SEASONAL AND LONG-TERM CRUSTAL STRESS MODULATION DUE TO GROUNDWATER UNLOADING AND AQUIFER COMPACTION DURING THE 2007-2010 DROUGHT IN CALIFORNIA INFERRED FROM INSAR

Terrestrial water storage (TWS) variations cause deformation of Earth’s crust. In areas with large seasonal TWS oscillations, regional subsidence is observed in the wetter months 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, exacerbated by periods of intense drought. This loss has resulted in compaction of aquitard units, causing permanent reduction in storage and hazardous ground fissures. In contrast to the regional elastic response, the local poroelastic response to change in storage is uplift when the aquifer-system is experiencing recharge and subsidence when storage is reduced. We use vertical land motion obtained through multitemporal interferometric processing of large datasets of SAR images acquired by the ALOS L-Band satellite over the Central Valley during the 2007-2010 drought. We relate the observed vertical land motion to the volume of groundwater loss using a first order 1D poroelastic model. The groundwater volume loss is then used in an elastic loading model to calculate corresponding vertical displacement at the locations of GPS stations surrounding the Valley and are compared to true vertical GPS displacements. We find that at most sites, groundwater loading contributes less than 20% of vertical displacement, indicating many GPS stations are unable to pick up the groundwater loading signal. This suggests that GPS-based TWS estimates for the Central Valley may be underestimated. We also use the vertical land motion data to model local volumetric strain caused by aquifer-system compaction over the long-term and seasonally. Comparing the tensile stress obtained from our compaction model with rock tensile strength, we identify areas susceptible to tensile fissures. We then use both the elastic unloading and compaction models to examine Coulomb stress change at seismogenic depth. We observe that the combined elastic and hydrodynamic response to groundwater loss has an effect on both seasonal and long-term stressing rates along faults in California. This study highlights the importance of large-scale, high-resolution vertical land motion measurements in evaluating aquifer-system dynamics and hazards associated with overdraft.