Beyond Waveform Forward Modeling: The Lowermost Mantle Beneath the East of Australia

Seismic imaging of the lowermost mantle provides key information about its structure and dynamics, shaping constraints on mantle convection and heat transfer between the core and mantle. Ultra low velocity zones (ULVZs) sitting on top of the core-mantle boundary (CMB) are identified as small-scale structures with a sharp decrease in P- and S-wave velocity and an increase in density. Apart from small-scale features, it is also crucial to accurately image the large-scale features in the mantle because the dynamics of a boundary layer is closely coupled to the upwelling and downwelling motions of a convective system. Due to a high computational cost that more sophisticated inversion technique would impose, waveform forward modeling of the core-reflected and core-refracted waves is a widely used method for the investigation of ULVZs and other features of the lowermost mantle. In forward modeling, the density, velocity and thickness of layers are varied in a trial and error or simple grid-search fashion until they produce synthetic seismograms that match the main features observed in the seismic waveforms. It is often possible to convincingly model the observed waveforms by an ULVZ with different properties and geometry making forward solutions highly non-unique. It is also possible to generate a structural model that fits the waveform data, but is not necessarily required by the data. In order to address this problem we utilize transdimensional inversion, which is a Bayesian method that utilizes an ensemble of models representing the posterior probability distribution. The method treats the number of free parameters (e.g. the number of layers at the base of the mantle, their thicknesses, densities and velocities) as unknowns in the problem. Furthermore, the noise in the data is used to constrain the complexity of the model. This method thus carries the potential to advance our understanding about lowermost mantle structure and dynamics. Southwest Pacific subduction zones earthquakes recorded by the WOMBAT array short-period stations in southeast Australia and the WRA array broadband stations in Northern Territory enable us to achieve an unprecedented sampling of the CMB beneath east Australia. We examine more than 1000 (Mw>5.0) events and identify 27 events with clear onsets of core-reflected waves (ScP), out of which 9 events also contain clear PcP waves. We analyze the source-deconvolved ScP waveforms to identify subtle precursors and postcursors resulting from structures in the lowermost mantle. We can explain some of our observations with the existence of an ULVZ at the base of the mantle, but the transdimensional inversion reveals other classes of models as likely solutions overlooked by the limitations of conventional forward modeling.