Anisotropic block in the upper inner core’s quasi-eastern hemisphere

Abstract:

We analyze the dataset of teleseismic PKP and PKIKP waveforms that enable sampling the upper solid core below Southeastern Asia in polar and equatorial planes. Polar and equatorial measurements of differential travel times and relative amplitudes of PKP and PKIKP are compared with ak135 and PREM and modeled with respect to standard Earth models and their modifications. We find the inner core volume restricted by the range of North latitudes from 18° to 23°, East longitudes from 125° to 135° and depths exceeding 170 km exhibits specific properties as contrasted with its neighborhood. Polar PKIKP propagation velocity in the volume is by 1.4% above ak135 velocity profile, while the equatorial one is only by 0.3%. Amplitude ratio of PKP and PKIKP in the equatorial paths is 1.5 – 2 times as large as in the polar paths. The distinction between polar and equatorial paths in terms of both differential travel times and differential amplitudes indicates the sampled volume features pronounced anisotropic properties, whereas higher propagation velocity corresponds to stronger attenuation. On the contrary, distinction between polar and equatorial traces sampling the inner core outside the delineated volume is minor (in terms of both differential travel time and attenuation), which indicates weaker anisotropy. The observed pattern of localized anisotropic anomaly can be interpreted as an evidence for the plume mode of the inner core convection or in terms of the domain structure.