PKIKPPKIKP Versus PKiKPPKiKP : A New Pair of Seismic Phases for Mapping the Earth’s Inner Core

Abstract

The Earth’s uppermost inner core (IC) is a critical boundary region where thermal, compositional, and mechanical interactions between the liquid outer-core and solid IC influence solidification processes and the geodynamo. Previous studies relied primarily on PKIKP–PKiKP seismic phases recorded at epicentral distances of 130°–142°. Here, we introduce a complementary approach by analyzing PKIKPPKIKP–PKiKPPKiKP differential times near 90° from deep earthquakes. These rare, multiply reflected phase pairs sample the IC on both descending and ascending paths and share similar trajectories, minimizing the influence of mantle heterogeneity. We measure their differential arrival times via waveform cross correlation in the 0.7–2 Hz frequency band and compute residuals with respect to synthetic seismograms that account for earthquake source mechanisms. These residuals are directly sensitive to P-wave velocity anomalies in the IC. In addition, we identify a new small set of PKIKPPKIKP and PKPbcPKPbc arrivals near 60°, which could support future studies of deeper IC regions. Statistical analysis using Akaike and Bayesian information criteria favors a single-parameter isotropic model, suggesting that the observed residuals can be explained without introducing additional anisotropy. Our results highlight the untapped value of rare, multiply reflected core phase pairs in probing Earth’s deep interior.