PKPPcP–PcPPKP Splitting: Observations, Modeling, and Implications for 3‐D Earth's Mantle Heterogeneity

Seismic body waves provide valuable insights into the interiors of the Earth, the Moon, and Mars. By analyzing their waveform characteristics, we can effectively reveal deep seismic velocity structures and associated dynamic processes. For the Earth, incorporating various body waves helps improve the ray path coverage and therefore the resolution of seismic imaging. For other planets with limited data, identifying seismic phases beyond the main P and S arrivals can provide critical insights into the picture of the deep interior. PKPPcP and PcPPKP phases propagate along the major arc of the great circle path, and are theoretically degenerate—arriving simultaneously—in a 1‐D spherical Earth model. Their splitting offers a unique opportunity to resolve 3‐D mantle heterogeneity, yet this potential remains under‐explored. Here we compile a global data set of high‐quality PKPPcP waves from 94 large events. Using PKKP ab as a reference phase, we quantify the observed PKPPcP–PcPPKP splitting and evaluate it against predictions from 3‐D mantle tomography models. Mantle tomography models generally underestimate the observed splitting, but the spatial correlation suggests that the observed splitting is linked to large‐scale structures in the deep mantle. We further analyze the features of PKPPcP travel time sensitivity kernels and conduct numerical simulations with various heterogeneous models. The simulations indicate that large‐scale velocity anomalies, such as large low‐velocity provinces and high‐velocity subducted slabs, are the primary contributors to PKPPcP–PcPPKP splitting. Our results highlight both the potential and the limitations of the PKPPcP phase in imaging 3‐D mantle heterogeneities.