California’s drought has reached epic proportions. Nearly 60 percent of the state is in exceptional drought—the most severe category—and farmers are depleting groundwater reserves at record rates as wildfires break out north and south.

Now there’s something else to worry about: drought-triggered earthquakes.

If you want to sink a well in California and pump out as much water as you can, there historically hasn’t been a lot to stop you. Unlike other Western states, California has never regulated groundwater withdrawals. As a result, over the last 150 years, Californians have pumped nearly 160 cubic kilometers of groundwater from the Central Valley, the state’s agricultural heartland. That’s enough water to fill Lake Powell, Lake Mead, and all the Colorado River reservoirs downstream—twice, with some left over.

(For more on how California may run out of groundwater in 60 years, see the video above for an update to the documentary Last Call at the Oasis, produced by Participant Media, TakePart’s parent company.)

Groundwater withdrawal in the Central Valley was accelerating even before the current drought began in 2011. As much as 20 cubic kilometers of Central Valley groundwater may have been pumped out in just the last three years, according to one estimate. That’s about 12 percent of the last 150 years’ total depletion.

With less water in the aquifer beneath it to hold it up, the soil throughout the Central Valley is sinking. In some places, the land is dropping as much as a foot a year, damaging roads and other infrastructure and exposing communities to increased flood risk.

But the missing water wasn’t just holding up the soil; it may have been holding the earth down as well. A study published earlier this year in the journal Nature suggested that the more water gets pumped out of the ground in the Central Valley, the greater the chance of earthquakes on the nearby San Andreas Fault.

It’s no surprise that groundwater pumping can set off earthquakes. A May 2011 quake in Lorca, Spain, which killed nine people and caused extensive damage to historic buildings, is thought to have been sparked by overdrafting of the local aquifer.

It would seem the same physical processes operate in California.

In the Nature study, a team of geologists led by Colin B. Amos of Western Washington University examined data from more than 500 GPS recorders in the Central Valley and the mountains that surround it.

The recorders, which measured the degree to which their locations moved up or down, showed that without that 160 cubic kilometers of water weighing 176 billon tons tamping down the subterranean landscape—what geologists call the lithosphere—the California Coast Ranges, the Tehachapi Mountains, and the southern Sierra Nevada are now rising at up to three millimeters a year, or about an inch per decade.

“Groundwater pumping unburdens the lithosphere,” said William Hammond, a geologist at the University of Nevada, Reno, and coauthor of the Nature article. “When you pump that much groundwater, the load gets taken away and the landscape essentially bounces up. The Sierra Nevada is rising more quickly as a result of groundwater pumping in the Great Valley.”

In the Coast Ranges, that uplift increases the likelihood of quakes on the San Andreas Fault system. Geologists documenting the much studied Parkfield section of the fault have long reported more quakes of magnitude 1.25 or more in late summer and fall.

That’s right about when farmers in the Central Valley start firing up their pumps so their crops survive the state’s hottest months—especially in a drought year like this, when deliveries of surface water from California’s massive network of aqueducts have run dry.

“That seasonal change means loading and unloading on the lithosphere,” said Hammond. “The earth flexes up and down, and small earthquakes seem to respond to that.”

California isn’t the only place where people are overdrafting their aquifers, of course. A recent study by the NASA Jet Propulsion Laboratory and the University of California, Irvine, found that the drought-plagued Colorado River basin has lost 50 cubic kilometers of groundwater in the last nine years. That’s 17.3 trillion gallons of water.

Related Why California Could Run Out of Water in Two Years If the Drought Continues

So could that herald more earthquakes in the Southwest?

JPL geologist Donald Argus has his doubts. “The San Andreas Fault is very active,” he said. “It moves at about 33 millimeters a year. There are definitely earthquake faults in the Colorado Plateau area, but none of them move anywhere near that fast. The effect of groundwater depletion on those faults would probably be much less notable.”

That’s not to say that California would be earthquake-free if the Central Valley didn’t overuse its groundwater.

Though emptied aquifers might influence the frequency and strength of quakes, the reason for the quakes remains the same. California sits where two tectonic plates rub inexorably against each other. That massive collision, ongoing for 30 million years, is the real cause of earthquakes on the San Andreas Fault.

“The total number of large quakes won’t change,” said Hammond. “But groundwater depletion might well tweak their timing.”

Still, the thought that groundwater extraction might make quakes more frequent is cause for reasonable concern, because it’s not as if the state is about to kick its pumping habit anytime soon.

Regulators have been struggling to craft rules to manage groundwater use in California. But a plan released in late July would give consumers of the state’s most-exploited groundwater basins 20 years to stop overdrafting aquifers. And a recent court ruling, which found that groundwater extraction can’t be allowed to affect surface water supplies on California’s North Coast, may take years to impact statewide water consumption.

In other words, it would seem groundwater extraction will be business as usual for some time. That gives Californians one more reason to bolt their bookshelves to the wall.