Berkeley Seismological Laboratory
Email:
mpanning@seismo.berkeley.edu
poster/oral: poster
| Many studies have documented the existence of anisotropy in the earth's upper mantle, concentrated in the top 200 km. This evidence comes from the study of surface waves as well as shear wave splitting. There is also evidence for shear wave splitting in D", at least in well sampled regions. There are some hints of anisotropy at the base of the transition zone. Tomographic models of the upper mantle have been developed with simplifying assumptions about the nature of the anisotropy, in order to minimize the number of free parameters in the inversions. Some assume transverse isotropy (e.g Ekström and Dziewonski, 1997), others include additional degrees of freedom with some realistic constraints on mineralogy (e.g. Montagner and Tanimoto, 1991). Our goal is to investigate anisotropy in the whole mantle, using the framework of waveform inversion, and the nonlinear asymptotic mode coupling theory (NACT), previously developed and applied to the construction of whole-mantle SH velocity models (Li and Romanowicz, 1996; Mégnin and Romanowicz, 2000). For this we require a 3 component dataset, and we have extended our automatic transverse (T) component wavepicking procedures to the vertical (Z) and longitudinal (L) component - a non-trivial task given the large number of phases present in the coupled P-SV system. A useful initial assumption is that of transverse isotropy, which is described by requires five elastic parameters. It is convenient to express this as isotropic P and S velocity, with three anisotropic parameters, similar to Montagner and Tanimoto, 1991, which can be reduced to three total parameters by assuming a d(ln Vs)/d(ln Vp) scaling relationship. We have been developing a degree 24 isotropic Vs model from all three components of data, building on the transverse component SH velocity model SAW24B16 (Mégnin and Romanowicz, 2000). With a good isotropic model starting point, we will perform inversions for values of anisotropic parameters in various depth ranges in the mantle, and try to determine where we can resolve anisotropic structure. A related area of interest is studying the validity of assumed d(ln Vs)/d(ln Vp) scaling relationships. While simultaneous models of shear and compressional velocity (or alternately shear and bulk sound velocity) with P sensitivity based primarily on relative and absolute travel times have been developed (i.e. Masters et. al., 1999), very little work has been done to explore Vp modeling using waveforms, primarily due to the higher dominant frequency of Vp data. Synthesizing waveforms at a higher frequency requires greater computational resources. We will present preliminary tests of the application of NACT waveform inversion to Vp modeling. |