Groundwater use in California’s San Joaquin Valley exceeds replenishment of the aquifer, leading to substantial diminution of this resource1,2,3,4 and rapid subsidence of the valley floor5. 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. Here we use vertical global positioning system measurements to show that a broad zone of rock uplift of up to 1–3 mm per year surrounds the southern San Joaquin Valley. The observed uplift matches well with predicted flexure from a simple elastic model of current rates of water-storage loss, most of which is caused by groundwater depletion3. The height of the adjacent central Coast Ranges and the Sierra Nevada is strongly seasonal and peaks during the dry late summer and autumn, out of phase with uplift of the valley floor during wetter months. Our results suggest that long-term and late-summer flexural uplift of the Coast Ranges reduce the effective normal stress resolved on the San Andreas Fault. This process brings the fault closer to failure, thereby providing a viable mechanism for observed seasonality in microseismicity at Parkfield6 and potentially affecting long-term seismicity rates for fault systems adjacent to the valley. We also infer that the observed contemporary uplift of the southern Sierra Nevada previously attributed to tectonic or mantle-derived forces7,8,9,10 is partly a consequence of human-caused groundwater depletion.
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Funding for this work comes from NSF EarthScope award number EAR-1252210 to G.B. and W.C.H. GPS data were collected using the EarthScope Plate Boundary Observatory, SCIGN, BARGEN, BARD, CORS and IGS networks. We are particularly grateful to UNAVCO for operating the vast majority of GPS stations used in this project. GPS data were processed using the GIPSY OASIS II software and data products from the Jet Propulsion Laboratory.
The authors declare no competing financial interests.
Extended data figures and tables
Map of vertical uplift rates spanning California and the western Great Basin, including stations within the Central Valley groundwater basin. El., elevation; GPS vert., GPS vertical velocity.
The top panel shows a histogram of the duration of GPS time series. 96% are longer than 4.0 years. The bottom panel shows a histogram of the percentage of complete time series. 92% are greater than 80% complete.
Example vertical GPS time series for stations P056 and P570. Blue dots are daily vertical position solutions, red lines are the model estimated to represent the time series. The station P056 is in the southern Central Valley near Porterville, California. The station P570 is near Weldon, California, east of Lake Isabella.
a, Model setup, where is the line-load rate, a is the load half-width, and θ1 and θ2 measure the angle downward from the positive x axis to any point (x0, z0) at depth. and give the stress rates and Coulomb stress rates, respectively. b, c, Stress rates at depths of 5 km (b) and 15 km (c) calculated from the long-term rate of unloading over the San Joaquin Valley with a = 30 km. Vertical dashed lines labelled SAF and CF represent the locations of the San Andreas Fault and the Coalinga blind thrust faults (including the Nuñez fault), respectively, relative to the load centre. Black and coloured lines indicate stress components and Coulomb stress changes, respectively. The blue curves labelled N show Coulomb stress calculations for favourably oriented faults with a 65° dip, representing the Nuñez fault. For the red curve representing the Coalinga fault (CF), the calculations are performed using a dip of 30° for unfavourably oriented faults. The green curve represents unclamping stress for vertically dipping faults such as the San Andreas Fault. d, e, Stress changes at depths of 5 km and 15 km from seasonal (peak-to-peak) load changes over the San Joaquin River Basin with a = 100 km (full width of 200 km). The position of the San Andreas Fault and Coalinga faults reflects displacement of the load centre by 30 km relative to the long-term load. Varying the load half-width and the load centre by ±10 km has only a small impact (<0.5 kPa) on the resolved seasonal stress changes on the faults.
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Amos, C., Audet, P., Hammond, W. et al. Uplift and seismicity driven by groundwater depletion in central California. Nature 509, 483–486 (2014). https://doi.org/10.1038/nature13275
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