HR: 0830h
AN: U51B-0007    [PDF]
TI: Complexities in D" anisotropy beneath the Caribbean: Evidence for a tilted symmetry axis of transversely isotropic media from data and synthetics
AU: * Garnero, E J
EM: garnero@asu.edu
AF: Arizona State University, Department of Geological Sciences 871404, Tempe, AZ 85287-1404 United States
AU: Maupin, V
EM: valerie.maupin@geo.uio.no
AF: Department of Geology, University of Oslo PO Box 1047, Blindern, Oslo, 0316 Norway
AU: Lay, T
EM: thorne@es.ucsc.edu
AF: University of California, Department of Earth Sciences 1156 High Street, Santa Cruz, CA 95064 United States
AU: Fouch, M J
EM: fouch@asu.edu
AF: Arizona State University, Department of Geological Sciences 871404, Tempe, AZ 85287-1404 United States
AB: The goal of this study is to evaluate detailed seismic anisotropy in D'' for a broad region beneath the Caribbean Ocean. Our dataset consists of broadband core-grazing and diffracted shear waves for deep South American earthquakes recorded by the Canadian National Seismic Network. The motivation for this work is to ultimately better constrain lowermost mantle dynamics and rheological properties. High quality data containing simple source-time functions and strong SH and SV energy are utilized, instrument-deconvolved to displacement, and rotated to the plane of the incident S wave using all 3 components to minimize any possible SV-P conversions. Finally, data are corrected for upper mantle anisotropy using either published or newly derived parameters. For most of our dataset, S and Sdiff phases exhibit differential lag times between the SH and SV components. Variations are generally simple, with SV energy arriving later relative to SH, but many records also show SV energy initiating with the wrong polarity compared to focal mechanism predictions. Small rotations in the plane of the incident S wavefield cause the precursory SV energy to dissipate in our cleanest data. This observation suggests the presence of anisotropy beyond the common assumption of transverse isotropy (TI) with a vertical axis of symmetry (VTI). To evaluate the complex nature of our observations, we have constructed synthetic seismograms for several end-member models of mantle seismic velocities, including (a) isotropy, (b) VTI, and (c) a simple anisotropic case: tilted TI. For case (c), SV behavior relative to SH depends heavily on the focal mechanism, azimuth of the incoming wavefield, and the tilt angle of the TI system. To first order, the eastern portion of our study area contains significantly more data that are incompatible with the simple VTI geometry. We will present data and synthetic comparisons, and the geographic distribution of data sampling locations best supported by either VTI or tilted TI.
UR: http://garnero.asu.edu/research/earthslowermantle.html
DE: 3909 Elasticity and anelasticity
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
DE: 7260 Theory and modeling
SC: U
MN: 2003 Fall Meeting