HR: 0800h
AN: S41C-1025    [Abstracts]
TI: Whole mantle Vp/Vs tomography derived from broadband S-P travel time measurements
AU: * Oki, S
EM: toko@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, Tokyo, 113-0032 Japan
AU: Fukao, Y
EM: fukao@jamstec.go.jp
AF: IFREE, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka-shi, Kanagawa, 237-0061 Japan
AU: Obayashi, M
EM: obayashi@jamstec.go.jp
AF: IFREE, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka-shi, Kanagawa, 237-0061 Japan
AB: The P to S velocity ratio carries important information about the composition and state of the mantle. Comparison of the existing P and S tomography models, however, may not be always justified because of large differences in their accuracy and resolution. Our attempt was focused on the use of S-P differential travel times to constrain the three-dimensional structure of V_p/V_s ratio. Since S-P times are measured on the seismograms at the same stations for the same earthquake, the S to P velocity ratios based on these measurements are free from the difficulties associated with those derived from mutually independent P and S velocity models, and do not contain the errors arising from the uncertainty of origin times. We measured S-P travel times on the unfiltered broadband seismograms by cross-correlating the observed S waveform with those synthesized from the observed P waveform. The synthesis was made by taking the physical dispersion into account, where we set the reference frequency at 2 Hz which was confirmed by Oki et. al., [2004] to be most relevant to the hand-picked onset times. We obtained about 13,500 S-P times and about 13,000 handpicked P times. The scheme of our V_p/V_s tomography starts with the specification of the initial V_p/V_s model, which is one-dimensional and taken to be the same as AK135. The initial V_p model is the three-dimensional WEPP2 model [Fukao et. al., 2001], which remains to be unchanged throughout the inversion. Although our handpicked onset times of P were not incorporated in the hypocentral determinations using this three-dimensional V_p model, they show remarkable consistency with this model. The initial guesses of the S-P times are calculated from the initial V_p/V_s model and the fixed three-dimensional V_p model, which are found to be more consistent with the measured S-P times than those calculated with the spherically averaged V_p model. The data to be inverted are the residuals of measured S-P times from their initial guess. The model parameters to be determined are the difference of the three-dimensional distribution of V_p/V_s ratio from its initial model. We will show some features of the whole mantle three-dimensional V_p/V_s distribution. As a reconnaissance study before practicing V_p/V_s tomography, the measured S-P and hand-picked P data are examined carefully. Assuming that the sources of their residuals are concentrated in one layer of the mantle, we locate their possible sources and calculate the variance of the lateral variation of V_p/V_s ratio in that layer. The variance calculated in this way takes its broad minimum in the mid lower mantle and rapidly increases with depth below 2000 km.
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7270 Tomography (6982, 8180)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Seismology [S]
MN: Fall Meeting 2005