Tiny earthquakes may follow groundwater loss

Sucking out groundwater could be moving and shaking California.

As humans drain aquifers in the state’s Central Valley, the land — free of water weight — flexes upward, lifting the surrounding mountain ranges and possibly triggering tiny earthquakes, researchers suggest May 14 in Nature.

It’s the first time scientists have linked the region’s extensive groundwater pumping to mountain uplift and seismic activity, says geophysicist Kristy Tiampo of the University of Western Ontario in London, Canada.

Researchers have known for years that, compared with wet months, dry times see more earthquakes near Parkfield, Calif., a tiny town smack dab in the middle of the state’s biggest seismic hazard, the San Andreas Fault. The fault is the main boundary between two tectonic plates, the rigid sections of Earth’s crust and underlying rocks that can slide past one another.

Scientists weren’t sure how to explain the uptick of quakes, most of which aren’t strong enough to rattle a teacup. But researchers have now tied the region’s seasonal shakes with another peculiar observation: The neighboring mountains are creeping slowly skyward.

“Everybody thought the uplift was due to tectonics —we thought it was all related to plate motion,” Tiampo says.

Instead, Colin Amos, a geologist at Western Washington University in Bellingham, and colleagues thought that the mountains’ uplift and the earthquakes might both trace back to the vast amounts of water people have pumped out of the Central Valley. Over the last 150 years, the region has lost nearly 160 cubic kilometers of groundwater — more than the volume of Lake Tahoe.

To test the idea, the researchers analyzed years of data from 566 GPS stations crossing a southern swath of the Central Valley, from the Pacific Ocean all the way into Nevada. The stations bob up and down as the earth moves, and on average, Amos says, stations in the Coast Ranges next to the San Andreas Fault are rising 1 to 3 millimeters per year — about half the width of a paper clip. 

That uplift matched the team’s prediction of just how much the mountains would grow if unburdened by water, a value Amos and colleagues calculated using the elastic properties of Earth’s upper crust.

The results suggest that groundwater loss might help explain the mountain’s rise, Amos says.

“I was surprised,” he says. “I’ve got to confess: I study tectonics — I look at the mountains and want tectonic answers.”

Although the movement doesn’t sound like much, it could be just enough to unsettle the fault. As the ground expands upward, it pulls away from the fault, no longer clamping the plates tightly together. That makes it a bit easier for the plates to slide, which might set off subtle shudders.

Because groundwater levels are especially low in summer, the idea could account for Parkfield’s boost in little earthquakes during dry months.

Caltech Geologist Jason Saleeby says Amos’ team is onto something, but he thinks tectonic activity is important too. “You’ve got two things going on,” he says. “The next step is to figure out which is dominant.”