What is the greatest earthquake at the seventh Vial?
At the seventh vial, Revelation describes an earthquake so mighty and great that nothing like it has ever happened since men have been on the earth, splitting the great city into three parts and causing every island and mountain to vanish.
When the seventh angel pours out his vial into the air, a great voice comes from the throne saying, It is done, and immediately there follow voices, thunders, and lightnings, and then the greatest earthquake this world has ever known. Revelation 16:18 says plainly that it is "so mighty an earthquake, and so great" that nothing like it has occurred since men were upon the earth.
The effects of this earthquake are staggering. The great city is divided into three parts, and the cities of the nations fall. Great Babylon is remembered before God, and He gives her the cup of the wine of the fierceness of His wrath. Every island flees away and the mountains are no longer found, showing that the very geography of the earth is reshaped by this shaking. Right along with the earthquake comes a great hail out of heaven, with stones about the weight of a talent, yet even in the face of this exceeding great plague, men still blaspheme God rather than repent.
Other scripture confirms the scale of this shaking. Ezekiel speaks of a day when there will be a great shaking in the land of Israel, so that the fish of the sea, the fowls of heaven, the beasts of the field, every creeping thing, and all the men on the face of the earth will shake at the Lord's presence; the mountains will be thrown down, the steep places will fall, and every wall will fall to the ground. Isaiah likewise describes men fleeing into the holes of the rocks and the caves of the earth for fear of the Lord and for the glory of His majesty, when He arises to shake terribly the earth, even casting away their idols of silver and gold to the moles and the bats.
So the greatest earthquake of the seventh vial is presented as a final, unprecedented, global shaking of the earth, unmatched by anything before it, marking the completion of God's wrath being poured out upon a rebellious world.
The scriptures
And the seventh angel poured out his vial into the air; and there came a great voice out of the temple of heaven, from the throne, saying, It is done. And there were voices, and thunders, and lightnings; and there was a great earthquake, such as was not since men were upon the earth, so mighty an earthquake, and so great. And the great city was divided into three parts, and the cities of the nations fell: and great Babylon came in remembrance before God, to give unto her the cup of the wine of the fierceness of his wrath. And every island fled away, and the mountains were not found. And there fell upon men a great hail out of heaven, every stone about the weight of a talent: and men blasphemed God because of the plague of the hail; for the plague thereof was exceeding great.
For in my jealousy and in the fire of my wrath have I spoken, Surely in that day there shall be a great shaking in the land of Israel; So that the fishes of the sea, and the fowls of the heaven, and the beasts of the field, and all creeping things that creep upon the earth, and all the men that are upon the face of the earth, shall shake at my presence, and the mountains shall be thrown down, and the steep places shall fall, and every wall shall fall to the ground.
And they shall go into the holes of the rocks, and into the caves of the earth, for fear of the LORD, and for the glory of his majesty, when he ariseth to shake terribly the earth. In that day a man shall cast his idols of silver, and his idols of gold, which they made each one for himself to worship, to the moles and to the bats; To go into the clefts of the rocks, and into the tops of the ragged rocks, for fear of the LORD, and for the glory of his majesty, when he ariseth to shake terribly the earth.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.