Seismic Study on the Properties of the Earth's Inner Core Boundary

Abstract:

The Earth's inner core boundary grows from the freezing of the liquid outer core. The information of the inner core boundary can help us understand the driving forces of the outer core convection and the evolution history of the Earth. We study the inner core properties by analyzing the travel time and waveform of the compressional waves reflected off the inner core surface (PKiKP) recorded in the pre-critical epicentral distances. We collect the large datasets recorded in two dense seismic arrays in Japan (Hi-net) and the United States of America (USArray) from 2004 to 2014, and perform a systematic search for high-quality pre-critical PKiKP phases. The PKiKP-PcP differential time residuals exhibit large variations in some regions indicating presence of irregular topography and nearly no variations in some other regions suggesting a flat boundary. In some regions, the comparison of PKiKP and PcP waveforms constrains the thickness of the inner core boundary to be no more than 3 km, and in combination with the PKiKP/PcP relative amplitude ratios, the inner core boundary density jump to be 0.6~1.8 g/cm³. We present geographic variation of seismic properties in various regions of the inner core boundary and discuss its possible implications to the thermodynamic behaviors of the inner core boundary.