HR: 0800h
AN: T11E-1319    [Abstracts]
TI: D$''$ anisotropy from differential {\it S-ScS} splitting
AU: * Wookey, J
EM: j.wookey@earth.leeds.ac.uk
AF: School of Earth Sciences, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT United Kingdom
AU: Kendall, J
EM: m.kendall@earth.leeds.ac.uk
AF: School of Earth Sciences, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT United Kingdom
AU: Rumpker, G
EM: rumpker@gfz-potsdam.de
AF: GFZ-Potsdam, GeoForschungsZentrum Potsdam Telegrafenberg, Potsdam, D-14473 Germany
AB: The boundary layers of the Earth have been shown to have the most significant seismic anisotropy. Anisotropy has been observed in the upper mantle, near the 660 discontinuity and in the lowermost mantle. Anisotropy in D$''$ has been variously attributed to the shape-preferred orientation of subducted materials (such as basaltic melt) and the lattice preferred orientation of MgSiO$_3$ perovskite and MgO magnesiow\"{u}stite, aligned by dynamic processes in the region. Observations of anisotropy in the lowermost mantle have been made from normal-mode oscillation data, but evidence comes predominantly from body-wave studies measuring the splitting in {\it S}, {\it ScS}, {\it Sdiff} and {\it SKS/SKKS} phases. A major problem in such studies is accurately accounting for anisotropy in the source region and in the upper mantle below the station. One method by which this can be circumvented is by using {\it S-ScS} differential splitting (in the distance range 65--85 degrees), that is, using the splitting in the {\it S}-phase of a seismogram as a pre-analysis correction for the {\it ScS}-phase. We study the feasibility of using this method to determine shear-wave splitting in the D$''$ layer. One problem we highlight is the complication associated with the phase-shift in the {\it ScS} across part of the distance range studied. This introduces complex particle motion which we show to contaminate the analysis of shear-wave splitting. We develop a correction for this using an estimate of the complex reflection coefficients at the CMB, and show that after this correction we can recover accurate shear-wave splitting parameters in synthetics. We also employ an automated shear-wave splitting analysis algorithm which uses cluster analysis to improve the robustness of the results. We apply this method to earthquakes from the Western Pacific region recorded at Canadian National Seismic Network stations; these sample D$''$ beneath the north Pacific. The residual splitting in {\it ScS}, which we attribute to D$''$, shows lag times between 0.8--3.25\,s. Given the near-horizontal raypath of {\it ScS} in D$''$, we interpret the recovered fast directions as the orientation of the fast shear wave in the vertical-transverse plane. The largest population of results shows a south-easterly dipping fast axis. The level of complexity shown in the results and the possible contributions to D$''$ anisotropy from lower mantle minerals, melt and subducted materials suggest that the current paradigm for simply resolving transverse isotropy is, at least in places, inadequate.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting