Boundary and volumetric sensitivity kernels of teleseismic receiver functions for mantle discontinuities in the transition zone

Abstract

Teleseismic receiver functions are widely used to map the depth and topography of various major discontinuities in the Earth's mantle. To determine what precisely contributes to the receiver functions, we applied the adjoint method of full waveform inversion to calculate their sensitivity kernels. These kernels illustrate the extend to which model parameters may influence the waveforms. We calculated synthetic data for a realistic event measured at a realistic receiver array, whereby we focused on the waveforms of the P410s and P660s phases, that convert a P to an S wave at the 410- and 660-discontinuity, respectively. We calculated both the volumetric sensitivity kernels for density, P- and S-wave speeds, as well as boundary kernels that illustrate receiver functions' sensitivity to topography on the discontinuity. In the boundary kernels, we observe that receiver functions are highly sensitive to a discontinuity's topography, in particular to an area surrounding the conversion point with a radius comparable to the Fresnel zone. However, the volumetric kernels illustrate a sensitivity to model parameters in large areas of the mantle. This includes sensitivity to the Fresnel zone of the converted wave far before the conversion, as well as sensitivity to scatterers of other phases. We therefore conclude that receiver functions are sensitive to the topography of discontinuities. However, effects of an incorrect velocity model, even far from the conversion point, may erroneously be projected onto the topography of the discontinuity. Therefore, a simultaneous inversion of topography and velocity parameters is required to image topography with high accuracy.