HR: 0800h
AN: S41C-1019    [Abstracts]
TI: Small scale lateral variations in azimuthally anisotropic D'' structure
AU: * Rokosky, J
EM: jrokosky@pmc.ucsc.edu
AF: Earth Sciences Department, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95060 United States
AU: Lay, T
EM: tlay@pmc.ucsc.edu
AF: Earth Sciences Department, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95060 United States
AU: Garnero, E
EM: garnero@asu.edu
AF: Dept. of Geological Sciences, Arizona State University, Box 871404 Arizona State University, Tempe, AZ 85287 United States
AB: Characterizing D" anisotropy has the potential to provide important information on lowermost mantle structure and dynamics. However, isolating anisotropy in D" remains challenging because it requires corrections for anisotropy along other parts of the ray path, which are often difficult to constrain. We present results from ScS splitting analyses for two contrasting regions of D", the anomalously fast region beneath the Cocos plate and the markedly slow Central Pacific. Both datasets are larger than in previous studies. Waveforms are instrument deconvolved, and then deconvolved by average source wavelets to improve temporal resolution of arrivals and to give uniformity in signals between events. Data are corrected for previously determined estimates of upper mantle anisotropy. To constrain possible contributions from source-side anisotropy, we perform polarization analyses on an expanded dataset of S waveforms, yielding average source anisotropy terms for each event. We then seek a D" anisotropy term that best linearizes ScS from a grid search over all possible azimuths, and shift times of up to 2.5 s between fast and slow components. Data are subsequently corrected for the previously determined source term before calculating the covariance matrix between the two orthogonal components of ground motion. The D" term that produces the smallest second eigenvalue maximizes particle motion linearity for each arrival. Our results for both study regions reveal azimuthal anisotropy with systematic lateral variations in fast direction occurring over less than 200 km. For example, results beneath the Cocos plate show an abrupt change from fast directions nearly transverse to raypath (i.e., SH fast) to fast directions approximately parallel to raypath (i.e., SV fast). This change occurs near 7 deg latitude. North of 8 deg latitude, fast directions transition into a more scattered behavior, although the bulk are nearly transverse to raypath, as in the south. The middle patch correlates well with an elevated D" discontinuity, suggesting a link between bulk D" structure the character of anisotropy within the layer. Taken together, these results suggests that anisotropy in D" is azimuthal in character and changes on scale lengths of less than 200 km, consistent with scales of heterogeneity presented in past high resolution imaging studies.
DE: 7203 Body waves
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Seismology [S]
MN: Fall Meeting 2005