Discussion on origin of Pn velocity variationin China and adjacent region

IntroductionInrecentyears,theresultsfromresearchesonwavevelocitystructureofuppermostmantleusingMohorefractionwavesindicatethattheclearlateralvelocityvariationandanisotropyexistintheuppermostmantle(Hearn,1996,1999;Mele,etal,1998;WANG,etal,2002).Astotheoriginofvelocitylateralvariation,thegeneralconsiderationisthatitmayberelatedtothemate-rialcomposition,pressureandtemperatureintheuppermostmantle(Hearn,1984,1999;Beghouletal,1993).Theseismicanisotropyoftheuppermostmantleisnowanestablishedfactan…

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

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, Earths 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 Earths 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 veloc-ity vP with pressure P, Pv/p, estimated from the velocity variation with crust thickness Hv/p, is close to the result obtained from the rock experiment under high pressure and high temperature. If the effect of crust thick-ness on Pn velocity is deducted from the velocity variation, then the low Pn velocity beneath Qinghai-Xizang pla-teau is more notable. The low Pn velocity regions well agree with the Cenozoic volcanic rock. In the several re-gions 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.