Groundwater Unloading in the Central Valley Modifies Crustal Stress

Grace Carlson, Manoochehr Shirzaei, Susanna Werth, Guang Zhai, & Chandrakanta Ojha

Published August 14, 2019, SCEC Contribution #9665, 2019 SCEC Annual Meeting Poster #200

The earth responds elastically to changes in surface mass such that when mass is added there is regional sinking of the land surface and when mass is lost there is regional uplift. These mass changes can disturb the regional stress field, which in turn, may influence earthquake activity. The pattern of these disturbances, though, is controlled by the weight and area of the mass that is applied. Here, we consider the mass loss and gain associated with groundwater withdrawal and recharge in California’s Central Valley, and also test three different load distribution patterns over the Central Valley aquifer system. We separate vertical land motion data obtained using InSAR directly above the aquifer into trend and seasonal components in order to isolate groundwater storage from total terrestrial water storage. In isolating the groundwater component, we show to what extent human pumping activities on both seasonal and long-term timescales are disturbing crustal stress and possibly influencing seismic hazard. We find that in the southern Central Valley, intense pumping activity and return flows from excess irrigation water cause a phase lag in the seasonal deformation signal when compared to the northern Central Valley, where recharge and discharge are primarily driven by natural sources. We also find that the stress change rate ranges from ~-5 to 5 kPa/yr along the San Andreas fault with seasonal fluctuations of up to ~7 kPa. We further show that the spatial resolution of groundwater volume change estimates is a crucial factor for stress calculation and particularly affects the calculated normal stress change rate. Finally, we find that periods of increased unclamping stress correlate with periods of increased seismicity, while periods of increased shear stress correlate with higher than average seismic moment release, indicating groundwater loading likely contributes to the modulation of seismicity in California.

Citation
Carlson, G., Shirzaei, M., Werth, S., Zhai, G., & Ojha, C. (2019, 08). Groundwater Unloading in the Central Valley Modifies Crustal Stress. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Stress and Deformation Over Time (SDOT)