HR: 0800h
AN: T11E-1321    [Abstracts]
TI: High-resolution investigation of lowermost mantle anisotropy beneath the Central Pacific
AU: * Rokosky, J M
EM: jrokosky@es.ucsc.edu
AF: Earth Sciences Dept. University of California Santa Cruz, 1156 High St, Santa Cruz, CA 95064 United States
AU: Lay, T
EM: tlay@es.ucsc.edu
AF: Earth Sciences Dept. University of California Santa Cruz, 1156 High St, Santa Cruz, CA 95064 United States
AU: Garnero, E
EM: garnero@asu.edu
AF: Dept. of Geological Sciences Arizona State Univ., Box 871404, Tempe, AZ 85287-1404 United States
AB: Analysis of lowermost mantle anisotropy is a crucial step in fully characterizing the deep mantle. While the majority of studies of shear-wave splitting in D'' report behavior compatible with vertical transverse isotropy (VTI), some observations indicate more complicated structure. Russell et al. (1999) propose that D'' beneath the Central Pacific exhibits azimuthal anisotropy with symmetry orientation relative to northwest trending ray paths changing from transverse to parallel over a few hundred kilometers. Such a rapid change in symmetry orientation warrants further investigation. Over the past few years a wealth of new data have become available, allowing us to further investigate this region with higher spatial resolution and to characterize the type, lateral variability, and depth extent of lowermost mantle anisotropy. We utilize a large number of Tonga-Fiji events recorded by California stations to assess shear-wave splitting of core reflections [ScS]. Waveforms are deconvolved by average source wavelets estimated by stacking transverse ScS arrivals for each event in order to improve the temporal resolution of arrivals and to give uniformity in signals between events. Corrections are also applied for lithospheric anisotropy beneath the receivers. Shear-wave splits [ScSH-ScSV] and differential travel times [ScS-SDATA - ScS-SPREM] are calculated for over 390 records from 37 events, a nearly five-fold increase in data from previous work in the region. There is trend of increasing ScS travel time delays from southwest to the northeast, suggesting that deep mantle shear velocity decreases in this direction. While ScS splitting is pervasive, it is not as simply organized as suggested in prior work, with substantial intermingling of ScSH or ScSV advances. Evaluation of focal mechanism effects, near-source anisotropy, and azimuthal anisotropy in D'' will be presented, and the relationship, if any, to volumetric shear velocity variations will be assessed.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 7207 Core and mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting