Regional tomography of Earth's inner core

The P-wave velocity of the inner core is not uniform but contains seismic anisotropy, aligned with Earth's rotational axis, which varies laterally and radially. There are also variations in isotropic velocity. These global-scale differences have been probed by both normal mode and body wave data. The dominant lateral variation is the hemispherical pattern whereby the anisotropy is much stronger in the western hemisphere than in the eastern hemisphere. High frequency body wave data are also sensitive to smaller scale variations in seismic velocity inner core. Here, we use these data to probe regional scale inner core structure. We perform a tomographic inversion of body wave travel-time data to probe the isotropic and anisotropic velocity structure under the northern and western Pacific Ocean. We use high quality, high frequency PKPbc-PKPdf differential travel-time data. PKPdf is a compressional body wave which turns in the inner core; PKPbc has a similar path but turns in the relatively homogeneous outer core. By measuring differential travel-times we are able to eliminate problems caused by source mis-location and crustal structure, as well as reduce the impact of heterogeneous mantle structure on our measurements. We use EHB locations where possible and correct the data for ellipticity and known mantle structure. Data are collected from earthquakes occurring from Tonga to south of New Zealand and from Kamchatka to Japan, recorded at stations 146-156 degrees from the earthquakes. The distribution of earthquakes and stations used provides us with crossing raypaths under the northern and western Pacific Ocean; the rays have a range of angles from Earth's rotational axis, permitting inversion for both the anisotropic and isotropic P-wave velocities. The region sampled lies over the boundary between the eastern and western hemispheres; we investigate this boundary as a function of latitude. We also investigate the properties of the eastern and western hemispheres which lie on either side of the boundary. A better understanding of the regional scale heterogeneities in the inner core will inform studies of the growth of the inner core and provide information about the dynamics of the core.