An Eleven Year History of Seasonally-Driven Stress Changes on Faults within the Plate Boundary Zone in California Inferred from cGPS Data

We quantify time-dependent horizontal transient strains inferred from cGPS data between 2007 and 2018 within the Plate Boundary Zone in California. We also calculate 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. In general, during the summer, a zone of dilatation and shear (involving general increase in Coulomb stress) develops along the San Andreas fault zone between 34° N - 37° N. The Great Valley and Sierra Nevada also experience dilatational strains and displace a total of 1-2 mm toward the Great Basin. During winter the patterns reverse: The Great Valley and Sierra Nevada experience compression, move 1-2 mm westward, and much of the San Andreas enters dilatational compression. Negative Coulomb stress changes are shown on much of the San Andreas fault during winter. To investigate our hypothesis that these horizontal long-wavelength anomaly patterns are related to seasonal hydrologic loading, weanalyze the UNAVCO hydrologic models derived from the Noah 0.125˚ grid Land Surface Model for NLDASand produce the strain and displacement patterns predicted by the hydrologic predictions. The solution inferred from UNAVCO hydrologic prediction show significant spatiotemporal similarities tothe long-wavelength anomaly patterns inferred from cGSP. However, evident disagreement appears between 2012 and 2016. During the severe drought (2012-2016), significantly diminished winter signals and augmented summer patterns are shown in the solution inferred from cGPS. 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. However, 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. This research, therefore, provides an important link between seasonal histories of hydrologic loading, including the effect of drought and the variations of stress on faults.