The interplay between fault fabric and frictional healing in altered serpentinite-rich fault gouge

Emma M. Armstrong, Monica R. Barbery, Alexis K. Ault, Greg Hirth, Srisharan Shreedharan, & Ainsley MacDonald

Submitted September 7, 2025, SCEC Contribution #14363, 2025 SCEC Annual Meeting Poster #TBD

Creep or aseismic deformation influences earthquake behavior and modulates stress on faults. Fault creep may be controlled by frictional properties such as healing and/or fabric development. In the brittle portion of fault zones, fabric develops by mechanical breakdown and alignment of strong phases and oriented growth and/or alignment of weak phases. An outstanding question is the relationship between fabric development and healing rate, which we address with rotary shear experiments using the Tullis Rotary Shear (TRS) apparatus on powdered fault gouge that involve slide-hold-slide protocols following 0 to 2.5 m of pre-slip at a constant sliding rate.

We target exhumed clay-rich, strongly foliated, serpentinite gouge along the Bartlett Springs fault (BSF) in the northern San Andreas fault system. The BSF creeps ~3.5 mm/yr and is capable of hosting MW7.5 earthquakes. We revisit the BSF locality north of Lake Pillsbury of Moore et al. (2018), where the serpentine-rich gouge zone cuts Pleistocene sediments and exhibits five structural and compositional domains across a 2 m wide transect. Domains comprise variable amounts of serpentine, chlorite, talc, smectite, and other minor phases. We first characterized the frictional properties of disaggregated gouge from each domain using a direct shear apparatus at near-plate rates. Steady state µ varies from 0.1-0.4 and weaker samples correlate with the presence of talc.

Based on outcomes from initial direct shear experiments, we targeted one sample for TRS experiments to investigate the role of fabric development. These experiments were conducted at 10 MPa normal stress, room temperature, and water saturated conditions and reveal µ is 0.1-0.2 regardless of the amount of pre-slip, consistent with direct shear tests. Experiments without pre-slip (i.e., no initial fabric development), yield a healing rate of ~0.007/decade. With increasing pre-slip, the healing rate decreases to ~0.002/decade but the absolute amount of healing (Δµ) increases, especially for short hold times. Scanning electron microscopy imaging reveals that deformation is distributed throughout the gouge, rather than in demonstrably localized slip surfaces. Together, these observations may suggest that comminution and alignment of phyllosilicates related to foliation development saturates the bulk volume contact area, thus allowing for significant short-term healing related to improvements in contact quality but limited long-term healing.

Key Words
frictional healing, fault fabric, altered serpentinite, deformation experiment

Citation
Armstrong, E. M., Barbery, M. R., Ault, A. K., Hirth, G., Shreedharan, S., & MacDonald, A. (2025, 09). The interplay between fault fabric and frictional healing in altered serpentinite-rich fault gouge. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)