Study Of The Velocity Structure And Seismic Anisotropy Of Upper Mantle Beneath East Asia

The seismic velocity in the upper mantle is controlled by mantle temperature,pressure and composition,and anisotropy structures are caused by lattice-preferred orientation(LPO)of upper mantle minerals.Understanding upper mantle velocity and anisotropy structures can help us infer mantle composition structure and the LPO of minerals.Furthermore,it can provide constraints for geodynamics study.In East Asia,the subduction of the Pacific and Philippine Sea plates resulted in a complex upper mantle structure.Detailed velocity and anisotropy structures in this region are important for us to understand the interaction between the upper mantle and the subducted slabs.In this study,we detected the upper mantle velocity and anisotropy structures beneath East Asia using waveform modeling of triplication data and analyzed our seismic results using mineral physics modeling method.Based on S triplication data of two earthquake events in East Asia,recorded by China Digital Seismogragh network,we study the upper mantle velocity structure beneath northeast Asia.The modeling results of these two events show that,both models have a low velocity zone above 250 km,a high velocity zone at the lower part of the mantle transition zone,which suggest the influence of the Pacific subduction slab in this area.The thickness of the mantle transition zone of both models are the same as PREM,because the influences of the slab near the two discontinuities are different.The modeling result of event 01(with ray paths across the sampling region from south to north)indicates the absence of radial anisotropy above the 410-km discontinuity;however,the modeling result of event 02(with ray paths cross the sampling region from east to west)suggests a strong radial anisotropy above the 410-km discontinuity.Based on the seismic inferred anisotropy structures,we analyzed the mineral compositions and the olivine LPO,and obtained a basic model of olivine alignment,which is useful to study the mantle dynamic model in this region.