HR: 0800h
AN: T11E-1332    [Abstracts]
TI: Structure Of The Ultra-low Velocity Zone In The Lowermost Mantle Derived From CMB Reflected Phases
AU: * Idehara, K
EM: idehara@sci.ehime-u.ac.jp
AF: Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama, 790-8577 Japan
AU: Yamada, A
EM: yamada@sci.ehime-u.ac.jp
AF: Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama, 790-8577 Japan
AU: Zhao, D
EM: zhao@sci.ehime-u.ac.jp
AF: Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama, 790-8577 Japan
AB: Analyses of reflected seismic waves from the core-mantle boundary (CMB) enable us to constrain the existence and properties of the ultra-low velocity zone (ULVZ) at the base of the mantle. Here we investigate the fine scale heterogeneity of the lowermost mantle beneath the western Pacific region using precursors and postcursors to PcP and ScP from deep earthquakes in the Philippine-Indonesian region recorded by the Hi-net seismic network in Japan and seismic array stations of the International Monitoring System in the western Pacific region. Clear arrivals of postcursor to ScP are detected for several contiguous events, which have bounce points beneath the Philippine Islands, while no significant precursors to PcP and ScP are detected in the same region. Our one-dimensional forward waveform modelings suggest that these postcursors are associated with an ultra-low shear-wave velocity layer in the lowermost mantle. The size of this ULVZ is about 500 km with velocity reductions of 7% for P-wave and 25% for S-wave, and with no or slight density increase (less than 4%). Garnero (2004) suggests the existence of ultra-low velocity patches with a spatial extent ranging from less than 10 km to hundreds of kilometers beneath the Central American subduction region. The properties of the ULVZ we found in this study are consistent with his result. The physical and/or chemical properties of this anomalous layer could be different from that of the ULVZ under South Pacific because clear precursors to PcP and ScP were reported there by several previous studies. The present results will be compared with other ULVZ studies conducted in various regions, focusing on the wavelength of the heterogeneity, seismic properties and physical constraints.
DE: 7207 Core and mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting