Image copyright SPL Image caption The San Andreas fault is part of the boundary between the Pacific and North American tectonic plates

Extracting water for human activities is increasing the number of small earthquakes being triggered in California.

A new study suggests that the heavy use of ground water for pumping and irrigation is causing mountains to lift and valleys to subside.

The scientists say this depletion of the water is increasing seismic activity along the San Andreas fault.

They worry that over time this will hasten the occurrence of large quakes.

The report has been published in the journal Nature.

It seems as though these small stress changes that happen on a yearly basis are causing more small earthquakes to occur on portions of the fault Dr Colin Amos, Western Washington University

The San Andreas fault runs for almost 1,300km through the western part of California and marks part of the boundary between the Pacific and North American tectonic plates.

Seismologists have mainly focussed on the movements of these plates as the critical factors in the build up of stress that can lead to large earthquakes, such as the one that destroyed San Francisco in 1906.

This paper looks at another factor - the impacts of humans on the Earth's surface.

The researchers have used the well developed GPS system in the western US to analyse small lifts and dips in the topography of the San Joaquin valley.

San Joaquin is part of California's central valley, one of the most productive farm regions in the US. That productivity is based on access to ground water, extracted and pumped to irrigate crops.

So great is the demand that scientists estimate twice as much water is being consumed as is being returned through rain and snow.

Image copyright UNAVCO Image caption The researchers have taken data on the topography from hundreds of GPS stations like this

All this extraction is having a significant impact on the shape of the Earth. The floors of the valleys are subsiding, the researchers found, while the surrounding mountains are on the rise.

"We are removing a weight from the Earth's crust and it is responding by flexing upwards and literally moving mountains," lead author Dr Colin Amos told BBC News.

"It seems as though these small stress changes that happen on a yearly basis, are causing more small earthquakes to occur on portions of the fault."

Dr Amos and his colleagues stress that there is a natural pattern to these tiny rises and falls along the mountain ranges - the extraction of water is a small but significant impact that researchers haven't recognised in this area before.

In a commentary on the research, Dr Paul Lundgren from the California Institute of Technology (Caltech) says the movement of the mountain serves to unclamp and increase the sliding on the San Andreas fault system.

Image copyright SPL Image caption Irrigation canals like this are used to channel billions of litres of water to farms in the central valley

"There is both a seasonal variation in and long term promotion of seismicity associated with the water extraction," he writes.

"The latter may hasten the occurrence of future large earthquakes in the San Andreas fault system."

In another part of the region along the southern Sierra Nevada mountain range, scientists had believed that the crustal uplift was due to tectonic forces.

This new research indicates that it too is partly a consequence of groundwater depletion.

Dr Amos believes the study shows that we need to think more broadly about the impact of our actions in relation to nature.

"Human activities are changing things that we hadn't appreciated before - its a wake up call to the far reaching implications for the things that we are doing that may affect systems that we didn't know that we could affect."

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