HR: 16:15h
AN: T14B-02 INVITED     [Abstracts]
TI: SEISMOLOGICAL EVIDENCE FOR GENERAL ANISOTROPY IN THE D" LAYER BENEATH THE CARIBBEAN
AU: * Garnero, E J
EM: garnero@asu.edu
AF: Arizona Dtate University, Department of Geological Sciences , Tempe, AZ 85287-1404 United States
AU: Maupin, V
EM: valerie.maupin@geo.uio.no
AF: University of Oslo, Department of Geosciences PO Box 1047, Blindern, Oslo, 0316 Norway
AU: Lay, T
EM: thorne@es.ucsc.edu
AF: University of California, Santa Cruz, Department of Earth Sciences 1156 High St. , Santa Cruz, CA 95064 United States
AU: Fouch, M J
EM: fouch@asu.edu
AF: Arizona Dtate University, Department of Geological Sciences , Tempe, AZ 85287-1404 United States
AB: Current models of anisotropy in D" are based on time delays observed between the SH and the SV components of core-reflected or core-grazing phases, which can be explained by the simplest form for anisotropy, vertical transverse isotropy (VTI). In a detailed analysis of high-quality broadband S (or Sdiff) phases, we find that many recordings do not show such simple delays; rather, some coupling between the SH and SV components is apparent: SV components have a small initial pulse with the polarity opposite to that predicted by isotropic or VTI structures. We analyze deep South American events recorded by the Canadian National Seismic Network, sampling D" under the Caribbean Ocean and Central America. The data are corrected for upper mantle anisotropy and compared to full wave theory synthetic seismograms calculated from isotropic, transversely isotropic, and azimuthally anisotropic structures. While restrictions in azimuthal sampling of this region limit a complete characterization of D" anisotropy, the need for azimuthal anisotropy is robustly established. A simple form of azimuthal anisotropy is explored which entails tilting the symmetry axis of transverse isotropy away from the vertical (TTI). Anomalous data are reproduced by a 20 degree tilt, and discriminate between eastward and westward tilting orientations, as well as zero tilts (i.e., isotropy or VTI). However, we note this does not preclude other forms of azimuthal anisotropy. Out of the 80 highest quality records analyzed, 22 are uniquely explained by the eastward TTI structures (with 34 records compatible with such a structure); 16 are compatible with westward TTI, with 2 uniquely requiring such a tilt. This gives strong evidence for lateral heterogeneity in azimuthal anisotropy at a lateral scale of a several 100 km. Further interpretation of the results requires identification the physical mechanism behind seismic anisotropy in D". Together with the recent discovery of the possibly anisotropic post-perovskite phase near D", the possibility of observing general anisotropy in D" opens completely new perspectives for mapping flow in the lowermost mantle.
DE: 7260 Theory and modeling
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
DE: 3924 High-pressure behavior
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting