HR: 1340h
AN: T23A-1218    [Abstracts]
TI: Upper mantle anisotropy beneath Indochina block and adjacent regions from shear wave splitting analysis of Vietnam array data
AU: * Bai, L
EM: bai@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Iidaka, T
EM: Iidaka@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Kawakatsu, H
EM: hitosi@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Morita, Y
AF: Earthquake Research Institute, Univ of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Dzung, N
AF: Institute of Geophysics, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Quan Cau Giay, Hanoi, 10000, Viet Nam
AB: Indochina block is located at the junction of Eurasia, Indian and Pacific plates. It has a close relationship with the uplift of Tibet plateau and the spreading of South China Sea basin in the geological evolution histories. The anisotropy is considered to directly relate to deformation in the mantle, and therefore provide constraints on tectonic and geodynamic processes. In this study, we present upper mantle anisotropy beneath Indochina block and adjacent regions from shear wave splitting analysis of Vietnam array and IRIS data. The data used here recorded by Vietnam array during the period between February 2000 and October 2005. The array consists of 6 broadband seismometers. We employed a compact HDD for data storage, which enables long-term and stable observation. We also used data form three IRIS stations around Vietnam for the same period. All records are band-pass filtered between 0.02 and 1.0 Hz to eliminate background noise. The dominant time windows used are 10-15 s around SKS phases based on the predicted travel times from the iasp91 model. We used the methodology of Silver and Chan (1991) as modified by Walker (2004) to determine the fast polarization direction and the delay time between the fast and slow components. In order to find the best splitting parameters, a grid search over possible values is performed to linearize the shear wave particle motion when the effect of the anisotropy if removed. The error estimation of each combination of splitting parameters is given by 95 percent confidence level of F test. We are able to characterize the splitting patterns in greater detail because of the use of new data. The average value of delay times in Indochina block is about 1.35 s, larger than that in adjacent regions. Tomographic studies also revealed low velocity anomalies in upper mantle beneath study area (Lebedev and Nolet, 2003). We estimate that the Indochina block might be affected by the mantle convection produced by the spreading of the South China Sea. Fast polarization directions vary significantly from south to north, indicating complex deformation in the upper mantle beneath study area. We suggest that the Indochina block might be divided into two sub-blocks according to the historic seismicity and tectonic activities. References Silver P.G., and W.W. Chan, Shear wave splitting and subcontinental mantle deformation, J. Geophys. Res., 96: 16429-16454, 1991. Lebedev, S., and G. Nolet, Upper mantle beneath Southeast Asia from S velocity tomography. J. Geophys. Res. 108, 2048-2074, 2003. Walker K.T., G.H.R. Bokelmann, and S.L. Klemperer, Shear-wave splitting reveals mantle upwelling beneath eastern Nevada, USA, Earth Planet Sci. Lett., 222, 529-542, 2004.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7294 Seismic instruments and networks (0935, 3025)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 9320 Asia
SC: Tectonophysics [T]
MN: 2007 Fall Meeting