Groundwater Extraction Boosts Likelihood of California Earthquakes

Extracting groundwater for irrigation in California's agricultural belt can stress the San Andreas Fault, potentially increasing the risk of future small earthquakes, a new study suggests.

GPS readings of parts of the San Joaquin Valley floor indicate that the heavy use of groundwater is causing mountains to lift and valleys to subside. The Associated Press (AP) reported.

"We are removing a weight from the Earth's crust and it is responding by flexing upwards and literally moving mountains," lead researcher Colin Amos, a geologist at Western Washington University, told BBC News.

In the last century, the amount of groundwater lost in the Central Valley via pumping and crop irrigation is equal to the volume of Lake Tahoe, and the ongoing drought only exacerbates the problem. According to the Los Angeles Times, a US Geological Survey reported that 20 percent of the nation's groundwater comes from Central Valley aquifers.

The San Andreas Fault runs for about 800 miles (1,300km) through the western part of California and marks part of the boundary between the Pacific and North American tectonic plates.

Seismologists note that the movements of these plates are the critical factors responsible for the build up of stress that can lead to large earthquakes, such as the one that devastated San Francisco in 1906 and reduced it to rubble.

"The magnitude of these stress changes is exceedingly small compared to the stresses relieved during a large earthquake," Amos said.

BBC reports that, according to scientists, a magnitude-7.8 earthquake on the southern San Andreas - a so-called "Big One" - could kill 1,800 people and cost $200 billion in damage repairs.

Researchers stress that there are tiny, natural rises and falls along the mountain ranges, and that excessive groundwater pumping is a small but significant impact.

Amos believes this study shows that humans need to think more carefully about the influence we have on nature.

"Human activities are changing things that we hadn't appreciated before - it's a wake up call," he said.

The study was published in the journal Nature.