SCEC Award Number 16270 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Textural, Geochemical, and Thermal signatures within the Southern San Andreas Fault: Evidence for Thermally Activated Dynamic Weakening?
Investigator(s)
Name Organization
James Evans Utah State University
Other Participants Kelly Bradbury
Amy Moser
one undergraduate
SCEC Priorities 3b, 3c, 4b SCEC Groups FARM, Geology
Report Due Date 03/15/2017 Date Report Submitted 04/26/2017
Project Abstract
 This study provides direct evidence for thermally activated weakening mechanisms in faults associated with SAFOD and the Southern San Andreas Fault, Mecca Hills, California. Characterization of fault zone structure and composition using a range of field and laboratory-based observations of exhumed fault zones of the San Andreas system and the SAFOD core. To examine slip surfaces, we employ optical and scanning electron microscopy and X-ray Adsorption Near Edge Spectroscopy (XANES) and X-ray Adsorption mapping methods. Focused alteration of Mg- or Fe- rich clays, silica, pyrite, and/or carbonaceous matter are common in both the SAFOD core and exhumed slip surfaces within the Mecca Hills. Spatial variations in elemental distribution are associated with the presence of asperities at the micro-scale.
Intellectual Merit Documenting meter to sub-meter scale heterogeneities within fault zone structure and permeability architecture is important to understanding earthquake cycle. We address the evolution of fault zone structure and composition using a range of field and laboratory-based analyses of exhumed fault zones of the San Andreas system. The spatial distribution and characterization of on-fault geometric complexities (slip surface textures, fault zone fabric, and composition) and off-fault damage variations in rock properties (fracture intensity and composition/alteration variations) are examined. On-fault, evidence for thermal pressurization and diffusion processes are observed whereas off-fault damage elements reflect significant changes in damage zone elasticity. In actively deforming faults, frictional properties evolve with variations in fluid migration pathways and fluid chemistry, and pore fluid pressures (permeability architecture). These properties ultimately influence future rupture propagations and/or arrests as most thermally activated mechanisms result in low coefficients of dynamic friction (Rowe and Griffith, 2015).
Broader Impacts Three undergraduate and 2 graduate students have been involved in working on portions of this project spanning various stages of field and laboratory work, and data analysis. The participating students are all female, including one minority, and are gaining critical technical skills and research experience that will make them excellent future graduate school candidates, academic researchers, or STEM workforce. One Msc. degree has been completed by a graduate student that has worked in the Mecca Hills and whose work has informed the 2016 SCEC project.
Public presentations about Earthquake Geology with a focus on fault rock textures and mineral alteration products were given to the local Utah community at the USU Science Unwrapped Series and the Dept. of Geology’s Rock n’ Fossil Day. Several hands-on activities with our SCEC-related fault rock samples were conducted and ~ 100 or so community members per event, ranging in age from toddlers to senior citizens, were engaged in learning about fault rock deformation processes, fault rocks, and alteration minerals from hand-samples, micro-scale images, or through demonstrations using physical analog models or microscopes. Informal geology lessons were also provided to citizen audiences in Bombay Beach and in the Mecca Hills trails.
Exemplary Figure Figure 7. Sample from finely coated, Mn-rich surface of the SSAF fault zone. Spectra images illustrate the distribution of elemental phases present on the slip surface. Warm colors (red) represent highest abundance, cool (blue) the lowest. Elemental runs (Si, S, Cl, K, Ca, Ti, V, Cr, Mn, and Fe) are listed under each window. Localized, irregular patches of Si occur with other notable increases of S, Cl, K, Ca,Cr, Mn, S, and Fe.