Seasonal Water Storage, the Resulting Deformation and Stress, and Occurrence of Earthquakes in California

Abstract:

In California the accumulated winter snow pack in the Sierra Nevada, reservoirs and groundwater water storage in the Central Valley follow an annual periodic cycle and each contribute to the resulting surface deformation, which can be observed using GPS time series. The ongoing drought conditions in the western U.S. amplify the observed uplift signal as the Earth’s crust responds to the mass changes associated with the water loss. The near surface hydrological mass loss can result in annual stress changes of ~1kPa at seismogenic depths. Similarly, small static stress perturbations have previously been associated with changes in earthquake activity. Periodicity analysis of earthquake catalog time series suggest that periods of 4-, 6-, 12-, and 14.24-months are statistically significant in regions of California, and provide documentation for the modulation of earthquake populations at periods of natural loading cycles. Knowledge of what governs the timing of earthquakes is essential to understanding the nature of the earthquake cycle. If small static stress changes influence the timing of earthquakes, then one could expect that events will occur more rapidly during periods of greater external load increases. To test this hypothesis we develop a loading model using GPS derived surface water storage for California and calculate the stress change at seismogenic depths for different faulting geometries. We then evaluate the degree of correlation between the stress models and the seismicity taking into consideration the variable amplitude of stress cycles, the orientation of transient load stress with respect to the background stress field, and the geometry of active faults revealed by focal mechanisms.