Varying Seismic Property of the Earth’s Inner Core Boundary

Abstract:

The seismic property of the Earth’s inner core boundary (ICB) is important for our understanding of the evolution and dynamic process of the Earth’s core. We study the seismic properties of ICB by analyzing pre-critical PKiKP phase, a compressional wave reflected off the inner core surface. We collect seismic data recorded by two dense seismic arrays, Hi-net and USArray, in the distance range of 0-100 degree for more than 3000 events from 2004 to 2014 with magnitude greater than 5.8, and perform a systematic search for high-quality PKiKP phases. The PKiKP-PcP(P) differential travel time residuals exhibit variations as large as ±2 s in some regions and nearly no variations in some other regions. We rule out the possible effects of lower mantle heterogeneity and CMB structure, and attribute the observed PKiKP-PcP(P) variations to the irregular topography of inner core surface. Comparisons of regional stacking waveforms and spectral ratios of PKiKP/PcP(P) suggest a possible ICB transition zone in some regions. The thickness of the ICB transition zone is constrained to be no more than 3 km and exhibits variations in 50 km scale. The geographic variations of ICB seismic properties indicate complex thermo-dynamic process near the ICB.