Albert Barnes Commentary Revelation 16:18

"and there were lightnings, and voices, and thunders; and there was a great earthquake, such as was not since there were men upon the earth, so great an earthquake, so mighty." — Revelation 16:18 (ASV)

And there were voices, and thunders, and lightnings, accompanying the voice that was heard from the throne. See Revelation 4:5 and Revelation 11:19.

And there was a great earthquake, etc. See Revelation 6:12; see also Revelation 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.