Complex velocity structure in the uppermost inner core constrained by differential travel times and differential ray parameters

Abstract:

Various studies over the last decades have revealed complex velocity structures of Earth’s inner core. Especially, researches using differential travel times of body waves mainly contribute to finding more sophisticated and precise structures in the inner core out. However, observed differential travel times compiled from different studies showed wide deviations with respect to the mean values and it made reliable determination of heterogeneous structures difficult. Recently, using well-constrained differential travel times of PKPbc-PKPdf from dense Hi-net array, we reported that Vp in the uppermost inner core (top 350 km of the inner core) is regionally perturbed by about 1% within each quasi-hemisphere. Although our well-constrained data reliably showed the existence of regional heterogeneity, the detailed 2-D or 3-D heterogeneity structures cannot be constrained by only perturbed differential travel times. To overcome this limitation, we utilize not only the differential travel times but also the differential ray parameters of PKPbc-PKPdf to study Vp structure of the uppermost inner core. The differential ray parameters measured by slant-stacking of waveforms recorded at Hi-net array also show regional variations in both quasi-hemispheres. This observation is important because it supports previous results obtained from differential travel time measurements and provides additional information to better constrain a characteristic parameter representing a high-dimensional Vp structure. We test many models with different characteristic levels of heterogeneities to explain the observed regional variation of the differential travel times and differential ray parameters. The level of heterogeneities estimated in our study can be helpful to understand the nature of the uppermost inner core.