What type of sentence structure is this (specifically, the “war ... zu hören”):
Die Explosion war auf zwei Kilometer [Entfernung] zu hören.
When would one use it? I haven’t (consciously) seen it before, so is it commonly replaced with something else?
This is a simple main clause with a predicative. You can tell those sentences by the special verbs sein, werden and bleiben, if used as full verbs. These three always need a predicative. (There are some more verbs which can go with predicatives, but these three are most common.)
Predicatives may be adjective phrases, nominative noun phrases, or verb phrases.
Die Explosion war sehr heftig.
Die Explosion war eine Gefahr.
Die Explosion war zu hören.
The latter option does not exist in English —at least to my knowledge— , that's why it feels alien to you. English turns the verb phrase into an adjective phrase instead. This is possible in German, too, but it sounds awkward:
Die Explosion war auf zwei Kilometer [Entfernung] hörbar.
Let me start with the meaning and what's behind that sein.
Die Explosion war zu hören = man konnte die Explosion hören
Das Formular ist auszufüllen = man muss das Formular ausfüllen; das Formular muss ausgefüllt werden
See the modal verbs? They answer your question whether "it is commonly replaced with something else".
Now look at the English equivalent:
The explosion could be heard.
See the modal verb here, too? Well, that's it. No reason to be astonished. ;-)
By the way. Sentences like Die Explosion war noch in einer Entfernung von 20 Kilometern hörbar, auch die Erschütterungen waren dort spürbar. are not awkward at all. :-)
