Albert Barnes on Revelation 16:18

And there were voices, and thunders, and lightnings. Accompanying the voice that was heard from the throne. Rev 4:5; 11:19.

And there was a great earthquake, etc. Rev 6:12; Rev 11:19. The meaning is, that a judgment followed as if the world were shaken by an earthquake, or which would be properly represented by that.

So mighty an earthquake, and so great. All this is intensive, and is designed to represent the severity of the judgment that would follow.

(c) "city" Rev 14:8 (d) "cup" Isa 51:17,23, Jer 25:15,16