UPLIFT AND SEISMICITY DRIVEN BY GROUNDWATER DEPLETION IN CENTRAL CALIFORNIA

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 (~160 km³) 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 3 mm yr^-1 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 constrained by GRACE satellite data, 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 modeling suggests that long-term and late-summer flexural uplift of the Coast Ranges also affects stresses on faults paralleling the San Joaquin Valley. Estimated Coulomb stress evolution on the San Andreas Fault totals 1-2 kPa per decade, with seasonal variations of ~1 kPa at seismogenic depths. The seasonal stress change provides a viable mechanism for observed seasonality in microseismicity at Parkfield, and the trend potentially affects 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. We are currently exploring constraints from seasonal and interannual vertical motion and a more realistic viscoelastic Earth model to estimate spatial and temporal patterns of groundwater unloading, as well as potential impacts on apparent fault slip rates estimated with horizontal GPS.