Vp/Vs Anisotropy and Implications for Crustal Composition Identification and Earthquake Prediction

The ratio of P‐ to S‐wave velocities (V_p/V_s) is regarded as one of the most diagnostic properties of natural rocks. It has been used as a discriminant of composition for the continental crust and provides valuable constraints on its formation and evolution processes. Furthermore, the spatial and temporal changes in V_p/V_s before and after earthquakes are probably the most promising avenue to understanding the source mechanics and possibly predicting earthquakes. Here we calibrate the variations in V_p/V_s in dry, anisotropic crustal rocks and provide a set of basic information for the interpretation of future seismic data from the Wenchuan earthquake Fault zone Scientific Drilling (WFSD) project and other surveys. V_p/V_s is a constant (Φ₀) for an isotropic rock. However, most of crustal rocks are anisotropic due to lattice‐preferred orientations of anisotropic minerals (e.g., mica, amphibole, plagioclase and pyroxene) and cracks as well as thin compositional layering. The V_p/V_s ratio of an anisotropic rock measured along a selected pair of propagation‐vibration directions is an apparent value (Φ_ij) that is significantly different from the value for its isotropic counterpart (Φ₀). The usefulness of apparent V_p/V_s ratios as a diagnostic of crustal composition depends largely on rock seismic anisotropy. A 5% of P‐ and S‐wave velocity anisotropy is sufficient to make it impossible to determine the crustal composition using the conventional criteria (V_p/V_s≤1.756 for felsic rocks, 1.756<V_p/V_s≤1.809 for intermediate rocks, 1.809<V_p/V_s≤1.944 for mafic rocks, and V_p/V_s>1.944 fluid‐filled porous/fractured or partially molten rocks) if the information about the wave propagation‐polarization directions with respect to the tectonic framework is unknown. However, the variations in V_p/V_s measured from borehole seismic experiments can be readily interpreted according to the orientations of the ray path and the polarization of the shear waves with respect to the present‐day principal stress directions (i.e., the orientation of cracks) and the frozen fabric (i.e., foliation and lineation).