SCEC Award Number 18189 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Collaborative Research: The role of mineralogy and fabric in the frictional properties of the shallow San Andreas fault
Name Organization
Heather Savage Columbia University Christie Rowe McGill University (Canada) Randolph Williams McGill University (Canada)
Other Participants
SCEC Priorities 2c, 3c, 3d SCEC Groups FARM, SAFS, Geology
Report Due Date 03/15/2019 Date Report Submitted 03/15/2019
Project Abstract
The main goal of this project is to provide a more complete assessment of the relationship between fault rock mineralogy, fabric, and friction in the San Andreas fault (SAF), with a particular emphasis on improving our understanding of rupture propagation through the uppermost crust. We are studying fault rock from recently acquired drill core across the Mojave segment of the SAF and from surface exposures of small subsidiary faults. More specifically, we are analyzing the geochemistry, mineralogy, and microstructure of these fault rocks, and pairing those results with triaxial friction tests of both intact and reconstituted samples. Core samples were collected as part of a recent Los Angeles Department of Water and Power effort to assess seismic hazards in the area of Elizabeth Lake, CA. Our microstructural, mineralogical, and geochemical results indicate that neither mechanical or geochemical evolution of fault rock can be considered in isolation. Upcoming friction tests and further comparative analysis will seek to understand the positive feedbacks between these mechanisms as they affect the frictional stability and strength of fault rocks in the uppermost crust. We hypothesize that this approach will reveal a non-monotonic weakening path throughout fault-rock evolution, and that exceeding certain threshold values of cataclasis/grain-size reduction, clay formation, and fabric development in fault rocks will result in large changes in frictional stability and strength. Ultimately, these combined data sets will constrain the controls on the frictional properties of a locked seismogenic fault in the shallow most crust.
Intellectual Merit This study is examining newly acquired drill core from a locked portion of the San Andreas fault in the upper most crust. To date our results have begun to constrain the mechanical and geochemical processes of fault-rock formation, which are first-order controls on fault friction. These results contribute to several key questions listed by the SCEC Science Plan, including: P3.c Assess how shear resistance and energy dissipation depend on the maturity of the fault system, and how these area expressed geologically; P3.d - Determine how damage zones, crack healing and cementation, fault zone mienralogy, and off-fault plasticity govern strain localization, the stability of slip (creeping vs. locked), interseismic strength recovery, and rupture propagation; and P3.f Study the mechanical and chemical effects of fluid flows, both natural and anthropogenic, on faulting and earthquake occurrence, and how they vary through the earthquake cycle. Moreover, seismic processes at shallow depths ranges are relatively poorly understood, despite their importance in a variety of earthquake processes (e.g. rupture arrest and seismic wave dampening). As such, our research also addresses P3.g - Assess the importance of the mechanical properties of the near-surface in the commensurability of geodetic and seismological images of fault slip at depth with fault offset expressed at the surface.
Broader Impacts This project provided analytical costs for an early-career researcher, Randy Williams, in pursuit of his post-doctoral research at McGill University. In addition, Williams worked closely with an undergraduate research assistant, Erin Eves, on a variety of sample preparation and analysis procedures, allowing her to obtain critical research experience in her chosen career path of geosciences. Moreover, this research has strengthened ties between three university research institutions (Columbia, McGill, and Utah State), the United States Geological Survey, and the Los Angeles Department of Water and Power. We anticipate that the results of our study will greatly improve our understanding of the evolving mechanical properties of faults in the upper most crust. As seismic processes at shallow depths include rupture arrest and seismic wave dampening, our work is contributing to a fundamental (but understudied) area of fault mechanics and as it relates to a significant seismic hazard in southern California.
Exemplary Figure Figure 3. a) Bulk-rock XRD spectra for protolith and gouge. b) Gouge XRD results for <4 µm and <1 µm grain-size fractions after glycolation. c) Elemental MgO as a function of SiO2 with 95% confidence ellipses. d) Same as (c), but plotting total Fe concentration. e) Sample loss on ignition as a function of MgO concentration. f) Bootstrapped probability distributions showing elemental enrichment Mg and Fe in fault rocks relative to protolith.