EarthquakesPumping Central Valley’s ground water increases number of California’s earthquakes

Scientists at Western Washington University, University of Ottawa, University of Nevada, Reno, and University of California, Berkeley have offered  a new theory explaining the steady increase in the number of small earthquakes in parts of Central California. They say that the quakes are partly due to the pumping of groundwater. “These results suggest that human activity may give rise to a gradual increase in the rate of earthquake occurrence.”

They published their findings in the journal Nature.

The Los Angeles Times reports that using new GPS data, the scientists found that the mountains closest to California’s Central Valley were growing at a faster-than-expected rate compared to ranges further away, by a rate of one to three millimeters a year, enough to lift the mountains by less than six inches over the last 150 years. Scientists attribute the growth gap to the loss of groundwater in the Central Valley as it supplies nearby farms and cities. Groundwater is heavy, and depresses the Earth’s upper crust like a weight. Without that weight, the earth springs upward and the change in pressure can trigger more small earthquakes.

“It reduces the forces that are keeping the fault clamped together — leading to more small earthquakes during dry periods of time,” said Colin Amos, assistant professor of geology at Western Washington University, the lead author of the study.

“During wet periods of time when the fault is loaded down, the forces that are keeping the fault clamped down are greater. It inhibits the sliding of the fault,” Amos said. “Over the long term, because we’re losing more groundwater, it could give rise to more seismicity by reducing these overall forces. Our model of what the groundwater is doing might explain those two things: showing that humans may have a hand in changing the state of stress on the fault, and therefore, rates of small earthquakes over time.”

Since the mid-1800s, the Central Valley has lost roughly thirty-eight cubic miles of water. The U.S. Geological Survey notes that 20 percent of the nation’s groundwater comes from Central Valley aquifers, which makes it the second-most-pumped aquifer system in the United States. The region also produces 25 percent of the nation’s food, including 40 percent of fruits, and nuts. It is unlikely that demand for the region’s groundwater will decrease in the coming years.

Supporting the new study is previous research that suggests there are seasonal and long-term increases in small earthquakes, those under magnitude 5, in the Parkfield area of Central California. There are more small earthquakes in dry months than in wet months. Also, the number of small earthquakes has doubled annually between 1984 and 2005.

The study  does not suggest that future large earthquakes will be caused by human activity. “Large earthquakes are going to occur on the San Andreas fault no matter what we do,” Amos said. “But what is important is the idea human activity could trigger more seismic activity.”

“It’s really opening up a possibility that humans are changing stresses on faults,” Amos said. “It’s a simple realization that human use of groundwater is having small but perhaps measurable impacts on the San Andreas fault.”