What led to the man-made catastrophe found in Gathering Blue?

We are given only sparse information concerning "The
Ruin," as it has come to be known in this excellent novel. It is clear that the precise
nature of the destruction that mankind brought upon itself has been forgotten, but there
are vague hints that we can pick up from the song that the Singer sings each year,
charting the history of mankind, that we can use to suggest a possible scenario. Note
the following terrifying details of The Ruin that are given in Chapter Three, that are
so horrific that even though the whole community is supposed to listen to The Ruin Song
every year, mothers will cover the ears of the very young at this
point:

Verses
told of smoky, poisonous fumes, of great fractures in the earth itself, of the way huge
buildings toppled and were swept away by the
seas.

We can either infer
therefore that civilisation as we know it ended in a nuclear apocalypse or some kind of
environmental oblivion, characterises by massive earthquakes and tsunamis that wreaked
havoc with earth as we know it.