Contemporary vertical uplift and modulation of seismicity due to groundwater removal in the southern San Joaquin Valley of California

GPS stations surrounding the southern San Joaquin Valley in California reveal a broad pattern of uplift concentrated along the basin margins. Peak vertical velocities reach values up to 1 - 3 mm/yr in the adjacent central Coast Range and southern Sierra Nevada, relative to the comparatively stable Great Basin to the east. We use a simple elastic model to explore the vertical response to unloading driven by sustained, anthropogenic groundwater depletion in the southern San Joaquin Valley as a possible driver of contemporary uplift. The model uses a line load centered along the valley axis, a range of elastic parameters, and published estimates of the integrated rate of mass loss due to groundwater removal over the last decade. Predicted uplift centered along the valley axis matches well with patterns of GPS motion, with the upward vertical rates decaying away from the valley margins. Observed seasonal variability in the vertical GPS positions lends support for this model, showing peak uplift for stations surrounding the valley during the dry summer and fall months. On the other hand, stations in the San Joaquin Valley show larger seasonal uplift accompanying aquifer recharge during winter months. As a consequence of net uplift in the Coast Range to the west, the San Andreas Fault experiences unclamping, or a reduction in the effective normal stress, given its orientation parallel to the valley axis. Seasonal unloading due to groundwater removal therefore may provide a viable mechanism to explain previously observed peaks in seismicity during dry months along this portion of the fault, and perhaps pushes the fault closer to failure. Our results also suggest that the signal of vertical flexural uplift from groundwater unloading overlaps with the possible signal of modest, contemporary uplift of the southern Sierra Nevada from tectonic and/or mantle flow derived forces. Thus, at least some of the signal previously interpreted as tectonic uplift may be a consequence of human-caused groundwater changes in the adjacent San Joaquin Valley.