Western Pacific Subduction - New Results from Regional Joint P and S Travel-Time Tomography

A detailed regional tomographic inversion of the Western Pacific region has been performed using P and S travel times from common sources and receivers, with a joint inversion in terms of bulk-sound and shear wave speed variations in the mantle. This parameterization allows the separation of the influence of bulk and shear moduli, and hence a more direct comparison with laboratory information. The study region was parameterized by cells of 0.5° to 2° and 19 layers to a depth of 1500 km, while the surrounding whole mantle was parameterized by cells of 5° x5° and 16 layers. A simultaneous inversion was made for regional and global structures to minimize the influence of surrounding structures on the regional image. The iterative linerized inversion was used with updates of 3-D ray tracing. Penetration of a subducted slab below the 660 km discontinuity is seen at the Kurile-Kamchatka trench, while the flattening of slabs above this depth is observed in the Japan, Ryukyu, and Izu-Bonin subduction zones on both the bulk-sound and shear images. The penetration of a subducted slab down to a depth of at least of ~1500 km is observed below the southern part of Bonin trench, Mariana, Philippine, and Java subduction zones. The shear wave speed perturbations of the subducted slab, down to the 410 km transition zone, is larger than the bulk-sound perturbations for all these subduction zones (except the Philippines). The bulk-sound signature of the subducted slab is more pronounced than for shear in the Philippines, Talaud, New Guinea, Solomon, and Tonga subduction zones where penetration of the slab into the middle mantle is observed. A statistical analysis of the bulk-sound and shear wave-speed anomalies in the Western Pacific associated with the labs shows that the average amplitude of the velocity perturbations generally decreases at depths corresponding to the major transition zones, i.e. ~410, ~660 and ~900 km, in both moduli. These local minima could reflect a change in the physical properties of the subducted slabs as they cross the transition zone boundaries.