Why does Zeus send a flood to the earth and who are the two survivors?

Answer 1

Apologies for being Socratic, but in answer to the first part of your question would be: What was God's reason for sending down the Great Flood? (Answer is in Genesis if you're interested in religious links etc.)

This is a universal question, and the answer is the same: to cleanse humanity of sin. If you read Ovid's Metamorphoses as I did (bearing in mind that I was 14 at the time and I didn't die of boredom) you will come to a greater understanding by far on a number of different myths (may I suggest that you read Melville's translation, otherwise you may indeed die of boredom - no offence to classicists to whom my comments may seem to imply that classicists are in any way boring: I do not intend to be derogatory in any way). The story is described there, and I will paraphrase it here.

Zeus (Jupiter in Ovid) is fed up with mankind's ingratitude towards him, as they have grown evil and greedy. He therefore orders his brother, Poseidon, to flood the land. The two people who survive are Deucalion and his wife Pyrrha.

Though I'm not sure whether this is applicable to the Greek version, this is overall, hopefully, a useful answer for you (I'm sorry if it's not, but remember that Roman and Greek mythologies are very similar - and the Romans take many of their 'own' ideas from the Greeks).