SCEC Award Number 15005 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Seasonal Loading, Deformation and Seismicity in California
Investigator(s)
Name Organization
Roland Bürgmann University of California, Berkeley
Other Participants Chris Johnson (PhD Student, UC Berkeley), Pierre Dutilleul (collaborator, McGill University)
SCEC Priorities 2b, 2c, 2d SCEC Groups Seismology, Geodesy, SDOT
Report Due Date 03/15/2016 Date Report Submitted 03/07/2016
Project Abstract
 Seasonal crustal stress variations result from hydrospheric loads, pore pressure fluctuations, and temperature gradients. In California, these processes follow an annual periodic cycle and each contributes to the crustal deformation. The near surface hydrological mass changes can result in stresses >1kPa at seismogenic depths, a small but comparable stress perturbation that is associated with rate changes in seismicity. Presented herein are results for a multi-year project plan to test the sensitivity of earthquake occurrence with respect to small external stressing due to hydrospheric loading. We examine 26 years of M≥2.5 catalog data and find statistically significant periods of about 4-, 6-, and 12-months for seismicity located near Parkfield and a 12-month significant period in the Sierra Nevada –ECSZ. The modulation of seismicity with a period of one year supports our hypothesis of hydrospheric seasonal loading and warrants our continued efforts to model the seasonal deformation associated with these time varying loads. We model the deformation using the near-surface water storage derived from vertical GPS displacements. Our results suggest a seasonally varying stress on the central SAF with a peak-to-peak amplitude of ~0.7 kPa. Currently, we are evaluating the degree of correlation between model stress cycles and 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. Our initial results from the seismicity analysis suggest dipping faults in the Coast Ranges are more sensitive to the changing loads.
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 and 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 Velocity 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 3 (A) Seasonal Coulomb stress resolved on the SCEC Community Fault Model. This one-month snapshot indicates a peak stress change of 1.5 kPa during the fall months in 2009. (B) Representative time series of seasonally modulated Coulomb stress on the central SAF indicate a peak-to-peak stress cycle of ~700 Pa. (C) Faulting mechanism resolved from 1984-2014 M≥2.0 focal mechanisms for area outlined in the map. Distributions indicate strike-slip and reverse events occur more often in the spring and fall months.