HR: 08:20h
AN: T51B-02    [PDF]
TI: Seismic Structure of the Mantle Beneath Western Pacific Using 3D Fr\'echet Kernels
AU: * Chen, L
EM: liangjun@usc.edu
AF: Univ. of Southern California, Dept. of Earth Sciences, Los Angeles, CA 90007
AU: Zhao, L
EM: zhaol@usc.edu
AF: Univ. of Southern California, Dept. of Earth Sciences, Los Angeles, CA 90007
AU: Jordan, T
EM: tjordan@usc.edu
AF: Univ. of Southern California, Dept. of Earth Sciences, Los Angeles, CA 90007
AB: Two decades of seismic tomography studies have yielded earth models with three-dimensional (3D) velocity heterogeneities in the mantle on both global and regional scales. We have developed a tomography technique which uses 3D Fr\'echet kernels in setting up the linearized inverse problem. We present a 3D model of the shear-speed structure for the mantle beneath the Western Pacific Ocean. Over 1600 three-component recordings of earthquakes (Mw $>$ 5.5) from the seismic zones around the western Pacific rim were processed to obtain $\sim$30,000 frequency-dependent phase delays for various of seismic waves, including S, SS, upper-mantle guided and surface waves, and ScS reverberations. The 3D Fr\'echet kernels for these delay times are computed by the coupled normal mode theory described by Zhao, Jordan, and Chapman (2000), and the measurements were inverted for a 3D radially anisotropic shear-speed model. The model parameters include shear-speed variations throughout the mantle and perturbations to radial shear-wave anisotropy in the uppermost mantle. The resolving power has been investigated through a series of inversion tests, which indicate that the horizontal and vertical resolving lengths are about 750 km and 250 km, respectively, in the upper mantle. Our results for the large-scale variations in the isotropic shear speeds are generally consistent with published global tomographic models. Comparison of inversion results using 2D and 3D Fr\'echet kernels shows a good correlation between 2D and 3D inversion, and somewhat lower amplitude in 3D inversion. Our model reveals greater lateral heterogeneity than the global models, especially in the 200-400 km depth range, suggesting a complex 3D mantle flow in the western Pacific upper mantle.
DE: 7218 Lithosphere and upper mantle
DE: 8180 Tomography
SC: Tectonophysics [T]
MN: 2003 Fall Meeting