HR: 0800h
AN: DI41A-1248    [Abstracts]
TI: Fine Mapping of the Mantle Discontinuities Beneath the Japanese Islands Using Short Period Hi-net Data
AU: * Kawakatsu, H
EM: hitosi@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Watada, S
EM: watada@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: We have conducted a receiver function analysis to finely map the seismic discontinuity structure beneath the Japanese islands using Hi-net data. From earthquakes occurred between 2001 and 2004, we select those in a distant range of 30-90 degrees with Mw > 5.5, which results in a total of 404. Receiver functions (RFs) are constructed for instrument response corrected seismograms employing the water level correction (C=0.001) in a frequency range of 0.1-0.5Hz. Out of 238,574 RFs, we select RFs with decent S/N ratio to make 74,083 RFs. 1-D IASPEI91 based imaging of mantle discontinuities beneath the Japanese islands shows remarkably clear continuous features corresponding to the 410km and 660km transition zone discontinuities, as well as the top boundary of the subducting Pacific plate traceable down to at least 300 km depth. The 660km discontinuity is deflected downwards as much as ~40km with a width of approximately 500 km where the Pacific plate appears to stagnate in recent tomographic models; The 410km discontinuity is elevated 20-30km where the Pacific plate penetrates through it. While these features are expected for a simple penetrating subduction, the narrow depression of the 660km discontinuity beneath southwest Japan does not appear to be consistent with a flat stagnating slab near the boundary (Bina and Kawakatsu, 2005). The high velocity anomaly there may be ``floating" in the transition zone. Encouraged by the continuous image obtained by the simple RF analysis, we have further applied a Kirchhoff type migration to the same data set. As a full migration does not give reasonable image, we have restricted to recover the wavefield sub-horizontal (conversion plane within 15 degrees from horizontal) and sub-parallel to the slab dip (within 18 degrees from the local slab dip estimated from the slab contour of Gudmundsson & Sambridge, 1998). Some of the features seen this image are: (1) weak slab signature ~100-150km depth, which may correspond to the location of basalt to eclogite transition in the subducting oceanic crust, (2) ~10km elevation of the 660km discontinuity at the ocean side of the Japanese subduction zone, which appear to coincide with the slow P-wave velocity anomaly in a recent tomographic model (Obayashi and Fukao, 2001), (3) though highly speculative, a possible deep (~800km) structure.
UR: http://gachon.eri.u-tokyo.ac.jp/~hitosi/outgoing/kawakatsu+watada2005.jpg
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Study of Earth's Deep Interior [DI]
MN: Fall Meeting 2005