Stress models of the annual hydrospheric, atmospheric, thermal, and tidal loading cycles on California faults: Perturbation of background stress and changes in seismicity

Christopher W. Johnson, Yuning Fu, & Roland Bürgmann

Submitted August 3, 2017, SCEC Contribution #7395, 2017 SCEC Annual Meeting Poster #206

Deformation of the lithosphere arises from multiple natural loading sources that include both surface and body forces. The largest surface loads include near-surface water storage, atmosphere pressure, ocean loading, and temperature changes. The solid earth is also deformed by celestial body interactions and variations in the earth’s rotation rate. We model the deformation in California from 2006 through 2014 for seven different loading sources with annual periods to produce an aggregate stressing history for the study area. Our modeling efforts show that the pole-tides, atmosphere, temperature, and water loading cycles should be evaluated to fully describe seasonal stress changes. Time series analysis indicates the hydrological loads and earth pole-tides are the largest source of seasonal stress changes. The in-phase nature of these two loads constructively interferes and contributes to the largest resolvable stresses for many locations. We explore the seasonally induced stresses with respect to the background principal stress orientation and the timing of seismicity. The long-term background stress orientation is resolved using >30 years of focal mechanism data. There is no resolvable seasonal variation to the ambient stress orientation in the crust. Nine years of M≥1.7 seismicity reveals the timing of earthquakes modestly correlates with a ≤5 kPa seasonal normal-stress perturbation in the local orientation of the maximum compressional background stress direction. The results suggest faults in California are optimally oriented with the background stress field and at any given time a population of faults are near failure as evident from earthquakes triggered by these slight seasonal stress perturbations.

Citation
Johnson, C. W., Fu, Y., & Bürgmann, R. (2017, 08). Stress models of the annual hydrospheric, atmospheric, thermal, and tidal loading cycles on California faults: Perturbation of background stress and changes in seismicity. Poster Presentation at 2017 SCEC Annual Meeting.


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