[SIT28-P02] Inversions for radially anisotropic upper mantle structure with the new fifth anisotropic parameter ηκ using multi-mode surface waves
Seismic anisotropy estimated from surface waves provides us with fundamental information to unravel dynamics and structure of the Earth’s mantle. Radial anisotropy is described by five elastic parameters; four parameters related to seismic wave speeds (βv, βh, αh, αv) and an additional fifth anisotropic parameter (η). One of the anisotropic parameters, η, was originally defined by Anderson (1968), but its physical proprieties have been rather unclear compared with other four parameters related to elastic velocity.
A newly proposed definition of the fifth anisotropic parameter ηκ by Kawakatsu et al. (2015) makes it easier to understand its physical proprieties compared with the conventional parameter η. The introduction of ηκ causes non-negligible influence on shape of sensitivity kernels of Rayleigh wave phase speeds with respect to ηκ, and PH-wave speeds αh and PV-wave speeds αv (Kawakatsu, 2016b). Since the sensitivity kernel for ηκ becomes higher than that for η, we may have a possibility of resolving ηκ . However, since the inverse correlation between the sensitivity kernels of SV-wave speed βv and ηκ becomes rather stronger, the trade-off between βv and ηκ may easily occur, which makes it difficult to interpret the resultant model.
In this study, by incorporating ηκ with several combinations of a priori parameters, we performed inversions for five elastic parameters in the upper mantle, based on an iterative nonlinear least-squares inversion method (Tarantola and Valette, 1982). We employed multi-mode dispersion data sets of surface waves in the Australian region to construct a preliminary 3-D anisotropic model. Regional variations of ηκ can be observed between Coral/Tasman seas and Australian continent . Beneath the continent, a positive anomaly of ηκ at asthenospheric depth was observed, which is located deeper than that beneath the oceanic region. In this preliminary model, the depth where ηκ is close to 1.0 seems to coincide well with the lithosphere-asthenosphere boundary. Care needs to be taken, however, for the interpretation of the ηκ model, since it can readily be affected by SV wave speed βv due to the strong trade off, which should be examined in more detail.
A newly proposed definition of the fifth anisotropic parameter ηκ by Kawakatsu et al. (2015) makes it easier to understand its physical proprieties compared with the conventional parameter η. The introduction of ηκ causes non-negligible influence on shape of sensitivity kernels of Rayleigh wave phase speeds with respect to ηκ, and PH-wave speeds αh and PV-wave speeds αv (Kawakatsu, 2016b). Since the sensitivity kernel for ηκ becomes higher than that for η, we may have a possibility of resolving ηκ . However, since the inverse correlation between the sensitivity kernels of SV-wave speed βv and ηκ becomes rather stronger, the trade-off between βv and ηκ may easily occur, which makes it difficult to interpret the resultant model.
In this study, by incorporating ηκ with several combinations of a priori parameters, we performed inversions for five elastic parameters in the upper mantle, based on an iterative nonlinear least-squares inversion method (Tarantola and Valette, 1982). We employed multi-mode dispersion data sets of surface waves in the Australian region to construct a preliminary 3-D anisotropic model. Regional variations of ηκ can be observed between Coral/Tasman seas and Australian continent . Beneath the continent, a positive anomaly of ηκ at asthenospheric depth was observed, which is located deeper than that beneath the oceanic region. In this preliminary model, the depth where ηκ is close to 1.0 seems to coincide well with the lithosphere-asthenosphere boundary. Care needs to be taken, however, for the interpretation of the ηκ model, since it can readily be affected by SV wave speed βv due to the strong trade off, which should be examined in more detail.