HR: 1340h
AN: S33A-1075    [Abstracts]
TI: P to S scattered-wave imaging of the subducting slabs beneath Japan
AU: * Niu, F
EM: niu@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main Street, Houston, TX 77005 United States
AU: Levander, A
EM: alan@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main Street, Houston, TX 77005 United States
AU: Obayashi, M
EM: obayashi@jamstec.go.jp
AF: IFREE, Japan Marine Science and Technology Center, 2-15 Natsushima-Cho,, Yokosuka, 237-0061 Japan
AB: Recent developments in seismic observations make it possible to apply scattered wave imaging techniques developed for petroleum exploration, such as the pre-stack depth migration, to investigate the crustal and upper mantle structures with unprecedented detail. In this study, we used P to S scattered waves to image the top 1000 km beneath Japan islands. We are particularly interested in imaging the fine structures related to the descent of the Pacific slab, for example, whether the seismic velocity between the slab and the surrounding mantle changes rapidly or gradually, and how the boundary changes with depth. Our dataset includes 7903 three-component seismograms at the distance range of $35^{\circ}$ to $85^{\circ}$ recorded at more than 500 borehole short-period seismometers from a total of 20 earthquakes with Mw $>$6. We generate receiver functions from the radial components of the rotated seismograms. Ham {\it et al.} (this session) show the CCP stacked images from the same dataset. While the 410-km and 660-km discontinuities are very clearly shown in the CCP images, it is difficult to identify the subducting Pacific slab. The absence of the descending Pacific slab in the image is probably due to the nature of CCP stacking process, which assumes a horizontally layered structure in calculating time corrections that degrades the P to S scattered wave signals associated with the dipping slab. Analysis of the data using diffraction migration, yields clear dipping events. Strong scattering associated with the subducting slabs are found in 2D profiles across southwest Japan, northeast Japan, and Hokkaido. However, the preliminary images show that scattering locations are $\sim$20-50 km below the Wadati-Benioff zone. For simplicity we have used the 1D iasp91 velocity model in back projecting the P to S scattered waves. The 1D reference model appears to misposition the events in the subduction region, which has been reported to be very heterogeneous. It is well known that the absolute depths of migrated events in the images depend strongly on the migration velocity model. For example, our synthetic test shows that if the reference velocity model differs from the true structure by 3%, the subducting slab can be mislocated by about 30 km. As the seismic velocity in the mantle wedge is usually several percent slower than the global average, the observed discrepancy between the scattering locations and the Wadati-Benioff zone could be much smaller, if an accurate 2D migration velocity model is used. A 2D Kirchhoff depth migration of the data is ongoing. The migration is a scalar form of the Rayleigh-Sommerfeld diffraction integral, a far field approximation of the Kirchhoff integral and has be successfully applied to the waveform data collected in PASSCAL experiments such as CD-ROM and the Kaapvaal Seismic Array.
DE: 7260 Theory and modeling
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting