Antipodal Seismic Observation and Sensitivity Kernel for the Liquid Region on the Earth’s Inner Core

It is considered that a part of the inner core surface where iron in the fluid outer core is precipitated may have 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 the PKIIKP phase reflected beneath the inner core boundary. 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 of the amplitude of these precursor waves for the shear wave velocity structure on the inner core surface was calculated by the adjoint method, using theoretical seismic waveforms. Our results might be used to locate regions of the inner core surface where the shear wave velocity may be close to zero.