HR: 16:45h
AN: S34C-04    [Abstracts]
TI: Geographic Boundary and Shear Wave Velocity Structure of the`Pacific Anomaly' Near the Core-Mantle Boundary Beneath Western Pacific
AU: He, Y
EM: ymhe@mail.igcas.ac.cn
AF: Yumei He Tianyu Zheng, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O.Box 9825, Beijing, 100029 China
AU: * Wen, L
EM: wen@earth.geo.sunysb.edu
AF: Lianxing Wen, Department of Geosciences, State University of New York at Stony Brook, Stony Brook, New York, NY 11794-2100 United States
AU: Zheng, T
EM: tyzheng@mail.igcas.ac.cn
AF: Yumei He Tianyu Zheng, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O.Box 9825, Beijing, 100029 China
AB: We determine geographical boundary and shear velocity structure of a very-low velocity province at the base of the Earth's mantle beneath western Pacific (we term it the `Pacific anomaly') based on waveform modeling and travel time analysis of ScSH-SH phases. Our seismic data are collected from those recorded in the China National Digital Seismographic Network, the F-net in Japan, the Global Seismographic Network and several PASSICAL arrays. The observed ScS-SH differential travel time residuals allow the northwestern geographic boundary of the anomaly to be clearly defined. The seismic data also suggest that the average shear velocity reduction inside the anomaly reaches -5% in the lowermost 300 km of the mantle. Waveform modeling of the seismic data sampling the edge of the anomaly suggests that the northwestern boundary is characterized by a shear velocity model with a velocity jump about 2% at about 100-145 km above the core-mantle boundary and a thin (30-km thick) basal layer with a shear wave velocity reduction of -13%. Stacked seismic data sampling the middle of the anomaly, however, show no evidence for any internal discontinuity with a velocity decrease greater than -2% in the middle of the anomaly. Overall, the seismic data sampling the base of the `Pacific anomaly' can be explained by a negative shear velocity gradient from 0% to -1% (top) to -13% (bottom) in the lowermost 200 km of the mantle, similar to those of a very-low velocity province beneath the South Atlantic Ocean and the Indian Ocean. Such a strong negative shear velocity gradient can be explained by partial melt of a compositional anomaly produced early in the Earth's history situating in a bottom thermal boundary layer. Our travel time data also exhibit small-scale variations inside the anomaly, indicating existence of internal small-scale seismic heterogeneities inside the `Pacific anomaly'.
DE: 7200 SEISMOLOGY
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 9355 Pacific Ocean
SC: Seismology [S]
MN: Fall Meeting 2005