This massive earthquake is mentioned several times. It is like God saying "The End". It is a sign that Jesus and the army of the Lord are about to arrive. If someone says Jesus is here or there, don't believe them, for this is the description of the moment of Christ's return. The sun became black as sackcloth, the full moon became like blood, and the stars of the sky fell to the earth as the fig tree sheds its winter fruit when shaken by a gale. The sky vanished like a scroll that is being rolled up, and mountains and islands will be moved from their places. There is a lot more to this earthquake than meets the eye. The day of the Lord is here. There is a reasonable amount of information about what things are like when the Lord has returned and the period after. I cover this in more detail in The Day of the Lord.

Seventh Seal
This is recorded not in Revelations 6 or 7 but in Revelations 8, and it is the introduction to the trumpets. Revelations 7 goes on to tell us about the 144,000 and some details about the 100 million before the throne.