New Insights into ULVZs Using a Bayesian Inference: A Case Study for ULVZs Beneath the East of Australia

Abstract:

Large Low Shear Velocity Provinces (LLSVPs) are the most common features of shear wave velocity models of the lowermost mantle. In contrast, the existence of thin layers with strong decrease in P- and S-wave velocity and increase in density - known as ultra low velocity zones (ULVZs) - is still ambiguous at some locations of the core mantle boundary (CMB). Moreover, ULVZ parameters are highly non-unique because of the strong parameter correlations (i.e., trade-offs). The parameter uncertainty estimation is crucial for appropriate physical interpretation. These challenges cannot be addressed using a traditional forward waveform modeling. We thus previously developed a Bayesian inversion to address these difficulties. In a Bayesian inversion, the prior knowledge on the model parameters is combined with the data information described by the likelihood function. Ultimately, the ensemble of solutions is obtained in terms of posterior probability density (solution of the inverse problem). To investigate the unknown parameterization (i.e., the number of layers), we utilized the Bayesian Information Criterion and applied it to study the ULVZ beneath the east of the Philippines [Pachhai et al., 2014, revised]. Here we build on the previous study and develop a trans-dimensional approach in which the number of free parameters (ULVZ layers) is treated as an unknown parameter in the inversion. We confirm that the ULVZ beneath the east of the Philippines is multi-layered, with dense and melt-rich iron material sitting on the bottom. Furthermore, we analyze ScP waves recorded by the Australian arrays that sample the CMB beneath the east of Australia. We examine more than 1000 (Mw>5.0) events and identify 36 events with clear onsets of core-reflected waves (ScP), out of which 9 events also contain clear PcP waves. These data enable us to achieve an unprecedented sampling of a significant area near the edges of the LLSVP under the southwest Pacific Ocean and give insights in fine details of the CMB. We interpret our findings in the context of known ULVZ morphology and our previous findings of different types of ULVZs, and discuss the spatial relationship relative to the Pacific LLSVP.