Structural anisotropy of normal fault surfaces

Abstract

Precise description of natural fault surfaces is indispensable to understanding the geometry, mechanics and fluid transport properties of faults. Profiles of fault surfaces in the Wasatch fault zone and Oquirrh Mountains, Utah, are measured at 30° increments within the fault plane to determine the directional anisotropy of surface roughness at wavelengths between 10 −3 m and 30 m, and then compared with profiles of larger-scale fault surfaces. Surface anisotropy and an increasing ratio of surface amplitude to wavelength are consistent with self-affine fault topography at wavelengths between 1 mm and approximately 5 km. Fractal dimension of surface profiles generally decreases systematically as the angle to the slip direction increases. Directional anisotropy is described by an azimuthal scaling function γ φ = K sin( φ) + γ 0 or AF φ = ( AF max −1) sin( φ) + 1, where γ φ and AF φ are the amplitude to wavelength ratio and anisotropy factor respectively at azimuth φ, measured clockwise relative to slip direction within the fault surface, and γ 0 is the amplitude to wavelength ratio parallel to slip direction. K = ( γ 90 − γ 0) is an anisotropy coefficient and increases systematically with spatial wavelength on the fault surface. Characterization of natural fault surfaces provides parameters such as fractal dimension ( D), intercept (log( C)) of power spectra, profile variance, and variation in anisotropy factor ( AF), which are needed to generate fractal models of natural fault surfaces using spectral synthesis. We generate sample models which illustrate the differences between fault surfaces characterized by constant versus azimuthally varying fractal dimension. The latter model surfaces contain low amplitude corrugations superimposed on elongate ridges which parallel slip direction. This surface texture resembles that of natural fault surfaces that refract across lithologic layering or are cut by secondary faults such as R and R′ shears.