SCEC Award Number 22082 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Documenting the formation, frictional properties, and slip history of shallow fault damage with natural and experimental mixed hematite-clay faults
Name Organization
Alexis Ault Utah State University Greg Hirth Brown University
Other Participants Alexandra (Alex) DiMonte (graduate student; Utah State University); Dr. Cameron Meyers (Scientific Research Engineer and Lab Manager; Brown University)
SCEC Priorities 2c, 3d, 3g SCEC Groups Geology, FARM, SDOT
Report Due Date 03/15/2023 Date Report Submitted 06/23/2023
Project Abstract
We investigate the frictional behavior of mixed clay-hematite faults, critical for characterizing shallow fault damage and deformation processes along the southern San Andreas fault (SSAF). We report field observations and fault rock mineralogy; frictional properties and rate-and-state slip behavior from rotary shear experiments of unconsolidated gouge derived from hematite slip surfaces and SSAF red clay gouge; and scanning electron microscopy (SEM) of natural and experimental faults. The Coachella Valley segment of the SSAF is delineated by Fe- and Mn-oxide-rich, sandstone phacoid-bearing, clay gouge with internal, indurated slip surfaces. Exhumed Mecca Hills basement faults, including the Painted Canyon fault (PCF) are comprise brown metallic gouge, white and green cataclasite, and hematite damage zone slip surfaces. SEM and energy dispersive spectroscopy show SSAF gouge at the mouth of Painted Canyon comprises clay in a scaly fabric with microscale quartz and feldspar clasts and metallic Mn-oxide patches. PCF clay gouge exhibits local euhedral, neo-formed hematite nanoplatelets. Bulk and <2 μm size-fraction X-ray diffraction data indicate SSAF and PCF clays are primarily illite with lesser smectite, kaolinite, and minor chlorite, making them suitable targets for on-going K-Ar geochronology. Gouge deformation experiments were conducted at room temperature and humidity; 5, 8.5 and 10 MPa; 1-1.5 cm displacement; and 1 μm/s-1 mm/s velocity. SSAF and hematite gouge yield coefficients of friction of ~0.58 ± 0.07 and ~0.73 ± 0.06, respectively, and both exhibit dominantly velocity-strengthening behavior at these conditions. Results suggest these materials could inhibit propagation of seismic slip at depths shallower than the groundwater table.
Intellectual Merit Developing probabilistic earthquake hazard maps for southern California requires integration of community-derived models for crustal rheology, thermal structure, stress, velocity, deformation rates, and fault geometries. Some of these models are underpinned by inferences about the distribution of slip, mechanical behavior, and thermal properties of the shallow crust that collectively influence how earthquake energy is distributed. Models of on and off-fault deformation require direct observations of the shallow portions of fault zones above the seismogenic zone. To accomplish this, this SCEC project (1) trades space for time and examines shallowly exhumed fault damage to characterize the rheology and distribution of past slip and (2) compares natural faults with controlled laboratory deformation experiments to benchmark fault friction. Investigation of common San Andreas fault zone minerals, such as clay and hematite, at shallow fault conditions is critical for documenting how these phases form and deform throughout the earthquake cycle.
Our research plan directly addresses the SCEC5 2022 Science Plan’s research objectives. (Q2) Off-fault inelastic deformation impacts on strain accumulation, radiated seismic energy: Frictional properties derived from experiments of hematite-clay fault gouge, together with comparative textures from natural and experimental faults, have the potential to constrain the creation and evolution of past fault damage and how earthquake energy is distributed in the shallow crust today (P2c). (Q3) Evolving structure, composition, and physical fault zone properties impact seismic and aseismic slip: These data help inform how the mineralogy, structure, and friction govern evolving slip stability along the SSAF (P3d). Realistic characterization of near-surface mechanical properties is critical for reconciling geodetic and seismological estimates of fault slip at depth with offset at the surface (P3g).
Broader Impacts This SCEC project continues the successful collaboration between a female scientist at Utah State University (Ault) and scientist at Brown University (Hirth) and the enhanced research and educational connections between all PIs and participants. Research activities and SCEC funding support the continued research and training of female USU PhD student Alexandra (Alex) DiMonte. This project also provides support for Senior Research Technician Dr. Cameron Meyers (Brown), who mentored DiMonte on operation of the Instron rotary shear apparatus at Brown University. In addition to supporting the USU Microscopy Core Facility, which houses the field emission scanning electron microscope, this project has also resulted in method development for friction studies. The new approach developed by DiMonte and Meyers to analyze unconsolidated gouge on the Instron rotary shear apparatus is transportable to future frictional studies of homogeneous or heterogeneous gouge.
Exemplary Figure Figure 5. Frictional properties and velocity-dependent slip behavior of SSAF red clay gouge (A-C) and hematite gouge (D-F) from Instron rotary shear apparatus deformation experiments conducted by USU PhD student Alexandra DiMonte at Brown University. A, D. Examples of a single-velocity deformation experiments of SSAF and hematite gouge showing coefficient of friction (µ) as a function of displacement. B, E. Friction coefficient as function of slip velocity for different normal stresses. C,F. Calculated values of a-b as a function of final velocity from steps in velocity-step experiments.SSAF and hematite gouge yield coefficients of friction of ~0.58 ± 0.07 and ~0.73 ± 0.06, respectively, and both exhibit dominantly velocity-strengthening behavior at these conditions.