How do Mineralogy and Fractures Affect Seismic Velocities and Anisotropy of the Alpine Fault Mylonites: a Numerical Study

Abstract

P-wave anisotropy can be significant in fault zones due to intrinsic rock texture such as crystallographic preferred orientation (CPO), and it can be exacerbated by the presence of fractures. Understanding and quantifying P-wave anisotropy helps us unravel the physical rock properties that should be accounted for in seismic data processing to produce the best tomographic inversions and earthquake locations. At the Alpine Fault, elastic wave anisotropy is known to exist in the shear zone mylonites and schists, but the underlying controls on P-wave anisotropy at the micro-scale are mostly unknown. Electron back-scattered diffraction (EBSD) and synchrotron X-ray microtomography (micro- CT) data are acquired on exhumed schist, protomylonite, mylonite and ultramylonite samples. Samples are composed of quartz, plagioclase and mica (inferred from non-index phase). In this study, mica cannot be indexed with EBSD experiments on the sample cut although two cut directions (30o and 90o) with respect to foliation are analyzed. Based on ambient condition micro-CT data analysis, it is estimated that the samples contain less than 5% porosity with an average pore aspect ratio of 5:2:1. Two numerical models are compared: wave propagation model (EWAVE) and a Voigt-Reuss-Hill average elastic model (MTEX) combined with differential effective medium (DEM) for inclusions. Both models have comparable pore-free wave speed predictions. All lithologies also have comparable pore-free elastic wave anisotropies (10-15%) as the volume of mica is similar among the samples and the assumed CPO for mica constant. Aligned fractures reduce P-wave velocity normal to foliation significantly more than for waves traveling parallel to it. The P-wave anisotropy increases with alignedfracture volume. As I increase the volume of aligned fractures, modeling 3D fractures (DEM) decreases velocities more significantly than when 2D fractures are included in the wave propagation modeling (EWAVE). I show that the volume and orientation of mica have the highest impact on the rock P-wave anisotropy, followed by fracture volume and its 3D shape. The study is concluded by comparing several numerical models to experimental wave velocities. Parameter sensitivity studies show that sample heterogeneity and pore shape and volume are critical to constrain to fully predict the observed data.