HR: 0800h
AN: S41C-1022    [Abstracts]
TI: Seismic anisotropy and velocity discontinuity in the lowermost mantle beneath the Antarctic Ocean
AU: * Usui, Y
EM: yusuke@hakusan.s.kanazawa-u.ac.jp
AU: Hiramatsu, Y
EM: yoshizo@hakusan.s.kanazawa-u.ac.jp
AU: Furumoto, M
EM: furumoto@hakusan.s.kanazawa-u.ac.jp
AB: We analyze seismograms from 54 deep earthquakes in South America from 1990 to 2004 recorded by 21 broad-band seismographic stations located in New Zealand, South America, Antarctica and Australia to study the anisotropy and velocity structure of shear waves in the D'' layer beneath the Antarctic Ocean. Their source-receiver distances are in the range of 65°~125°. We investigate three regions in the Antarctic Ocean (Region 1: beneath the southwest Pacific basin, Region 2: beneath southeast Pacific basin, Region 3: beneath Scotia Sea). The region 1 is high-to-low velocity transition zone at CMB, while the regions 2 and 3 correspond to the high velocity regions in model S16U6L8 (Liu and Dziewonski, 1998). We rotate the waveform data of the two horizontal components to the radial (SV) and transverse (SH) components of the shear wave signals. Then times of phase onsets are read on the seismograms. We measure SV-SH differential travel times (ΔTSV-SH) of ScS in the range of 60°~83° and those of S (Sdiff) splitting of beyond 83°. We generally observe VSV < VSH (SH leads SV) such as previous works beneath Alaska and the Caribbean Sea regions. ΔTSV-SH is -2.0~+6.0s in all data. The estimated anisotropy is -0.8~1.4%, -0.2~1.5% and -0.6~1.6% in the region 1, 2 and 3, respectively. There is small-scale lateral variations in the anisotropy and it suggest that the existence of the different type of anisotropy and possibly reflect the strong lateral variation of the lowermost mantle shear flow in the D'' layer. For SH, observations show D'' reflector (Scd) between S and ScS at 65°~92° in the region 2 and 3 due to triplication at the top of D'' layer. We also observe sScd phase between sS and sScS at the same range. We perform waveform modeling and the differential travel time analysis among S (Sdiff), Scd, ScS and SKS phases to constrain the anisotropic velocity structure in the lowermost mantle beneath the Antarctic Ocean. We focus on the region 2 and 3 because we have a sufficient number of data enough to constrain a unique anisotropic shear wave velocity model. We calculate synthetic waveforms using the Direct Solution Method (DSM: Takeuchi et al, 1996). Model PREM (Dziewonski and Anderson, 1981) is adopted as reference models. Analysis of SH-SKS differential travel times indicates that the lowermost mantle underlying the Antarctic Ocean must be faster than the PREM structure. The result of S-Scd and Scd-ScS shows that it is the possibly of D'' discontinuity at the depth of 2550km~2650km. In near and around the Antarctic Ocean, Olivieri et al. (1997) proposed the D'' discontinuity model (usb2) with 3% velocity jump at the depth of 2580km in the south part of our region 2 and model SYYM (Usui et al., 2005) with 2% at 2550km beneath the Antarctic Ocean, off the south coast of Australia. Although trade-off exist in the depth of D'' discontinuity, the degrees of velocity jump and D'' anisotropy, we consider that the D'' layer in the region 2 and 3 has the possibility of 1.5~3.0% velocity discontinuity at the top of D'' layer. Thick and anisotropic D'' layer may be exist beneath all regions of the Antarctic Ocean. We prefer may be associated with large-scale downwelling in the lower mantle and older subduction history than Alaska and/or Caribbean Sea regions.
UR: http://hakusan.s.kanazawa-u.ac.jp/
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 9310 Antarctica (4207)
SC: Seismology [S]
MN: Fall Meeting 2005