Imaging the Mantle Transition Zone Using Multimode Waveform Tomography

The mantle tranisition zone (TZ) is defined by discontinuities in seismic wave velocities at 410 and 660 km depth, and is thought to play an important role in governing thermo-chemical transport between the upper and lower mantle, in addition to the dynamic evolution of the planet. Furthermore, it has long been at the center of debate between whole-mantle and multi-layered convection models. Although not so well resolved in early tomographic models, improvements in imaging methods and availability of high-quality seismic data have provided increasingly detailed images capable of capturing TZ structure. The variations in isotropic velocity and presence of anisotropy can be used to infer constraints on the thermal and chemical nature of the TZ, and ongoing deformation and transport. For anisotropy in particular, its detection in the deep upper mantle and TZ has remained elusive, with few global models showing significant azimuthal anisotropy at these depths.

We utilize automated, multimode waveform inversion to extract structural information from surface, S, and multiple-S waveforms, resulting in a dataset of more than 3/4 of a million successful vertical-component fits. Using a more precise regularization of anisotropy, tuned to honour the amplitude and orientation of anisotropic terms uniformly, we have generated two new models with improved TZ sensitivity to variations in isotropic heterogeneity (SL2013sv), and the detection of azimuthal anisotropy (SL2013svA). For instance high-velocity isotropic anomalies associated with subducted Tethyan lithosphere are detected beneath Eurasia, as well as subducted high-velocity oceanic lithosphere beneath North America. We quantify the requirement for TZ anisotropy through analysis of SL2013svA. Finally we also compare our recent tomography models with other global models (both isotropic and anisotropic), and discuss the implications of their similarities and differences on mantle transition zone properties.