The Jonah Mystery

So I was listening to a sermon on Jonah when a question came to mind. Well, two questions.

1. Why did the Ninevites respond to Jonah's message?
2. How did they know that judgment had been averted?

What was it about Jonah or the current situation that made them respond to such a wacky message? Did they have a basic knowledge of and fear of God? Did Jonah's strange arrival generate a fearful following? Or was there some visible, physical threat, such as a plague, atmospheric/astronomical anomalies, distant rumblings from enemy nations? Or was the king having scary dreams?

So they're repenting in sackcloth and ashes. How did they know that God had pardoned them, unless there was a threat of the abovementioned variety that could be visibly or experientally "turned off"?

Update: Found this:

This city was spared. Why did they listen to Jonah's message? Well, I think this would always be a mystery to us were it not for clues supplied by the Lord Jesus Christ himself. In the Gospel of Luke in chapter 11, our Lord refers to this account: "For as Jonah became a sign to the men of Nineveh, so will the Son of man be to this generation." (Lk. 11:30) He said, "Jonah -- the man, the prophet -- was himself a sign to the city of Nineveh, and in just the same manner, I, the Lord Jesus Christ, will be a sign to the whole generation." He referred to Israel but he meant the whole race of man beyond that; and as Jonah was a sign to Nineveh, so the Son of Man will be a sign to this generation.

There are Bible scholars who feel that what happened to Jonah was that his features were changed by his experience in the whale's belly.

That seems to work. Another view is that it was Jonah's "resurrection" from the dead, or being coughed up on the beach from the fish, that was the sign.

Still doesn't answer how they knew that the crisis had been averted, though.