While earthquake-prone California weighs the future of expanded quake-linked fracking in the state, a new report says that humans have already raised the seismic threat-level through groundwater extraction.

A study of the San Joaquin Valley published in the journal Nature last week found the valley floor has been sinking for decades while the surrounding mountains rise as wells suck water out of the region's aquifers. The activity causes stress to the San Andreas Fault and others nearby.

The San Joaquin Valley is the lower two-thirds of the Central Valley (the Sacramento Valley is the rest), the nation's top agricultural producing region. Twenty-five percent of the nation's food is grown there. Farmers irrigate a lot of land, and residents draw their drinking water primarily from the ground in a mostly-arid area.

The valley sits on an ocean of water that in some respects buoys it up. A 2009 study by the U.S. Geological Survey (USGS) found that flow through the Central Valley aquifer increased six-fold between 1962 and 2003. That's a guess, because no one keeps track. “Pumpage is physically possible to measure; yet in the Central Valley it is one of the least certain components of the entire water budget,” according to the agency.

That said, USGS used a scientific model to estimate that 12 million acre/feet of water was flowing, much of it from the northern Sacramento Valley portion of the aquifer to the south, as a result of pumping. The report noted what this week's study amplified: “Significant land subsidence (more than 1 foot) due to the withdrawal of groundwater has occurred in about half of the San Joaquin Valley.”  

The water pumpage is affected by seasonal factors (like snow pressing down on the mountains) and, together with long-term flexing in the valley and mountains, brings the San Andreas Fault “closer to failure,” the researchers wrote in the journal. That stressing won't by itself cause a major quake that would rock Los Angeles, which sits atop it. But the smaller seismic activity it generates will be a factor.

The report also noted that previous studies of the Sierra Nevada uplift were wrong to attribute that entirely to tectonic or mantle-driven forces. Groundwater depletion by humans plays a role.

While seasonal effects are given a fair amount of weight, the researchers said that long-term removal of groundwater could increase its importance.

Or not.

Paul Lundgren, a geophysicist at NASA's Jet Propulsion Laboratory in Pasadena, wrote a commentary in Nature about the study and said the “unclamping” caused by all this movement might cause smaller, although more frequent, earthquakes. But it is unclear if that will result in any significant for the San Andreas.

The report does, however, show that humans, “whether it’s water withdrawal or things like oil extraction … can potentially have some other unintentional effects” on the Earth's crust, he wrote.

–Ken Broder

To Learn More:

Groundwater Extraction Boosts Likelihood of California Earthquakes (by Jenna Iacurci, Nature World News)

Study Says Irrigation Can Stress San Andreas Fault (Associated Press)

Increase in Earthquakes May Be Tied to Groundwater Pumping (by Rong-Gong Lin II, Los Angeles Times)

Uplift and Seismicity Driven by Groundwater Depletion in Central California (by Colin B. Amos, Pascal Audet, William C. Hammond, Roland Bürgmann, Ingrid A. Johanson and Geoffrey Blewitt, Nature)

Groundwater Availability of the Central Valley Aquifer, California (U.S. Geological Survey) (pdf)