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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(Φ_0) 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(Φ_0).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, 1.7561.944 fluidfilled porous/fractured or partially molten rocks) if the information about the wave propagationpolarization 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).
The ratio of P- to S-wave velocities (V_p / V_s) is considered as one of the most diagnostic properties of natural rocks.It has used as a discriminant of composition for the continental crust and may 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. Hee 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 (Φ_0) for an isotropic rock. What, most of crustal rocks are anisotropic due to lattice-preferred orientations of anisotropic minerals (eg, mica, amphibole, plagioclase and pyroxene) and cracks as well as thin compositional layering. The V_p / V_s ratio of an a nisotropic 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 (Φ_0). usefulness of apparent V_p / V_s ratios as a diagnostic of crustal 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 1.944 fluidfilled porous / fractured or partially molten rocks) if the information about the wave propagationpolarization directions with respect to the tectonic framework is unknown.However , the variations in V_p / V_s measured from borehole seismic experiments can be ready interpreted according to the orientations of the ray path and the polarization of the shear waves with respect to the present-day pr incipal stress directions (i.e., the orientation of cracks) and the frozen fabric (i.e., foliation and lineation).