Seasonal and long-term crustal stress modulation due to aquifer compaction and groundwater unloading during the 2007-2010 drought in California

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

Submitted August 1, 2018, SCEC Contribution #8245, 2018 SCEC Annual Meeting Poster #135

Fluctuations in terrestrial water storage (TWS) cause deformation of Earth’s crust. In regions with large seasonal TWS oscillations, subsidence is observed in the wet seasons and uplift in the dry, as the lithosphere responds elastically to changes in load. In California’s Central Valley, groundwater storage maintains natural seasonal fluctuations, but also experiences long-term loss that is exacerbated by periods of intense drought. This loss has resulted in compaction of aquifer grains, causing permanent reductions in storage and hazardous ground fissures. In contrast to the regional elastic response, the local poroelastic response to changes in groundwater storage shows uplift when the aquifer is experiencing recharge and subsidence when storage is reduced. Here, we use vertical land motion measurements obtained through multitemporal interferometric processing of large datasets of SAR images acquired by the ALOS L-Band satellite over the Central Valley during 2007-2010 drought to study both the regional elastic loading response and local volumetric strain caused by groundwater storage change both seasonally and over the long-term. Using a first order 1D poroelastic model, we relate the observed vertical land motion to the volume of annual groundwater loss. Next, the groundwater volume loss is used in an elastic loading model to calculate corresponding vertical displacement at the locations of GPS stations and are compared to true vertical GPS displacements. We find that at most sites, groundwater unloading contributes less than 20 percent of vertical displacement, indicating GPS-based TWS estimates likely lack part of the groundwater component. We also examine the impact of aquifer compaction and elastic groundwater unloading on the stress field at the surface and at seismogenic depth. We find that the combined elastic and hydrodynamic response to groundwater loss on Coulomb stress has an effect on both seasonal and long-term stressing rates along faults in California. Comparing the tensile stress obtained from aquifer compaction modeling with the rock tensile strength, we also identify areas susceptible to surface fissures. This study highlights the importance of large-scale, high-resolution vertical land motion measurements in evaluating aquifer system dynamics and hazards associated with overdraft and how we can use these tools to better understand the influence of non-tectonic deformation on seasonal stress change and long-term stressing rates.

Citation
Carlson, G., Shirzaei, M., Ojha, C., & Werth, S. (2018, 08). Seasonal and long-term crustal stress modulation due to aquifer compaction and groundwater unloading during the 2007-2010 drought in California. Poster Presentation at 2018 SCEC Annual Meeting.


Related Projects & Working Groups
Tectonic Geodesy