Irrigation systems in California may be putting additional strain on geological formations there. Researchers believe these forces could increase the risk of earthquakes along the San Andreas Fault. 

Irrigation equipment supplies water to crops, as well as directly to the population of 38 million people. 

The Central Valley in the Golden State is fertile farmland, growing 25 percent of the food consumed in the United States. However, this area is quickly losing water to irrigation systems. This is causing the land itself to rise, which can affect neighboring regions. 

Environmentalists believe the amount of water drained from the region since 1860 would be enough to fill Lake Tahoe. This has caused the Central Valley to rise roughly six inches since groundwater pumping from the region started over 150 years ago. 

This flexing of regions of the Earth's crust could also make earthquakes more likely.

"Other studies have shown that the San Andreas Fault is sensitive to small-scale changes in stress. These changes appear to control the timing of small earthquakes on portions of the fault, leading to more small earthquakes during dryer periods of the year. Previously, such changes were thought to be driven by rainfall and other hydrologic causes," Colin Amos of Western Washington University, and lead author of the study, said. 

"The real importance of this research is that we are demonstrating a potential link between human activity and deformation of the solid Earth, which explains current mountain uplift and the yearly variation in seismicity," he added.

Rising ground levels in the region were measured by a large network of GPS sensors. These have also shown the small, seasonal changes which took place in the valley. Winter snows push down on the ground, and this pressure is relived when the white cover melts. Land in the Central Valley then rises up in warmer months, revealed of its massive burden, just as pumping of groundwater starts to peak for the year.

The difference in ground level between summer and winter is between 0.04 and 0.13 inches. The shrinking and swelling from groundwater draining is believed to be much greater than the contribution from natural causes, according to the study. 

Although rising land masses could increase stress levels along the San Andreas Fault making an earthquake happen more likely, researchers do point out there is no evidence that such an event is likely to occur in the near future. 

A similar rise-and-fall of ground level also occurs in the Himalayan mountain range. The effect there is triggered by torrential rains from summer monsoons. This causes the region to experience greater number of earthquakes in the winter than in summertime. 

Study of how changes in the Central Valley ground level could affect earthquakes was profiled in the journal Nature.