Healing and frictional behavior of the shallow southern San Andreas fault gouge reveals ability to host geodetically observed creep events

Alexis K. Ault, Alexandra A. DiMonte, Srisharan Shreedharan, & Greg Hirth

Published September 8, 2024, SCEC Contribution #13857, 2024 SCEC Annual Meeting Poster #137

The southern San Andreas fault (SSAF) may be at the end of its earthquake cycle, but it releases some shallow strain as spontaneous and triggered creep events with cumulative slip of ~10 mm/event. The SSAF in Coachella Valley and associated subsidiary structures in Mecca Hills are defined at the surface by shallowly exhumed, compositionally heterogeneous, phacoid-bearing, clay-rich fault gouge. To evaluate the influence of this material on slip behavior, we performed deformation experiments and acquired X-ray diffraction and microscopy data that collectively characterize its frictional and healing behavior. X-ray diffraction analyses (n = 25 samples) show the gouge comprises ~20-50% clay (~50% illite, ~20% kaolinite, ~20% smectite), ~30% quartz, and ~10-30% feldspar. A sample of this clay-rich gouge from the mouth of Painted Canyon was deformed in a direct shear apparatus at Utah State University using a series of slide-hold-slide and velocity steps at room temperature and 10 MPa normal stress. Our results show that the clay-rich gouge is weak, with a coefficient of friction of ~0.45-0.50 at room humidity and 0.25-0.30 when water-saturated, exhibits a near-zero healing rate, and transitions from unstable stick slip at plate rates (~5 nm/s) to stable sliding above ~1 μm/s. Gouge develops localized slip surfaces comprising micro- to sub-microscale clay platelets with slickenlines during laboratory experiments; these surfaces are similar to observed networks of natural slip surfaces developed in clay that envelope phacoids in the main SSAF and related gouge zones.

The velocity dependent frictional stability and low healing rates of SSAF gouge are controlled by its clay content. Field observations suggest that this clay gouge is present at depth along the SSAF and subsidiary structures in sedimentary rocks and a similar clay-rich gouge is likely present where the SSAF extends into the underlying crystalline basement. This clay-rich material, especially when wet, provides throughgoing zones of weakness from seismogenic depths to the near-surface that could facilitate along-strike and up-dip rupture propagation during the next large earthquake in Coachella Valley. The frictional and healing behavior of this material is also conducive to the nucleation of the geodetically observed spontaneous and triggered creep events.

Key Words
southern San Andreas fault, clay, healing, friction, creep events

Citation
Ault, A. K., DiMonte, A. A., Shreedharan, S., & Hirth, G. (2024, 09). Healing and frictional behavior of the shallow southern San Andreas fault gouge reveals ability to host geodetically observed creep events. Poster Presentation at 2024 SCEC Annual Meeting.

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