Stress Changes on the San Andreas Fault due to Groundwater Fluctuations in the Central Valley

Molly Zebker, David T. Sandwell, Adrian A. Borsa, & Yehuda Bock

Submitted September 7, 2025, SCEC Contribution #14824, 2025 SCEC Annual Meeting Poster #TBD

Several studies have proposed that lithospheric unloading of the Central Valley due to extensive groundwater withdrawal over the past 150 years causes a rebound of the lithosphere that can be observed in GNSS and InSAR data [Amos et al., 2014; Lundgren et al., 2022]. This long-term uplift is modulated by seasonal hydrologic loading also observed in GNSS data [Johnson & Burgmann, 2017]. The earth response is viscoelastic -- an elastic response from the seasonal loading and a larger viscoelastic response from the 150-year unloading. Both types of response alter the crustal stress on nearby faults and thus may modulate the seismicity and perhaps even the earthquake cycle.

We are revisiting the effects of hydrologic loading on the San Andreas fault (SAF) from both modeling and observational perspectives. We use a semi-analytic model consisting of an elastic plate over a viscoelastic half space to rapidly calculate the 3-D surface deformation caused by both long-term and seasonal loading. The model also calculates the time-dependent Coulomb-stress changes. The model is driven by USGS groundwater storage change estimates from (1860-2003) as well as seasonal loading changes from more recent hydrologic and geodetic measurements. Initial results of ~3 mm/yr uplift rates on the perimeter of the Central Valley are consistent with a viscoelastic model having a 30 km thick elastic plate over a half space with a viscosity of 1e20 Pa*s and in basic agreement with the Lundgren et al., [2022] analysis.

Furthermore, a 10-year, high spatial resolution surface deformation model is being developed by combining sparse GNSS displacement time series from the SOPAC daily analysis with dense InSAR data from the C-band Sentinel-1 and L-band ALOS-2 missions. Preliminary results show up to 1.7 cm/yr and 2.2 cm/yr LOS displacement using Sentinel-1 and ALOS-2, respectively, along the SAF. The 10-year time series represents the elastic response from recent groundwater loading and slip distribution and can be used to infer stress changes on the fault.

Amos, C., et al. (2014). Uplift and seismicity driven by groundwater depletion in central California. Nature, 509(7501), 483–486.
Lundgren, P., et al. (2022). San Andreas Fault stress change due to groundwater withdrawal in California’s Central Valley, 1860–2010. Geophysical Research Letters, 49.
Johnson, C., & Bürgmann, R. (2017). Seasonal water storage, stress modulation, and California seismicity. Science, 356(6340), 1161–1164.

Citation
Zebker, M., Sandwell, D. T., Borsa, A. A., & Bock, Y. (2025, 09). Stress Changes on the San Andreas Fault due to Groundwater Fluctuations in the Central Valley. Poster Presentation at 2025 SCEC Annual Meeting.

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