Seismic Evidence for Lateral Heterogeneity in the Basal Layer of the Outer Core

Abstract:

Seismological studies have proposed that the bottom 200-400 km of the outer core, known as the F-layer, has a lower P velocity gradient than the rest of the outer core. However, it is still controversial whether the reduced velocity gradient is a global feature. Most seismic studies of the inner-outer core boundary (ICB) region have used single pairs of differential PKP waves and do not consider simultaneously all the waves that potentially sample the region. Using just two PKP phases can result in ambiguity between F-layer and upper inner core structure. Here we present a new seismic velocity model for the ICB of a portion of the eastern hemisphere beneath the western Indian Ocean that is determined by simultaneously fitting the waveforms of all branches of PKP between 130-162^o distance sampled at 0.2^o intervals. By modelling the complete set of PKP waves we minimize trade off between outer and inner core structure. We created our well sampled and high signal to noise dataset by slant stacking seismograms recorded by dense seismic networks in China and Mongolia. We then inverted that dataset for the velocity and attenuation around the ICB 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 provides information on the uncertainty of each model parameter. Our results show that to simultaneously fit all phases, our model cannot have a reduced velocity gradient in the F-layer. In fact, our best fitting model has F-layer velocities very similar to those in PREM. These results along with seismic observations elsewhere suggest that the F-layer is laterally heterogenous which has important implications for core dynamics.