The Earth's upper inner core has a quasi-hemispherical velocity structure. One of the two quasi-hemispheres has a high-velocity region centered beneath Southeast Asia and is referred to as “East”. The remaining low-velocity region is referred to as “West”. The boundary positions between the quasi-hemispheres are rather scattered but well estimated at low- and mid-latitudes, whereas discrepancies between seismological hemisphere models increase at high latitudes. However, the boundary position at high latitudes is important when considering the origin of the quasi hemisphericity. Recent deployment of a temporary broadband seismic array in Thailand (Thai Seismic Array; TSAR) provides a means to analyze the inner core structure in the north polar region because rays from Central American earthquakes to the network are transmitted through the inner core and have their turning points approximately 150–400 km beneath the north polar region of the inner core boundary. We measured the differential traveltimes of earthquake waveforms between a seismic phase transmitted through the inner core (PKIKP, DF) and a phase that turns in the lower outer core (PKPbc, BC). The observed differential times are approximately 0.2–0.3 s smaller than theoretical estimates predicted by the Preliminary Reference Earth Model (PREM) over the observed distance range and 0.3–0.5 s smaller than those of AK135 at larger distances, which suggests lower velocities in the analyzed region of the upper inner core. Our analyses show that models with velocities ~0.03 km/s lower than those of PREM in the upper 300 km of the inner core, with a velocity about midway between PREM and AK135 in the F layer, sufficiently fit the observed differential times. This low-velocity feature in the upper inner core is characteristic of the “West”. The observations are also largely explained by the model obtained for the structure beneath the northeast Pacific in mid-latitudes classified as “West”. Our previous study also shows that the south polar region of the upper inner core has “West” characteristics. Our results thus suggest that the north and south polar regions of the upper inner core have a “West” structure and that the Earth's upper inner core has an eyeball-shaped high-velocity anomaly called the “East”. The shape of the hemispheric boundary is possibly an ellipse shortened in the north-south direction rather than a circle.
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