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.