Antipodal seismic observation locates liquid region on the Earth’s Inner Core

Abstract The Earth's inner core is thought to have been formed by the precipitation of iron from the fluid outer core. It is considered that a part of the inner core surface where iron in the fluid outer core is precipitated may be melted and formed a mushy region, but its position is not well understood seismologically. We recently analyzed seismic waveforms observed at the antipodal station of the seismic source and showed that there are precursors to reflections beneath the inner core boundary from deeper in the inner core. It has been found that this precursory wave can be modeled as a reflection under the liquid/solid interface at a depth of 100 km below the inner core boundary. Here, we use these precursor waves observed at the antipodal station (> 179°). The sensitivity kernel for the shear wave velocity structure on the inner core surface was calculated by the adjoint method, using theoretical seismic waveforms. The location of the fluid region was identified using this kernel. Our results show two regions of the inner core surface where the shear wave velocity is close to zero. These results may provide seismological evidence to the origin of the earth's magnetic field.