of Western Washington University, used data from GPS networks to analyse the tiny movements in the Central Valley and the surrounding mountains. Scientists have known for years that the floor of the valley has been dropping as the groundwater is pumped out for irrigation.
An estimated 160 km3 of ground water in the Central Valley has been lost through pumping, irrigation and evaporation over the past 150 years. The rate of that depletion is accelerating, because of expanding population, increased demands for agriculture and recurring drought – which means that the groundwater can not be readily replaced.
Meanwhile, the mountains surrounding the valley have also been undergoing tiny shifts each summer and autumn, moving upward as the seasonal snowpack melts. Those competing pressures have brought the San Andreas fault closer to failure, the researchers said.
“The human effect is becoming the dominant effect,” said Paul Lundgren of Nasa’s Jet Propulsion Laboratory. “The more you deplete that groundwater, the more you keep promoting that fault towards failure.”
He said the human influence was fairly significant – around the order of the knock-on effect from other large earthquakes of relatively close faults. Growing demand for groundwater – because of drought – would put the fault under more pressure.
But it was impossible to say at this point when the next big earthquake might occur.
“We don’t know how far that fault has to go before we have the next large earthquake,” Lundgren said. “It’s kind of like going up an escalator. You are going up a little faster than you would ordinarily go but you still don’t know how far it is to the top.”