The ratio of P- to S-wave velocities（Vp/Vs） 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 Vp/Vs 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 Vp/Vs 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.Vp/Vs 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 Vp/Vs ratio of an anisotropic rock measured along a selected pair of propagation-vibration directions is an apparent value(Φij) that is significantly different from the value for its isotropic counterpart（Φ0）.The usefulness of apparent Vp/Vs 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（Vp/Vs≤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 Vp/Vs 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）.