SCEC Award Number 21068 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Collaborative Research: Characterizing shallow fault friction with slow slip hematite deformation experiments and hematite (U-Th)/He thermochronometry
Name Organization
Alexis Ault Utah State University Greg Hirth Brown University
Other Participants Alex DiMonte (graduate student, USU)
Dr. Cameron Meyers (Scientific Research Engineer, Brown)
SCEC Priorities 3d, 2d, 3g SCEC Groups FARM, Geology, SDOT
Report Due Date 03/15/2022 Date Report Submitted 06/14/2022
Project Abstract
This project tests the hypothesis that hematite promotes shallow, transient slow slip in the southern San Andreas fault (SSAF). Transient slow slip is a fundamental part of the earthquake cycle but a critical unknown is how fault material properties govern this phenomenon. We combine hematite microstructural analyses with hematite (U-Th)/He thermochronometry from natural and experimental faults to constrain hematite rate-dependent friction behavior, deformation mechanisms and textures, and the timing and rates of past slip in fault damage of the SSAF system. In Mecca Hills, networks of minor hematite slip surfaces occur in basement fault damage of oblique strike-slip faults that connect to the southern San Andreas fault in a positive flower structure. Scanning electron microscopy shows these mm-thick surfaces exhibit basal hematite injection veins and layered veinlets, both comprising nanoscale, high-aspect ratio hematite plates akin to phyllosilicates. New slow slip rotary shear experiments demonstrate that hematite is weak and exhibits velocity strengthening to velocity neutral rate-state frictional behavior. Combined microstructural, experimental, and hematite (U-Th)/He thermochronometry data indicate fault damage and associated hematite mineralization events that occurred from ~0.8 Ma to 0.4 Ma at <1.5 km depth. Minor hematite faults initially formed via fluid overpressure, and then accommodated repeated subseismic slip. These processes collectively created zones of shallow off-fault damage up to 4 km orthogonal to the trace of the SSAF. We suggest the development of distributed hematite slip surfaces governs transient and/or triggered slow slip that may dampen earthquake energy in the uppermost crust.
Intellectual Merit Constraints on slow slip deformation mechanisms and stress state in the shallow crust are critical for predictions of earthquake propagation and arrest, where faults are in the seismic cycle, ground shaking intensity, and associated hazards with large seismogenic faults, such as the southernmost San Andreas Fault. Transient slow slip is a fundamental part of the earthquake cycle. In shallow portions of fault zones, slow slip may dampen radiated seismic energy, inhibit nucleation, and promote arrest of earthquakes. A critical unknown is how fault material properties govern this phenomenon. Addressing this problem requires integrated data from natural and experimental faults. Here we focus on the mineral hematite, because iron is the 4th most common element in the crust and thus hematite is ubiquitous in fault zones. Importantly, hematite textures and grain morphologies reflect the conditions and depth at which hematite formed and deformed. In this work, we leverage and continue to develop a new tool, hematite (U-Th)/He thermochronometry from natural and experimental slip surfaces, to constrain the timing, depths, and rates of fault slip in the shallowly exhumed southern San Andreas fault system in the Mecca Hills.

Research activities and outcomes directly align with SCEC research objectives. These include addressing: (Q3) Structure, composition, and physical fault zone properties impact aseismic slip: Comparing textures and hematite (U-Th)/He thermochronometry from natural and experimental faults, together with frictional properties derived from experiments, informs how fault zone mineralogy and friction govern the stability of slip along the SSAF (P3d). Realistic characterization of the rheology of shallow portions of fault systems, including rate-dependent frictional properties, improve interpretations of geodesy-based analyses of fault slip and dynamic rupture models (P3g). (Q2) Off-fault inelastic deformation impacts on dynamic rupture and radiated seismic energy: Identifying shallow slow slip and associated deformation mechanisms in Mecca Hills fault rocks, part of the SSAF system, is critical for understanding how shallow fault zones dampen radiated seismic energy, as well as predictions of earthquake propagation and arrest in SSAF during the seismic cycle (P2d). (Q4) Relationship between strong ground motion and nonlinearities in earthquake systems: Integrated observables of frictional properties and rheology from experimental and natural perspectives inform how fault friction and crustal material heterogeneities may impact strong ground motions (P4a).
Broader Impacts This SCEC project continues a successful partnership between a female scientist at Utah State University (Ault) and Brown University (Hirth) and enhanced science and educational connections between all PIs and collaborators across multiple institutions (USU, Brown, University of Arizona). Research activities and funding support the education and training of female USU MSc student Alex DiMonte. Owing to successful research outcomes and experiences, supported by SCEC, DiMonte will continue this work as a PhD student at USU. This project has also supported USU MSc student Emma Armstrong and PhD student Jordan Jensen, who accompanied DiMonte, Ault, and Hirth in the field in the Mecca Hills and along the southern San Andreas fault. The project also provides critical support for Senior Research Technician Dr. Cameron Meyers (Brown), who trained DiMonte to prepare samples for deformation experiments, operate the Instron rotary shear apparatus, and how to acquire frictional data from this instrument. This project has supported the USU Microscopy Core Facility, which houses the field emission scanning electron microscope (SEM). Analytical approaches developed during this project by DiMonte for prescreening hematite aliquots with the SEM for purity and grain size analysis are now being used in other fault rock investigations.
Exemplary Figure Figure 5. Results of hematite rotary shear experiments on specular hematite at slow slip rates, 5 MPa (circle) or 8.5 MPa (triangle) load, and different upper annuli (diabase, granite). Slip rates vary from 1 um/s to 320 mm/s for 1 cm to 3 cm displacement. Experiments yield hematite coefficient of friction of 0.25-0.45. Hematite exhibits velocity strengthening to velocity neutral behavior (i.e., a-b). Data from published deformation experiments on various phyllosilicates for comparison: 1-talc, Moore and Lockner (2008); 2-talc, 3-montmorillonite, 4-biotite schist, 5-chlorite schist, 6-serpentinite, 7-kaolinite; Ikari et al. (2011).