12 And I beheld when he had opened the sixth seal, and, lo, there was a great earthquake; and the sun became black as sackcloth of hair, and the moon became as blood;
13 And the stars of heaven fell unto the earth, even as a fig tree casteth her untimely fig, when she is shaken of a mighty wind. 
14 And the heaven departed as a scroll when it is rolled together; and every mountain and island were moved out of their places.

There is a lot happening here.  This earthquake could be a natural earthquake, or it could be the marking of World War 3 as the earth trembles and shakes from the massive explosions all around the world. Either way, I believe these verses 12-17 are a reference to WWIII.  The earth shakes, missiles may have appeared to John as falling stars, the smoke blackens the sun and darkens the moon, and it has been stated by witnesses of atomic blasts, that the atmosphere rolls back like a scroll.  The heavens will appear to be cut off by the black smoke rolling across the sky from the many explosions.

15 And the kings of the earth, and the great men, and the rich men, and the chief captains, and the mighty men, and every bondman, and every free man, hid themselves in the dens and in the rocks of the mountains;
16 And said to the mountains and rocks, Fall on us, and hide us from the face of him that sitteth on the throne, and from the wrath of the Lamb:
17 For the great day of his wrath is come; and who shall be able to stand?

During or at the end of WWIII there is going to be a global earthquake.  Do not confuse this with the greatest quake of Revelation in which the mountains flee away and the islands are not found.  Notice, Revelation has not yet declared that Gods wrath is being poured out, it is the men of the earth, especially and specifically the rich men and the kings saying that the wrath of God has come, for those who do not believe, this appears to be the battle spoken of in the Bible in which if Jesus does not return when He does, no life would be left.  The world is in turmoil, the people are in shock, and now the earth is shaking enough to move mountains, either by a natural earthquake, or caused by man.

This event will mark the beginning of the tribulation.  You will see as we follow the sequence of verses that John says, "after these things I saw...." thus securing that this is not a reference to the end times earthquake and events of REV 16:18-20.  As we move into chapter 7 you will see that the tribulation period is right now beginning.

Dano