Three-dimensional Structure of the Mantle Transition Zone Revealed by High Resolution Wavefield Imaging

We applied 3D plane wave migration to image P to S conversions scattering bodies in a volume covering the entire lower 48 states to a depth of 900 km. The result is arguably the highest resolution image ever produced of the transition zone. We migrated two different versions of the available data to appraise reliability. (1) we deconvolved all data with picks made by the Array Network Facility using the generalized iterative deconvolution method. (2) we reshaped 141,080 sets of high quality receiver functions from the Earthscope Automated Receiver Survey. We migrated these data with all available high resolution tomography models that have full coverage of the study area. Point source scattering summation simulations showed that compared to the widely used common conversion point stacking method, our method is capable of resolving details of amplitude and topography variations at scales near the diffraction limit. We imaged topography on both the 410 and 660 discontinuities within a vertical range of ±30 km at all scales down to our resolution limit at about 200 km. Our simulations suggest that the actual d410 and d660 have even smaller scale topography variations that are below the scale of resolution. In addition, we consistently recovered negative P to S scattering potentials throughout most of the transition zone that do not resolve into horizontally coherent structures (Figure 1). We are able to approximately model the observed phenomenon with negative perturbation, point scatterers randomly distributed throughout the transition zone to match the mean and standard deviation retrieved from the observation. These randomly distributed low velocity bodies may relate to the heterogeneity caused by the subducted slab, water content or a mix of both.