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Some branches like PKPbc are rather short in terms of distance range over which they exist according to ray theory. However, PKPbc is observed over a larger distance range as a diffracted wave (PKPb-diff resp. PKPc-diff). This hasn't been taken into account so far and the distance interval of 1° further reduces the usable range.

Therefore when PKPbc is used for locating earthquakes or even if it is only just associated with zero weight, a slight location change in a relocation can result in a PKPbc arrival leaving the distance range covered by the PKPbc table. This has been found to sometimes result in wrong or failed relocations.

The can be circumvented by using generic tables (as also for P and S) but in case of PKP with often several branches producing detectable arrivals, this will affect location accuracy or may even require disabling of PKP in the relocation (as done in SeisComP by scautoloc).

Ideally the PKPbc table would be augmented by including PKPb-diff and PKPc-diff.

Note that this issue does not exclusively affect PKPbc but also other phases.