[SIT21-P24] Sharpness of the hemispherical boundary in the inner core beneath the northern Pacific
Keywords:Inner core, Waveform inversion
In the analysis, waveform inversion approach is employed for each event data to measure traveltime and attenuation in the inner core. As a result, traveltime anomalies observed by EU stations show obvious positive anomaly suggesting mainly propagating the eastern hemisphere, while traveltimes observed by USAray show relatively smaller (or no) anomalies suggesting effects from both of hemispheres. On the other hand, attenuation parameters for both arrays show similar trend that suggests high attenuation corresponding to the eastern hemisphere. To investigate a hemispherical transition boundary in the sampled region, we conduct a ray theory based forward modeling of traveltimes by changing a shape of boundary from meridian boundary to eyeball shaped boundary, and its position. We compute root-mean-square (RMS) of residual between observations and theoretical traveltimes computed with each boundary model. Then, an eyeball shaped boundary, that is located at longitude of 158°W on the equatorial line and latitude of 80°N at the northernmost point, obtained as the preferred boundary model. Finally, we investigate a sharpness of the boundary by varying a width of hemispherical transition. We assume linear variation from the eastern and western hemisphere in a given width and compute RMS. As a result, a width of 600 km at the surface of the inner core shows smaller RMS than sharp boundary. Geophysical significance of this finding will be further discussed.