Seismic Velocity Structures of a High-Velocity Area Near the Core-Mantle Boundary Beneath Eastern Eurasia

The shear and compressional velocity structures of a high-velocity area near the core-mantle boundary (CMB) beneath the eastern Eurasia are studied based on waveform modeling and differential travel time analysis of ScSH-SH and PcP-P phases. Our seismic data are obtained from the F-net in Japan, the Global Seismographic Network and several PASSCAL arrays. The observed ScSH-SH and PcP-P differential travel time residuals show similar patterns, but also a change of P/S velocity ratio across the region. The seismic data suggest that the average shear and compressional velocity increases reach 3% and 2% in the lowermost 300 km of the mantle, respectively. The waveform data from the shear and compressional waves sampling the region can be explained by a model with a D" discontinuity with a large variability of its structure. The data sampling southwest and southeast of the high-velocity area can be explained by simplified one-dimensional models with a D" discontinuity that are characterized by a shear velocity jump of 5% to 3.4% and a compressional velocity jump of 2.6% at about 220 km to 260 km above the CMB followed by a negative gradient toward the CMB. But, the shear wave data sampling the region between would exclude a D" discontinuity with a shear velocity jump greater than 1%. We will discuss effects of two-dimensional wave propagation on the inferred seismic structures and the D" discontinuity, and implication to the origins of the inferred seismic structures in the region.