Earthquakes May Be Triggered by Groundwater Pumping, Research in California Suggests

Researchers from Western Washington University, University of Ottawa, University of Nevada, Reno, and UC Berkeley discovered that human activity might be a major cause for the number and rate of Earthquakes in Central California.

Their study, "Uplift and seismicity driven by groundwater depletion in central California," was published on Wednesday in the journal Nature. The rising of Sierra Nevada and other central coast ranges in California has stymied scientists for years. They haven't been able to definitively figure out why there were an increasing number of earthquakes.

"The volume of groundwater lost over the past century and a half also represents a substantial reduction in mass and a large-scale unburdening of the lithosphere, with significant but unexplored potential impacts on crustal deformation and seismicity," reads the study's abstract.

The scientists used new GPS data of California's Central Valley and found that the mountains closest to the valley were growing at a rate of 1 to 3 millimeters per year, which is, unexpectedly, a faster-than-usual rate. They found that this was because of the loss of groundwater in the Central Valley. Groundwater is very heavy, and when removed, the Earth springs upward and reduces the forces that are keeping the fault clamped together, leading to more small earthquakes.

The glaring evidence points to the fact that groundwater has been slowly exhausted in the Central Valley since the mid-1800s. Since then, the valley has lost 38 cubic miles of water due to irrigation and other water supply. The U.S. Geological Survey says that the Central Valley is the nation's second-most-pumped aquifer system in the United States, contributing to about 20% of the nation's groundwater.

Colin B. Amos of Washington University is the lead author of the study, and he believes that the earthquakes striking the area (on the San Andreas fault) are a result of human activity. Not only that, but the activity is likely to also affect long-term seismicity rates as well as tectonic or mantle-derived forces.

The number of small earthquakes in the Parkfield area of Central California have nearly doubled each year between 1984 and 2005 - a statistic that supports the scientists' findings.