In Rev 8, each of the first four trumpets affects one- third of the earth; i.e., the civilized world, or one- third of the Roman Empire. We have to remember that the fourth terrible beast of iron, the Roman Empire, is the final beast which the kingdom of the Lord Jesus Christ strikes in the feet and topples, establishing His own kingdom in its place (Dan 2, Dan 7). It is this fourth beast that the seals, trumpets, and bowls of wrath are judging to bring about its fall (just as the ten plagues brought about the fall of the Egyptian kingdom so that His people were delivered).
The interpretation cannot mean that each trumpet affects a different third of the earth, because three thirds make one whole, and there are four trumpets.
Each trumpet afflicts a portion of the first third of the Roman Empire. The first trumpet afflicts the vegetation or the land in that one- third, the second trumpet afflicts the seas and shipping in that one- third, the third trumpet afflicts the rivers in that one- third, and the fourth trumpet afflicts the light in that one- third.
Here is a map of the Roman Empire at its greatest extent under Trajan. The barbarian invasions of the Western Empire represent the first four trumpets together, with Alaric hurting the vegetation, Genseric hurting the seas and shipping, Attila hurting the rivers, and Odacer hurting the light (for with his ascension began the Dark Ages). By 476, the Germans had overrun all the Roman Empire of the West from Britain to Italy, from the Atlantic to the Rhine and Danube.
Let us say for now that the first third represents Western Europe to the Rhine and Danube, those areas best known for German invasions, and then, if we shade those areas from Trajan’s map, there is still quite a significant portion of the Empire left, which we will assume to be the remaining two- thirds of the Empire. Then let us see what happens in the fifth and sixth trumpets (described in Rev 9), which I am assuming will afflict the second and third thirds, respectively.
Christine’s book The Revelation of Jesus Christ Revealed, based on these studies but greatly expanded, is now available at Revelation Revealed Online. You may also be interested in reading the Book Extras and joining in on the Discussion.