HR: 0830h
AN: U51B-0002    [PDF]
TI: Anisotropy of S wave velocity in the lowermost mantle using broad-band data recorded at Syowa in Antarctica
AU: * Usui, Y
EM: yusuke@hakusan.s.kanazawa-u.ac.jp
AF: Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, 920-1192 Japan
AU: Hiramatsu, Y
EM: yoshizo@hakusan.s.kanazawa-u.ac.jp
AF: Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, 920-1192 Japan
AU: Furumoto, M
EM: furumoto@hakusan.skanazawa-u.ac.jp
AF: Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, 920-1192 Japan
AU: Kanao, M
EM: kanao@nipr.ac.jp
AF: National Institute of Polar Resaerch, 1-9-10 kaga, Itabashi-ku, Tokyo, 173-8515 Japan
AB: We investigate the velocity structure of the lowermost mantle (D") beneath the Antarctic Ocean. We analyze seismograms from 16 deep earthquakes in south Pacific subduction zones from 1990 to 2001 recorded by STS-1 broad-band seismographs at Syowa station in Antarctica. The source-receiver combinations span distances range 85\deg-95\deg with associated S waves passing through D" beneath the Antarctic ocean. Differential travel times of split S waves are estimated to be up to 2s, showing that longitudinal components (SV) energy arrives earlier than transverse components (SH) energy. The absence of significant splitting for S waves with turning points more than four hundred kilometers above the core-mantle boundary (CMB) indicates that anisotropy is localized within the D" region. Differential travel times among S, ScS and SKS phases and waveform modeling are used to construct the velocity structure in D". We calculate synthetic waveforms by the {\it Direct Solution Method} (DSM: Geller and Ohminato, 1994; Geller and Takeuchi, 1995). SH shows a double arrival at the epicentral distance near 89\deg. However SV in this range remains a single arrival. Isotropic model_@can not explain these observation. We find that synthetics for transverse isotropic models with SH velocity discontinuity (SYYM model) explain well the observed differential travel times and waveforms. The thickness of the anisotropic zone, where SH wave is faster up to 2.0% than SV wave, estimated to be about 350 km. This study region corresponds to the high velocity region at the lowermost mantle by tomographic studies (Kuo et al., 2000; Masters et al., 2000). This kind of transverse anisotropy correlates with high velocity regions where paleo-slabs may descend into the lower mantle (Kendall and Silver, 1996; Garnero and Lay, 1997). We conclude that these observations may be explained by an anisotropic D" layer and D" layer anisotropy is attributed to the paleo-slab material subducted during 120Myr-180Myr.
DE: 7200 SEISMOLOGY
DE: 7207 Core and mantle
SC: U
MN: 2003 Fall Meeting