An improved approach to Sp phase common-conversion point stacking that incorporates scattering kernels was applied to the Anatolian region and resolves the boundaries of an asthenospheric low velocity layer. With the new stacking approach, Sp receiver function amplitudes are projected around the converted wave ray paths only to locations with strong sensitivity to horizontal discontinuities. An expression for accurately estimating the standard deviation of the stack amplitude was also derived. This expression is more efficient than bootstrapping and can be used for any problem requiring the standard deviation of a weighted average. We also developed a method to more accurately measure near surface compressional and shear wave velocities, which are used to separate P and SV waveform components by removing free-surface effects. We applied these improved approaches to data from the Anatolian region, using multiple bandpass filters to better image velocity gradients of varying depth extent. Common conversion point stacks of 23,787 Sp receiver functions contain a clear Moho and 410-discontinuity, but also reveal a less common positive velocity gradient at 80-150 km depth beneath most of the region. The latter is particularly prominent at longer periods (10-100 s), indicating that it is relatively gradual in depth. This feature represents the base of an asthenospheric low velocity layer that is consistent with high mantle temperatures and the presence of partial melt. At shorter periods (2-20 s), a negative velocity gradient corresponding to the lithosphere-asthenosphere boundary is observed at 60-90 km depth, marking the top of the asthenospheric low velocity layer.

We present the first continental-scale seismic model of the lithosphere and underlying mantle beneath Southeast Asia obtained from adjoint waveform tomography (often referred to as full-waveform inversion or FWI), using seismic data filtered at periods from 20 - 150s. Based on >3,000h of analyzed waveform data gathered from ~13,000 unique source-receiver pairs, we image isotropic P-wave velocity, radially anisotropic S-wave velocity and density via an iterative non-linear inversion that begins from a 1-D reference model. At each iteration, the full 3-D wavefield is determined through an anelastic Earth, accommodating effects of topography, bathymetry and ocean load. Our data selection aims to maximize sensitivity to deep structure by accounting for body-wave arrivals separately. SASSY21, our final model after 87 iterations, is able to explain true-amplitude data from events and receivers not included in the inversion. The trade-off between inversion parameters is estimated through an analysis of the Hessian-vector product. SASSY21 reveals detailed anomalies down to the mantle transition zone, including multiple subduction zones. The most prominent feature is the (Indo-)Australian plate descending beneath Indonesia, which is imaged as one continuous slab along the 180-degree curvature of the Banda Arc. The tomography confirms the existence of a hole in the slab beneath Mount Tambora and locates a high S-wave velocity zone beneath northern Borneo that may be associated with subduction termination in the mid-late Miocene. A previously undiscovered feature beneath the east coast of Borneo is also revealed, which may be a signature of post-subduction processes, delamination or underthrusting from the formation of Sulawesi.