Relentless pumping of water to irrigate farms in part of California's Central Valley is boosting the risk of earthquakes on the San Andreas fault, geologists said on Wednesday.

A century and a half of water extraction has bit by bit released a massive weight on a local part of the Earth's crust, causing it to spring up and ease a brake on the notorious fault, they said.

"This process brings the fault closer to failure," the experts said in a study published in the journal Nature.

They did not say if a large quake could result, or when or where it may strike.

The region, the San Joaquin Valley, is one of the world's breadbaskets, providing a huge and plentiful variety of crops.

But it has very little rainfall, so the irrigation water is not being replenished.

According to the scientists' calculations, farmers in the valley have extracted around 160 cubic kilometers (38 cubic miles) of groundwater since 1860.

This is slightly more than Lake Tahoe, the 27th biggest lake in the world by volume.

The extraction has caused the crust surrounding the valley to lift by about one to three millimetres (0.04 to 0.12 inches) per year, according to Global Positioning System (GPS) measurements.

At the same time, southern parts of the valley have subsided as the porous rock below, deprived of water, starts to compact.

The long-term rebound, along with seasonal uplift in late summer when groundwater levels are at their lowest, is slyly easing the vertical clamp on the San Andreas fault running down southern California, the team believe.

Evidence for this comes from seasonal episodes of mini-quakes at a monitoring site at Parkfield in late summer and autumn, the paper said.

The findings should prompt a rethink of earthquake risk prediction, said the authors, led by Colin Amos of Western Washington University in Washington state.

Estimates are typically based on a fault's seismic history and knowledge of the friction mechanics of the crustal plates that are in contact with each other.

But man-made forces such as groundwater loss may be playing "a new and unappreciated" role in the process, the study said.