Seismic structure near the inner-outer core boundary inferred from waveform modeling of HiNet data

Knowledge of the velocity and attenuation structure at the base of the outer core and top of the inner core is crucial to understanding core dynamics. Previous seismic studies of the inner and outer core looking at the PKP family of waves have found an anomalous low velocity gradient at the bottom 200 km of the outer core and a dichotomy within the inner core separating a fast eastern hemisphere from a slow western hemisphere. Velocity models of this region are often made using travel times with large variances and have difficulty fitting both the timing and amplitude of the waves. Herein, we stack data recorded by the extremely dense HiNet of Japan at epicentral distances of 148-160^o to create very low noise PKP AB, DF and BC waveforms sampling the two hemispheres of the inner core. Relative timing and amplitude of these phases allows us to isolate our study region because of the close proximity of their raypaths in the mantle. We use AB to reduce the ambiguity between lower outer core and upper inner core structure which occurs if only DF and BC is used. Our data sampling the western hemisphere shows an abrupt increase in BC/DF amplitude ratio at 151^o which is not predicted by AK135. We invert for velocity and attenuation models of the bottom 400 km of the outer core and top 600 km of the inner core using the reflectivity method in conjunction with very fast simulated annealing (VFSA). VFSA is a random walk algorithm that utilizes a decreasing probability function to control both step size and acceptance criteria to find the optimal model. The advantage of VFSA over other inversion algorithms is that it allows for a much larger parameter space to be explored and is less dependent on what starting model is used. We find that in order to fit the timing and relative amplitude of the PKP phases traveling through the western hemisphere there must be a low velocity zone preceding a sharp 1% increase in velocity at around 200 km below the ICB.