Interpreting spatially stacked Sp receiver functions

(CCP) of Sp receiver revealed variations of lithospheric thickness across short horizontal receiver reflection, towards quantifying the interpretability of Sp receiver functions, especially in settings where large lateral structure variations are present. Using the spectral element method, we model wave propagation and S -to- P conversion through simple synthetic models with varying velocity interface topography. We systematically explore the effects of wave frequency content, seismometer spacing and illumination geometry on CCP stacked Sp receiver functions in settings where velocity interface depth varies laterally. We observe that the resolving power of Sp receiver functions decreases with decreasing frequency content, and that upward deflections of velocity interfaces are more difficult to observe than are downward deflections, an asymmetry that primarily arises due to corner diffractions. Furthermore, we document how the relationship between the angle of illumination and the orientation of the topography of the velocity interfaces largely determines the apparent interface slope and strongly affects the amplitude of Sp phases in the CCP stacks. Indeed, under certain illumination geometries, strong velocity contrasts across a dipping lithosphere–asthenosphere boundary may not produce detectable Sp phases at the surface. Furthermore, diffractions arising from corners of interface topography can produce artefacts in CCP stacks that masquerade as mid-lithospheric impedance jumps or drops, as well as gently sloped sublithospheric impedance drops. We find that estimates based on Fresnel zone arguments might, in some cases, underestimate the true resolution, and that they are likely to be only appropriate for situations in which abrupt lateral variations in structure do not produce waveform complexities. These results imply that the interpretation of Sp receiver functions and CCP stacks is not straightforward and that care must be exercised when inferring the presence or absence of lithospheric velocity interfaces.