Multifrequency Sp Stacking Reveals a Strong Asthenospheric Discontinuity Beneath the Anatolian Region

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.