Using Extensive Core-Reflected Phases to Constrain Sharp Inner Core Boundary Beneath East Asia

The physical properties of the Earth's inner core boundary (ICB) are crucial for understanding inner core growth and geodynamo generation. In this study, we analyze the differential travel time residuals (DTTRs) and waveform similarities of the core-reflected phases (PKiKP and PcP) to investigate fine-scale seismic structures of the ICB. To study the ICB beneath East Asia, we collect a total of 4,272 PKiKP and PcP phase pairs from 37 earthquakes occurring from January 2009 to December 2018 recorded by permanent stations in the Chinese Digital Seismic Network (CDSN). This extensive PKiKP dataset allows for a geographically continuous depiction of the ICB beneath East Asia, revealing a diverse scope on its topography and sharpness. There are three main findings in our study. First, we collect a comprehensive PKiKP and PcP dataset with extensive ray coverage across East Asia. Numerous weak precritical PKiKP signals are detected, enabling robust constraints on the ICB across wide regions. Second, the PKiKP-PcP DTTRs exhibit an average offset of −0.25 s (ranging from −1.5 to 1.0 s) relative to the PREM model, suggesting the outer core is approximately 1.3 km thinner than predicted by the PREM model, although it remains largely consistent with it. Third, the bin-stacked PKiKP and PcP waveforms show high similarity across most sampled areas, indicating that the ICB thickness is no more than 2 km mostly beneath East Asia, which aligns with the hypothesis of a rapid transition from the solid inner core to the liquid outer core in our study region.