SCEC Award Number 16006 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Seasonal stress modulation on active California fault structures
Investigator(s)
Name Organization
Roland Bürgmann University of California, Berkeley
Other Participants Chris Johnson (Ph.D. student)
Pierre Dutilleul (unpaid international collaborator, McGill University)
SCEC Priorities 2b, 2d, 2f SCEC Groups Seismology, Geodesy, SDOT
Report Due Date 03/15/2017 Date Report Submitted 07/04/2017
Project Abstract
 In California, the accumulation of winter snowpack in the Sierra Nevada, water in lakes and reservoirs, and groundwater in basins follow the annual cycle of wet winters and dry summers. These seasonal changes in water storage produce elastic deformation of the Earth’s crust and micro-earthquakes appear to follow a subtle annual cycle, possibly in response to the water load. Previous studies posit that temperature, atmospheric pressure, or hydrologic changes may promote additional earthquakes above background levels. Here we use GPS vertical time series (2006 – 2015) to constrain models of monthly hydrospheric loading and compute stresses on faults throughout California, which can exceed 1kPa. Depending on fault geometry the addition or removal of water increases the Coulomb failure stress. The largest stress amplitudes are occurring on dipping reverse faults in the Coast Ranges and along the eastern Sierra Nevada range front. We analyze M≥2.0 earthquakes with known focal mechanisms in northern and central California to resolve fault normal and shear stresses for the focal geometry. Our results reveal more earthquakes occurring during slip-encouraging stress conditions and suggest that earthquake populations are modulated at periods of natural loading cycles, which promote failure by subtle stress changes. The most notable shear-stress change occurs on more shallowly dipping structures. However, vertically dipping strike-slip faults are common throughout California and experience smaller amplitude stress change but still exhibit positive correlation with seasonal loading cycles. Our analysis suggests the annual hydrologic cycle is a viable mechanism to promote earthquakes and provides new insight to fault mechanical properties.
Intellectual Merit We directly address the SCEC4 Research Priorities and Requirements by exploring the response of seismicity to a well-characterized periodic forcing function. While hydrological loads are not explicitly mentioned, this work addresses several of the problems in earthquake physics called out by SCEC4 including stress transfer, stress-mediated fault interactions and earthquake clustering, and causes and effects of transient deformations. We rely on CSM andthe CFM products, 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. Seasonal average peak-to-peak (A) shear and (B) normal stress changes resolved on the UCERF3 California statewide fault model. (C) The Coulomb stress using a friction coefficient of 0.1. (D) The Coulomb stress using a friction coefficient of 0.7. (E) The times series of Coulomb stress changes on the central San Andreas fault for friction coefficients of 0.0, 0.1, 0.4, 0.7. (F) The central San Andreas Fault estimated annual stress (dark gray line) with one standard deviation (grad shading) for the hydrological induced Coulomb (µ=0.4) stress change (blue line). Modified from Johnson et al. (2017).
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