Mountains are on the move in the San Joaquin valley in California, with valley floors subsiding and the mountains rising. New research published in Nature suggests that human activity, particularly depletion of groundwater for irrigation, may be contributing to increased seismic activity. In the long run, this could mean an increase the frequency of small earthquakes. California is important agricultural land, and groundwater use has increased considerably since the late 1800s; some scientists estimate that up to twice as much water is being extracted as would naturally be replenished by rain and snow.
California is also home to many fault lines including the San Andreas. Groundwater plays an important role in shaping the landscape, adding weight to the earth’s crust. When this weight is removed, the land flexes upwards. Previous studies have shown that seasonal oscillations in groundwater levels cause natural topographical changes, and these movements in the lithosphere contribute to pressure on fault lines, potentially leading to increases in the frequency and magnitude of earthquakes. This finding is of particular interest given the high seismic in The Golden State, the site of the deadly 1906 earthquake.
For several years, satellites have collected high resolution vertical GPS data, mapping the topology of the landscape in the Western USA for surveillance purposes. Based on the hypothesis that both rainfall and groundwater depletion for irrigation impact movements in the crust and surface, a team of researchers led by Geoffrey Blewitt and the University of Nevada modelled the expected flexes in the lithosphere and used GPS data to test their results. The match was significant, indicating that human activity is a part of recent years’ changes. They also realised that a neighbouring area, Sierra Nevada, seems to have been affected in a similar way.
The researchers now hope to expand high resolution GPS coverage worldwide to facilitate data analysis and modelling of other high risk areas. This technique may become increasingly important as climate change affects rainfall levels and growing populations deplete groundwater stocks.
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