SCEC Award Number 17059 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Seasonal stress modulation on active California fault structures
Name Organization
Roland Bürgmann University of California, Berkeley
Other Participants Chris Johnson
SCEC Priorities 1c, 1e, 1d SCEC Groups SDOT, Seismology
Report Due Date 06/15/2018 Date Report Submitted 05/26/2018
Project Abstract
Stress and deformation in the lithosphere arise from multiple natural loading sources that include both surface and body forces. The largest surface loads include near-surface water storage, snow and ice, atmospheric pressure, ocean loading, and temperature changes. The solid Earth also de-forms from celestial body interactions and variations in Earth’s rotation. We model the seasonal stress changes in California from 2006 through 2014 for seven different loading sources with annual periods to produce an aggregate stressing history for faults in the study area. Our modeling shows that the annual water loading, atmosphere, temperature, and Earth pole-tides are the largest load-ing sources and should each be evaluated to fully describe seasonal stress changes. In California we find the hydrological loads are the largest source of seasonal stresses. We explore the seasonal stresses with respect to the background principal stress orientation, constrained with regional focal mechanisms, and analyze the modulation of seismicity. Our results do not suggest a resolvable sea-sonal variation for the ambient stress orientation in the shallow crust. When projecting the seasonal stresses into the background stress orientation we find the timing of microseismicity modestly in-creases from a ~8 kPa seasonal mean-normal-stress perturbation. The results suggest that faults in California are optimally oriented with the background stress field and respond to subsurface pres-sure changes, possibly due to processes we have not considered in this study. At any time, a popu-lation of faults are near failure as evident from earthquakes triggered by these slight seasonal stress perturbations.
Intellectual Merit We address the SCEC5 Research Priorities and Requirements by exploring the response of seismicity to well-characterized hydrological, tidal, atmospheric, and thermoelastic periodic forcing functions. While these external loading sources are not explicitly mentioned, this work addresses several of the problems in earthquake physics called out by SCEC5 including stress transfer, stress-mediated fault interactions and earthquake clustering, and causes and effects of transient deformations. We rely on the CFM product, our own stress inversions, and catalogs of focal mechanisms to refine our analysis by considering where the transient stress cycles act in parallel with the background stress.
Broader Impacts The seismic hazards of active plate boundary faults will affect more individuals as the population of California continues to increase. This project improves our ability to characterize the time-dependent stresses on active faults in California by investigating the seismicity response to seasonal loading along the plate boundary. An improved understanding of earthquake physics to better characterize the effect of transient loading will lead to improved forecast models in future endeavors.
The project has supported a graduate student and fostered collaborations for that student with more senior members of the SCEC scientific community. The collaborations developed by young scientist is integral to the advancement of new ideas and research directions. The student supported by this project is also actively participating in the SCEC Community Geodetic Model initiative.
Portions of this work are now included in classroom lectures and lab components on topics of active tectonics and structural geology to further the understanding of fault mechanics and fault interaction. Additionally the material is presented to middle school aged students through a graduate student outreach program led by the SCEC supported graduate student. These outreach activities are designed to raise public awareness at a young age and provide a basic understanding of the geologic environment in which they live and what to do in the event of an earthquake.
Exemplary Figure Figure 2. (a) The average annual peak-to-peak Coulomb stress (µ=0.4) change from the 4 largest loads (hydrological, atmosphere, pole-tides, and thermal) is shown on the UCERF3 fault model geometry with (b) the day of year of the peak stress. The aggregate stresses account for the phase differences in the loading cycles and provide a representative description of the seasonal stress change (From Johnson et al., 2017a).

Johnson, C. W., Y. Fu, and R. Bürgmann (2017), Stress models of the annual hydrospheric, atmospheric, thermal, and tidal loading cycles on California faults: Perturbation of background stress and changes in seismicity, Journal of Geophysical Research: Solid Earth, 122.