HR: 0800h
AN: DI41A-1251    [Abstracts]
TI: Modeling transition zone structure using multiple seismic discontinuity phases
AU: * Lawrence, J F
EM: jlawrence@ucsd.edu
AF: IGPP, Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093 United States
AU: Shearer, P M
EM: pshearer@ucsd.edu
AF: IGPP, Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093 United States
AB: Mantle transition zone discontinuities reflect and convert different amounts of seismic energy depending upon the wave type and incidence angle. In this study we use multiple seismic discontinuity phase observations to characterize the structure of the mantle transition zone. SS-precursors (SdS underside reflections) have long been used to resolve global impedance contrasts at about 410 km, 520 km, and 660 km depth. A recent global analysis of P-to-S converted phases (Pds) yields an average transition zone thickness (~ 242 km) and a pattern of lateral variations very similar to SdS results. While some individual stations may record P520s phases, globally the P520s phase is incoherent. Global analysis of PP-precursors (PdP underside reflections) demonstrates that while P410P and P510P have expected amplitudes, P660P has a very low amplitude, as noted by Estabrook and Kind (1996). The effects of interfering waves arriving simultaneously with P660P can be minimized by examining the radial component rather than the vertical component. Yet, even without interfering signals, P660P has a surprisingly low amplitude. We globally stack Pds, SdS, PdP, and Ppdp (topside P reflections) in order to constrain a best-fitting 1-D model of shear velocity, compressional velocity and density that explains all of the observed amplitude variations. Waveform modeling indicates that the 410-km discontinuity represents a large contrast in Vs (~ 7%), Vp(~ 6%), and density (~ 7%); at 520 km, Vp (~ 1.5%) and density (~ 1.5%) increase whereas Vs does not (~ 0%); at 660 km, Vs (~ 5%) and density (~ 5%) increase significantly whereas Vp has a small contrast (<1%) above a steep bottom-side gradient. This model of Vp, Vs, and density can be compared to elastic constants obtained through experimental and theoretical studies to infer rheologic and chemical state of the mantle.
UR: http://igpphome.ucsd.edu/~jlawrence/TZ/
DE: 4450 Nonlinear maps
DE: 4465 Phase transitions
DE: 7200 SEISMOLOGY
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
SC: Study of Earth's Deep Interior [DI]
MN: Fall Meeting 2005