"And they threw DUST ON THEIR HEADS and were CRYING OUT, weeping and mourning, saying,
This is not an expression of repentance (Job 42:1-6) but rather an expression of deep, CRYING OUT agony (Lamentations 2:1-10-13 and Job 2:11-12-13).
Dust is the material we are created from (Genesis 2:4-7-9) and ultimately we return to DUST (Ecclesiastics 3:16-20-22). Thus throwing DUST ON THEIR HEADS is a way to acknowledge that death/burial, both physical and spiritual, is near (verse 21).
Many scholars believe this event (verses 17b-19) is what the Ezekiel 27:25-36 prophesy about Tyre is all about.
WOE, WOE, THE GREAT CITY, in which all who had ships at sea became RICH by her WEALTH, for in ONE HOUR she has been laid waste!'
The service sector has the same lament as the manufacturing sector (verse 16). Note it only takes ONE HOUR for both to collapse.
This is the last verse of the rather lengthy discussion of the global economy collapse (verses 9-19). On the surface and in the viewpoints of many commentaries, we are only witnessing the collapse from a global economy viewpoint. However, a person must realize that the Revelation of Jesus Christ was written to seven churches (Revelation 1:4) and that just prior to this discussion, "my people" in verses 4 and 5 are to distance themselves from the materialistic influences of the harlot, Babylon. Looking at organized religion and Christianity, we wonder how much the playing with the harlot is taking place within these organizations? From this author's understanding, nearly all faiths have experienced some part of the Harlot.
That this passage (verses 9-19) is directed at religions and Christianity is confirmed in the next verse.