Amos to discuss research on earthquakes, groundwater use May 16 on campus

Western Washington University Assistant Professor of Geology Colin Amos and coauthors have published new research that shows a direct link between human-caused groundwater depletion and increased seismicity and mountain uplift in California’s Sierra Nevada and Coast Ranges.

Amos will speak about the research at noon Friday, May 16, in Environmental Science Room 100.

The research, published today in the journal “Nature,” shows that draining of the aquifer for agricultural irrigation in California’s Central Valley results in upward flexing of the earth’s surface and the surrounding mountains due to the to the loss of mass within the valley.

Widespread subsidence, or sinking of the ground surface, has been well documented in the Central Valley for decades. As groundwater is removed from the aquifer, compaction of the surrounding sediment causes the sinking motion. Subsiding ground has been costly for the state, wreaking havoc on canals, bridges, and levees throughout the agricultural valley.

In contrast with the sinking valley, Amos and coauthors use Global Positioning System data to explain the slow rise of the surrounding mountains as humans continue to use up groundwater resources in the San Joaquin Valley.

The total amount of groundwater lost from the valley over the past century and a half is approximately equal to the volume of Lake Tahoe on the California-Nevada border. And although the increase in mountain height over this period is relatively small (on the order of inches), this study is the first to ascribe this mountain growth directly to human activities.

The second and perhaps more worrisome finding by Amos and his peers was that these yearly ups and downs of the mountains and valleys may cause small disturbances to the nearby San Andreas Fault, which runs parallel to the San Joaquin Valley.

“Other studies have shown that the San Andreas Fault is sensitive to small-scale changes in stress. These changes appear to control the timing of small earthquakes on portions of the fault, leading to more small earthquakes during dryer periods of the year. Previously, such changes were thought to be driven by rainfall and other hydrologic causes,” Amos said. “But what our research suggests that the sustained loss of the groundwater and the resulting upward flexing of the ground surface may also contribute to or even drive these changes in stress.”

Amos cautioned that this research does not point to a correlation between groundwater depletion and an increased likelihood of a large earthquake along the fault.

“The real importance of this research is that we are demonstrating a potential link between human activity and deformation of the solid Earth, which explains current mountain uplift and the yearly variation in seismicity,” he said. “These are questions that lots of geologists have been puzzling over, and it’s a real eye opener to think that humans are the ultimate cause.”

“Given the current drought and most projections for climate change in California and elsewhere in the Western U.S., groundwater depletion will likely continue, and so will these phenomena,” said Amos.

Working with Amos on this research were Pascal Audet at the University of Ottawa, Canada; William Hammond and Geoffrey Blewitt at the University of Nevada, Reno; and Roland Bürgmann and Ingrid Johanson at the University of California at Berkeley.

The authors received funding from the National Science Foundation and UNAVCO for operating the GPS network.

The research of Amos and his peers is available online at www.nature.com.

For more information, contact Colin Amos at 360-650-3587 or colin.amos@wwu.edu.