High Frequency (6 Hz) PKPab precursors and their sensitivity to deep
We present observations on a new precursory phase of seismic waves
scattered in the deep Earth. This phase arrives prior to the PKPab wave
at epicentral distances larger than 155º, and we call it PKPab
precursor. We show that the presence of the PKPab precursor is a
necessary consequence of scattering in D’‘, which is the commonly
accepted cause of the PKPdf precursor at distances smaller than 145º.
PKPdf waves that propagate through the inner core should arrive before
the PKPab precursor but those, are strongly attenuated in the inner core
at frequencies between 4 Hz and 8 Hz used here, making the PKPab
precursor the earliest teleseismic signal at distances larger than 155º.
Calculated PKPab precursor sensitivity kernel shows that this phase is
mostly sensitive to scattering along the closest PKPbc path between
source and receiver. It can thus help to constrain the lateral
distribution of heterogeneity along D”.