HR: 1340h
AN: S33A-1073    [Abstracts]
TI: Mantle Discontinuities Beneath Japan Determined from CCP Stacking of Receiver Functions
AU: * Ham, S
EM: telnet2u@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main St., Houston, TX 77005 United States
AU: Niu, F
AF: Department of Earth Science, Rice University, 6100 Main St., Houston, TX 77005 United States
AU: Levander, A
AF: Department of Earth Science, Rice University, 6100 Main St., Houston, TX 77005 United States
AU: Obayashi, M
AF: IFREE, Japan Marine Science and Technology Center, 2-15 Natsushima-Cho, Yokosuka-city, Kanagawa, 237-0061 Japan
AB: Many seismic velocity discontinuities have been observed in Earth­_s mantle over the past several decades. Of these discontinuities the 410-km and 660-km discontinuities that mark the upper and lower boundaries of the transition zone exist globally, while others appear to be associated with specific tectonic regimes. For example, the Hales discontinuity at $\sim$80km is found only beneath the continents, and the mid-mantle discontinuity at $\sim$1000km has been reported specifically in the western Pacific. These discontinuities usually reflect the boundaries of layers that are distinct from each other in either composition or structure. With the unprecedented high quality and volume of seismic data recorded by the Japanese borehole seismic network (Hinet) deployed since 2000, we are in an ideal position to characterize the discontinuity structure beneath the Japanese islands, which is a ``type locale'' of a subduction region. In this study we used P-to-S converted waves to image to 1000 km depth beneath Japan. Our dataset includes 7903 three-component seismograms in the distance range of $35\deg$ to $85\deg$ recorded at more than 500 borehole seismometers that cover the Japanese islands. We used 20 earthquakes with Mw$>$6.0 to form the CCP images. Receiver functions were formed by deconvolution of the vertical components from the rotated radial components. To ensure a stable deconvolution from the short-period seismograms, the short-period instrument response was removed to recover the longer-period signals($\sim$6s) before the deconvolution. A revised common-conversion-point stacking technique was employed to enhance the signal-to-noise ratio. We varied the bin size and fixed the number {\it N} of conversion points in each bin to improve the horizontal resolution in densely sampled regions. The value for {\it N} in a given bin, 30 to 40 depending on the signal-to-noise ratio of seismograms, was chosen so that conversions for the 410-km and 660-km discontinuities were clearly visible. The bin size varies between $0.01\deg$ and $1\deg$ with an average of $\sim$$0.5\deg$. For a conversion depth {\it d}, we first calculated the ray path of converted phase {\it Pds} and its arrival time relative to {\it P} by ray tracing the 1D {\it iasp91} velocity model. We then summed the {\it N} seismograms using a 4$^{th}$-root stacking method, which provides a more efficient way to reduce the uncorrelated noise relative to the usual linear stack ({\it n=1}). The 410-km and 660-km discontinuities are clear on the CCP images. The 660-km discontinuity is found to be depressed as much as 40 km beneath southwest Japan and the Izu-Bonin arc, while the 410-km is uplifted by 20 km within the subducted Pacific slab and some of the fast regions in tomographic images. On the other hand, a thinner transition zone with a depressed 410-km and an uplifted 660-km is observed below the subducting slab, suggesting the possible existence of plume-like upwelling behind slabs, usually a slow region in tomographic images. A more quantitative comparison of the undulations of the two discontinuities with 3D velocity models will be presented in order to examine the origins of both the velocity anomalies and lateral variations of the two discontinuities. We observed some conversion at $\sim$560km depth only in the higher velocity regions, suggesting that the conversion may occur at top of the stagnant slab instead of the 520-km discontinuity. We also see some conversion energy below the 660-km discontinuity. These conversions, however, exhibit an intermittent feature rather than a continuous discontinuity.
DE: 7220 Oceanic crust
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 7200 SEISMOLOGY
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting