Literary/Textual: Verses 12-17 form a framed set of events. The beginning frame in verse 12 is “the whole face of the land was changed, because of the tempest and the whirlwinds and the thunderings and the lightnings, and the exceeding great quaking of the while earth.”
Verse 17 forms the obvious parallel and ending frame. The basic information is repeated, including the “face of the whole earth became deformed,” and the specific causes: “because of the tempests, and the thunderings, and the lightnings, and the quaking of the earth.”
The obvious paralleling of not only the descriptions but the order of the descriptions suggests that this is not likely to have been a spontaneously written piece. The information inside these brackets also repeats other information that has already been entered. One difference in the information presented is that these verses emphasize the loss of life as much as the physical elements themselves.
It is quite tempting to see this ordered section as some form of poetry, but if it was, there isn’t enough of the poetic structure remaining to note more than the opening and closing frames. Nevertheless, the frames themselves to suggest that this was a separate literary unit that was added into Nephi’s account.
We may be certain that Nephi wrote his account some time after these events. The timing was sufficient that he was able to have a report of all of the destruction of the various cities and lands. It is even probable that he is merging multiple accounts into his own record as he received information from various parts of the land. The formal bracketing of this particular section, coupled with the basic repetition of information already given, suggests that this was a separate account that Nephi merged into his own text. We cannot tell if Nephi or some other person was the original author of the unit.