HR: 0800h
AN: T11E-1329    [Abstracts]
TI: Seismic anisotropy in the lower mantle: a comparison of waveform splitting of SKS and SKKS
AU: * Perez, A M
EM: anisa@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main Street, Houston, TX 77005 United States
AU: Niu, F
EM: niu@rice.edu
AF: Department of Earth Science, Rice University, 6100 Main Street, Houston, TX 77005 United States
AB: Splittings of the {\it SKS/SKKS} waves are caused by seismic anisotropy along the receiver-side ray path from the core mantle boundary to the receiver. Yet, the splittings are usually assumed to be originated from the upper mantle anisotropy formed by tectonic processes, which is supported by both seismological and laboratory observations. Recent laboratory study, however, shows that dislocation creep occurs in $MgSiO_3$ perovskite at pressure-temperature conditions of the uppermost lower mantle. Seismic anisotropy is indeed observed at middle and lower mantle depths in some regions. In this study, we carried out a global investigation on seismic anisotropy in the lower mantle by comparing the waveform splitting of {\it SKS} and {\it SKKS} found at the same seismogram. The two shear waves have ray paths very similar in the upper mantle but different in the lower mantle. Any difference in splitting between the two indicates the presence of seismic anisotropy or dipping structures in the lower mantle. Since {\it SKS} and {\it SKKS} are indistinguishably used in measuring the upper mantle anisotropy to increase azimuth coverage, a systematic comparison of the splitting behavior of the two is also essential to the justification of the usage. We collected a total of 104 {\it SKS+SKKS} waveform data recorded at 76 stations. We chose the data based on the following criteria: (1) both phases are clear shown on the radial component. A signal-to-noise ratio (SNR) $>$ 3 is used for both $SKS_{R}$ and $SKKS_{R}$. (2) Amplitude of $SKS_{R}$ and $SKKS_{R}$ are comparable with each other. We limited the amplitude ratio $SKKS_{R}$/$SKS_{R}$ to the range of 0.5-2. This ensure us to obtain same precisions of the two in splitting measurements; (3) there are no other phases, such as the depth phase of {\it SKS}, arrive at the time window of {\it SKKS}. The {\it SKS+SKKS} data are first matched with a single anisotropic model (2 parameters; the fast polarization direction \phai and delay time $\delta{t}$) and two independent anisotropic models (4 parameters). We then applied the F-test to examine whether the 4-parameter models are really better than the 2-parameter ones in terms of error improvement. We found that the data from most of the stations can be explained by the simple 2-parameter models. While this observation provides the compelling evidence that vast part of the lower mantle below $\sim$1000 km (including the D" region) is transverse isotropic in most regions, it is still arguable that the uppermost lower mantle and the transition may partly contribute the {\it SKS/SKKS} splittings. We also found that the 4-parameter models provide a better fitting to the {\it SKS} and {\it SKKS} splitting at 8 stations, suggesting the existence of transverse anisotropy or anomalous dipping structures in some part of the lower mantle.
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
DE: 3902 Creep and deformation
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting