Sixth Seal is not the end of the Tribulation. It is a part of the last 3 and a half years of the Tribulation. For there are things that come out of the 6th seal and this takes time.
Sure it is, the Tribulation is 2-1/2 years and the Day of the Lord is 1 year. By common usage, the entire 3-1/2 year period has come to be known as the Tribulation. If you carefully read
Mat 24:29 Immediately
after the tribulation of those days shall the sun be darkened, and the moon shall not give her light, and the stars shall fall from heaven, and the powers of the heavens shall be shaken:
You see that these heavenly signs occur when? AFTER the Tribulation of those days.
Rev 6:12 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;
Rev 6:13 And the stars of heaven fell unto the earth, even as a fig tree casteth her untimely figs, when she is shaken of a mighty wind.
Rev 6:14 And the heaven departed as a scroll when it is rolled together; and every mountain and island were moved out of their places.
Rev 6:15 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;
Rev 6:16 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:
Rev 6:17
For the great day of his wrath is come; and who shall be able to stand?
God's wrath comes immediately after the heavenly signs. Right after the sixth seal (the heavenly signs) comes this...
Rev 8:1 And when he had opened the seventh seal, there was silence in heaven about the space of half an hour.
And that is composed of seven Trumpets, the Last of which is the return of Christ and the seven last plagues...