Uplift and Seismicity Driven by Groundwater Depletion in Central California

Colin Amos, Western Washington University
Pascal Audet, University of Ottawa
William Hammond, Nevada Geodetic Laboratory, Nevada Bureau of Mines and Geology, and Nevada Seismological Laboratory
Roland Burgmann, University of California, Berkeley
Ingrid Johanson, Berkeley Seismological Laboratory

Groundwater use in California's San Joaquin Valley exceeds replenishment of the aquifer, leading to substantial diminution of this resource and rapid subsidence of the valley floor. 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 GPS measurements to show that a broad zone of rock uplift up to 1 – 3 mm/yr 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, the majority of which is caused by groundwater depletion. Height of the adjacent central Coast Ranges and Sierra Nevada is strongly seasonal and peaks during the dry late summer and fall, out of phase with inflation 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 Parkfield and potentially affecting long-term seismicity rates for fault systems adjacent to the valley. We also infer that observed contemporary uplift of the southern Sierra Nevada previously attributed to tectonic and/or mantle derived forces is partly a consequence of human-caused groundwater depletion.