Discussion on origin of Pn velocity variation in China and adjacent region

Abstract

Pn velocity lateral variation and anisotropy images were reconstructed by adding about 50 000 travel times from the regional seismic networks to the datum set of near 40 000 travel times from National Seismic Network of China used by WANG,et al. We discussed the relation of Pn velocity variation to Moho depth, Earth’s heat flow, distribution of Cenozoic volcanic rock and the result of rock experiment under high pressure and high temperature. The result of quantitative analysis indicates that Pn velocity is positively correlated with the crust thickness and negatively correlated with the Earth’s heat flow. Two linear regression equations, one between Pn velocity and crust thickness, and the other between Pn velocity and heat flow, were obtained. The rate of variation of Pn velocityν p with pressureP, ∂ν p/∂P, estimated from the velocity variation with crust thickness, ∂ν p/∂H is close to the result obtained from the rock experiment under high pressure and high temperature. If the effect of crust thickness on Pn velocity is deducted from the velocity variation, then the low Pn velocity beneath Qinghai-Xizang plateau is more notable. The low Pn velocity regions well agree with the Cenozoic volcanic rock. In the several regions with significant anisotropy, the direction of fast Pn velocity is consistent with the orientation of maximum principal crustal compressive stress, and also with the direction of present-day crustal movement. It indicates that the fast Pn velocity direction may be related to the deformation or flow of top mantle material along the direction of maximum pressure.