Drought-stricken farmers in the Central Valley are pumping more and more water from the valley's huge aquifer beneath them, and the drainage is triggering unexpected earthquakes along the San Andreas Fault, scientists have discovered. During that same period of increased pumping, instruments at Parkfield in Monterey County detected unusual clusters of earthquakes along the quake-prone San Andreas Fault there. The unexpected links between the periodic drainage of the Central Valley's aquifer and the rise of the mountains that increase stresses on the San Andreas fault zone are reported in the May issue of the journal Nature. GPS sensitivityThe remarkable ability to measure tiny changes in the height of mountains is made possible by the extraordinary sensitivity of advanced global positioning systems, similar in principle to the GPS devices that tell car drivers where they're going in unfamiliar cities, block by block. 'Real eye-opener'"The periodic stress on earthquake faults would be very small, but in some circumstances even such small stress changes can be the straw that breaks the camel's back," BÃ¼rgmann said.