SCEC Award Number 17164 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Detecting asperity flash heating on hematite faults with laboratory experiments and hematite (U-Th)/He thermochronometry
Name Organization
Alexis Ault Utah State University
Other Participants Greg Hirth - Brown University
David L. Goldsby - University of Pennsylvania
Robert G. McDermott - USU PhD candidate
SCEC Priorities 2d, 3d, 1d SCEC Groups FARM, Geology, SDOT
Report Due Date 06/15/2018 Date Report Submitted 06/13/2018
Project Abstract
This goal of this SCEC project is to quantify the role of heat in hematite fault strength evolution during the seismic cycle. We integrate hematite low-to-high velocity, rotary-shear experiments (Instron, Brown University) and (U-Th)/He thermochronometry to quantify temperature, friction, microstructure, and He loss (T proxy) evolution over variable slip displacements and rates. Our workflow requires textural and (U-Th)/He characterization of hematite starting material, deformation experiments, and comparative microscopy and (U-Th)/He dating of experimental products. Scanning electron microscopy (SEM) shows undeformed hematite comprises specularite plates (~2-80 m-thick) with trace interstitial calcite. Hematite (U-Th)/He dates from individual crystals (41-77m-thick) extracted from the polycrystalline aggregate are ~207-284 Ma, are positively correlated with plate thickness, and, importantly, have a sufficient He budget to document He loss during slip. Preliminary experiments conducted at 2-5 MPa normal pressure, 0.085-340 mm/s sliding velocity, 50-600 mm slip distance use hematite and quartzite on the lower and upper plates, respectively. Experimental textures include a 20-100 µm-thick cataclastic band of micron-scale, angular clasts and subrounded nanoparticles. Velocity-step tests at sub-seismic slip rates indicate hematite exhibits a low coefficient of friction (0.25-0.44) and velocity-strengthening behavior at those conditions. High slip velocity test runs yield mm-wide, high-gloss, light-reflective patches analogous to natural hematite fault mirrors. SEM imaging shows these zones comprise sintered nanoparticles with polygonal or lobate grain boundaries. Nano-characterization and thermochronometry of on-going experiments will allow us to link experimental and natural hematite slip surface observables and identify thermally-activated dynamic weakening mechanisms.
Intellectual Merit Documenting fault properties that govern microearthquake physics is a research frontier critical for understanding dynamic rupture and propagation at the intersection of seismology, structural geology, and rock mechanics. Friction-generated heat is a primary by-product of seismic slip on faults. Heat activates various mechanisms that lead to low coseismic strength, with important implications for the earthquake energy budgets, the magnitude of strong ground motions, and earthquake self-similarity. This project documents the in situ physics of earthquakes on hematite-coated fault surfaces commonly exposed in fault damage zones. Nano- to micro-scale deformation textures on experimental hematite may reflect temperature and strength changes that are ultimately diagnostic for interpreting exposed fault surfaces in southern California locales. This project represents the first application of an emerging, novel low-temperature thermochronometry technique – hematite (U-Th)/He dating – to laboratory-generated earthquake rocks.

Research directly addresses the SCEC5 Science Plan’s research objectives including (Q2) Off-fault inelastic deformation impact on dynamic rupture and radiated seismic energy: high-gloss, light reflective or “mirrored” hematite-coated fault surfaces are common in fault damage zones. Experiments and detailed microscopic and thermochronometric characterization of experimental fault products informs how inelastic strain associated with evolving fault roughness and discontinuities influences earthquake physics (P2d). (Q3) Structure, composition, and physical fault zone properties impact on resistance to seismic slip: experiments and experimental fault rock thermochronometry document whether co-seismic weakening mechanisms – such as flash heating – occur on hematite-coated fault surfaces to evaluate how damage zones and fault zone mineralogy govern strain localization and rupture propagation (P3d). (Q1) Fault loading across temporal and spatial scales: experiments quantify friction evolution over different slip rates. Nano-to-micro-scale experimental fault surface characterization correlate stress concentrations with geometric asperities (P1d).
Broader Impacts This project supports the broader impacts of SCEC centered on generating useful knowledge of earthquake system science for stakeholders and society to ameliorate earthquake risk and improve community resilience, as well as promoting research, education, and career training. PI Ault is a female, early career scientist with expertise in fault rock thermochronometry but new to the field of earthquake mechanics and deformation experiments. Research activities engage and are spearheaded by USU postdoctoral fellow (PF, Calzolari), providing him with invaluable research, education, and training experiences. Calzolari is PI Ault’s first PF, provides her with a critical science and career mentoring opportunity. This project builds on established collaborations between USU and the University of Arizona (for (U-Th)/He analyses) and develops a new collaboration with Brown University. Thus, it affords PI Ault and PF Calzolari the opportunity to interact with a broader cross-section of the Earth science community and gain hands-on analytical experience vital for research and education advancement. The PF is Italian and his inclusion in the research team expands the diversity of the USU research engine. PF Calzolari reassembled collaborator Hirth’s Instron rotary-shear apparatus – previously disassembled during a move – to enable research progress. This provided Calzolari with invaluable insight into how to design and assemble a rotary shear apparatus, while enhancing the instrumentation and infrastructure of Brown University.
Exemplary Figure Figure 2. Caption: Figure 2. (A) Merged photo montage showing hematite run product from Run 11 (5 MPa, slip velocity 340 mm/s, displacement 4 cm) denoting the location of (B) cross-sectional SEM back-scattered electron image of cataclasite and (C) close-up of incipient fault mirror. (D) SEM secondary electron image of experimental mirrored surface in plan view with sintered polygonal and lobate grain boundaries. (E) Comparative SEM image from Sandia Mountains hematite fault mirror, also in plan view.