The widespread practice of extracting California groundwater to irrigate the state's agricultural belt could be stressing the San Andreas Fault, which means it could also increase the likelihood of earthquakes occurring in the region. 

In a study published in the scientific journal Nature on Wednesday, scientists reported that groundwater depletion in California's Central Valley could mean "significant but unexplored potential impacts on crustal deformation and seismicity." The authors add that “these results suggest that human activity may give rise to a gradual increase in the rate of earthquake occurrence."

Researchers have long-known that pumping groundwater has lowered the valley floor, but this is the first time that the agricultural practice has been linked to earthquakes in the region. Any earthquakes that do result from pressure on the faultline are likely to be very small, according to lead author Colin Amos. "The magnitude of these stress changes is exceedingly small compared to the stresses relieved during a large earthquake," said Amos. But his co-author, geoscientist Roland Bürgmann, warned that "in some circumstances such small stress changes can be the straw that breaks the camel's back," Bürgmann said. "It could just give that extra push to get a fault to fail."

According to the scientists, stress on the fault line results from small increases in nearby mountains, which are pushed up by the decrease in water reserves. The authors write: 

We use vertical global positioning system measurements to show that a broad zone of rock uplift of up to 1–3 mm per year surrounds the southern San Joaquin Valley. The observed uplift matches well with predicted flexure from a simple elastic model of current rates of water-storage loss, most of which is caused by groundwater depletion.

Basically, as the Los Angeles Times explains, the heavy groundwater weighs down the Earth's upper crust. As that water disappears, the crust is lifted, pushing up nearby mountains. This shift aggravates the fault line, which in turn can trigger earthquakes in the area. "[Groundwater removal] reduces the forces that are keeping the fault clamped together — leading to more small earthquakes during dry periods of time," Amos explained to the Times. "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." 

The Central Valley provides about one-fifth of the country's groundwater, and produces a quarter of the nation's food, so it's unlikely that officials will make moves to reduce the amount of water pumped out of the valley — especially as California continues to struggle with with one of its worst droughts in recent history.