Monitoring Seasonally-Driven Stress Changes on Faults within the Plate Boundary Zone in California using cGPS Observations

Jeonghyeop Kim, Alireza Bahadori, & William E. Holt

Submitted August 15, 2018, SCEC Contribution #8733, 2018 SCEC Annual Meeting Poster #156

Using horizontal continuous GPS data between 2007 and 2018, we have quantified time-dependent horizontal transient strains within the Plate Boundary Zone in California. We also determine associated Coulomb stress changes on existing fault structures through time. These long-wavelength anomalies highlight remarkable seasonal periodic motions throughout much of the entire Great Valley and Sierra Nevada over the eleven-year time interval. We have detected a seasonal positive dilatational strain and Coulomb stress transient along the San Andreas fault zone that occurs between 34° N – 37° N during most summers. The Great Valley and Sierra Nevada also experience dilatational strains during most summers and displace a total of 1-2 mm toward the Great Basin. During winter, the dilatational strain patterns typically reverse. The Great Valley and Sierra Nevada enter compression and move 1 – 2 mm westward and a zone of dilatational compression develops along much of the San Andreas fault. Negative Coulomb stress changes are also shown on much of the San Andreas fault during most winters. To investigate our hypothesis that these horizontal long-wavelength anomaly patterns are related to seasonal hydrologic loading, we analyze the UNAVCO surface-loading displacement models derived from NLDAS-Noah (Puskas et al, 2017). We produce the strain and displacement patterns predicted by the hydrologic predictions. Comparisons of predictions from this model show similar spatial and temporal correlations with the long-wavelength anomaly patterns inferred from cGPS. However, evident disagreements appear during the severe drought (2012-2016). The solution inferred from cGPS shows significantly diminished winter signals and augmented summer patterns. We infer that these patterns are due to the lower amounts of precipitation during the drought winter and more loss of water during the drought summer. The solution inferred from UNAVCO hydrologic model fails to predict the effect of drought because the model only represents the variation in loading near the surface. This implies that either ground water or deep soil moistures are mainly affected by the drought. We also analyze a model of variations in water storage inferred from the vertical GPS displacements (Argus et al, 2017) and we compare the horizontal field predicted by this model with our horizontal long-wavelength anomaly patterns.

Citation
Kim, J., Bahadori, A., & Holt, W. E. (2018, 08). Monitoring Seasonally-Driven Stress Changes on Faults within the Plate Boundary Zone in California using cGPS Observations. Poster Presentation at 2018 SCEC Annual Meeting.


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