  In this paper, we continue the study of Bos et al. (2018) regarding
statistical and numerical considerations of the Backus (1962) product
approximation. While the approximation is typically quite good for
seismological scenarios, Bos et al. (2018) demonstrate a physical scenario that
could, in spite of the stability conditions for isotropic media, lead to an
issue within the Backus average. Using the Preliminary Reference Earth Model of
Dziewo\'nski and Anderson (1981) and a case study in the upper oceanic crust,
we investigate whether this issue is likely to occur in the context of
seismology.
