Modelling and Interpreting Seismograms for 3D Earth Structure: A Study of Mid-Mantle Anisotropy

James Wookey. PhD thesis, School of Earth Sciences, University of Leeds, 2003.

The mid-mantle region of the Earth comprises the transition zone and the uppermost-lower mantle. The aim of this thesis is a study of mid-mantle anisotropy using shear-wave splitting analysis. New modelling techniques for 3D anisotropic models of the mid-mantle are used to interpret these observations.

A waveform modelling scheme based asymptotic ray theory and its extension, Maslov asymptotic theory is developed and applied to predict shear-wave splitting in em S-waves which turn in the mid-mantle (30-60 degrees epicentral distance). Models with an anisotropic layer below the 660 predict the largest splitting, assuming fixed levels of anisotropy.

em S-wave phases in 290 seismograms from earthquakes in the New Hebrides, Tonga and Kermadec subduction zones are analysed for evidence of shear-wave splitting at stations on the Australian mainland and Tasmania. Of these, 35 deep (deeper than 300km), well-constrained events show dt ranging between 0.85-7.1 s. On average the results suggest transverse isotropy, with SH-polarised waves leading SV. Modelling constrains the anisotropy to the uppermost lower-mantle, directly below the 660. A north-south variation in dt suggests that the SPO of melt inclusions or the LPO of perovskite by subduction-stresses are plausible explanations.

The effects of shear-coupled em P-waves on the measurement of shear-wave splitting are explored. Reflectivity modelling shows that the pollution of shear-wave splitting measurements is possible at short epicentral distances (around 30 deg). The Tonga-Australia dataset is reprocessed using a wavefield decomposition technique to remove these phases. The new results nevertheless agree with earlier inferences.

Isotropic body-wave travel-time tomography is an important source of information about the deep Earth, but the effects of anisotropy on it are uncertain. Constraining this requires accurate, efficient forward-modelling techniques. The properties of the Maslov-phase Theta in inhomogeneous, anisotropic media are exploited to develop a new 3D point-to-point raytracing scheme. The scheme is extended to allow the calculation of teleseismic body-wave travel-times and is used to demonstrate that, in a subduction-zone model, the effect of anisotropy on travel-time can be as significant as heterogeneity.

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