Both literally and figuratively.

 A literal earthquake, as the remainder of v. 18 implies (see on v. 1; cf. vs. 20, 21), but accompanied by a figurative earthquake, which shatters mystical Babylon (v. 19). As a literal earthquake leaves a literal city in ruins, so a figurative earthquake brings ruin and desolation to “great Babylon” (see on chs. 17:16; 18:6-8, 21).

 The threefold union of vs. 13, 14 collapses (cf. Isa. 28:14-22).

Or, “flashes of lightning.” Textual evidence attests (cf. p. 10) that this word precedes “voices, and thunders.”

Or, “peals of thunder.”

 Or, “sounds,” “noises.” Compare chs. 4:5; 8:5; 11:19. What the “voices” utter may be similar to the declaration of ch. 11:15 (cf. GC 640).