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We analyze the sensitivity of PP precursor traveltimes that are often used to infer lateral variation in the depths of the 410- and 660-km discontinuities in the mantle. Previous results were inconclusive due to complex wave phenomena, such as multiple energy conversions and focusing/defocusing, that hamper their interpretation. Using spectral-element synthetics and Frechet derivatives calculated with adjoint methods, we compute sensitivity kernels for volumetric and boundary parameters in a 1-D model for representative epicentral distances of past studies, and a dominant period of 11-25 s. Our results indicate that the boundary sensitivity of PP precursors is low and that these phases are not coherently seen in exact synthetics. Our most important finding is the strong sensitivity to both shear and compressional wave speeds, indicating that wave interference and wave conversions are dominant. The PP precursor traveltimes appear more sensitive to structural parameters, that is, compressional and shear wave speed, than to the boundaries; therefore, they are unlikely sources for valuable insight into discontinuity topography.

Plain Language Summary Seismologists frequently use traveltimes of seismic waves to understand how the Earth's internal structure changes at various depths. This study presents pictorial examples of how traveltimes of seismic waves sense the depth variations inside the Earth. We focus on waves that reflect from specific depths, and they are often used to explain topography along interfaces at 410- and 660-km depths. Along these depths, the ambient mantle conditions change significantly, indicating areas of major changes in material distribution, which enable vigorous movements within the Earth and cause geological phenomena observed at the surface. We aim at explaining why resulting models using these traveltimes are not in agreement. To achieve that, we use exact methods that give an accurate picture of the sensitivity of the traveltimes under investigation. Our contribution is important because we visualize many effects relating to how the waves travel, convert, and interact with each other. These effects, if not taken into account in an exact manner, could lead to inaccurate mapping of topography. Our explanations can help seismologists understand the reasons of disagreement between models produced using these traveltime data. We conclude that further interpretations relating to temperature and compositional variations should be considered with great care.