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 core1. 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 region2-4, 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 may be a clear precursor of lower reflections at the inner core boundary5. It has been found that this precursor wave can be successfully modeled as a reflection under the liquid / solid interface at a depth of 100 km below the inner core boundary. In this study, we use these precursor waves of the lower reflection at the inner core boundary 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 corresponding to these precursor waves, using theoretical seismic waveforms. The location of the fluid region was identified using the obtained sensitivity kernel. Our results show two regions of the inner core surface where the shear wave velocity is close to zero and is considered fluid. These regions are consistent with the westward shifts of geomagnetic anomalies observed on the Earth's surface and the regions of upwellings expected from geomagnetic dynamo simulations in the outer core3. These results may provide a seismological evidence that the inner core is locked to the mantle through the geodynamic motion in the outer core and provide new insights to the origin of the earth's magnetic field.