The Effects of Fault Composition and Microstructures on Fault Weakness: A Study of Synthetic and Natural Clay-Rich Fault Gouges

Abstract:

The mechanical weakness of faults has long been a fundamental issue in faulting and earthquake mechanics studies. Studies of core and outcrop samples obtained from major faults, including the San Andreas Fault and subduction megathrusts, and laboratory friction experiments on natural and synthetic fault gouges all document that clay minerals are one likely explanation for fault weakness. While laboratory experiments have shown that clays are frictionally weak, with friction coefficients (μ) as low as 0.08-0.2 for smectite family minerals, and that μ varies systematically with clay content, the effects of microstructure, composition, and the evolution of friction throughout shearing of clay-rich gouges is not well understood. To investigate these processes, we conducted shearing experiments on two-phase synthetic gouges of varying proportions of quartz and Ca-montmorillonite. We then studied the resulting microstructures to test the hypothesis that as clay-rich gouges are sheared, clay minerals align and form discrete through-going surfaces that lead to reduced strength.

Experiments were run in a double direct shear geometry at room temperature and normal stresses of 25 MPa. Samples were sheared at a constant velocity of 10μm/s. Sheared “wafers” were recovered for scanning electron microscope (SEM) analyses. Our preliminary results yield values of μ = 0.35-0.62 for mixtures ranging from 10%-90% clay, and document a decrease in μ with clay content, consistent with previous studies. Additionally, we observe a characteristic peak and then drop in shear strength at the beginning of shearing. The magnitude of this peak increases with clay content. Ongoing analysis of fabrics will systematically assess the relationship between microstructures, clay alignment, and the evolution of friction for our synthetic gouges as well as for a suite of natural clay-rich fault rocks from exhumed subduction thrusts. We expect that with clay content and shear strain increases, fabrics that form will control not only gouge weakness, but will also affect gouge permeability.