Why did Jonah say he had fled toward Tarshish?

Answer: He knew God would relent

Jonah said he fled because he knew God is gracious, compassionate, slow to anger and One who relents from sending disaster.

“So he prayed to the LORD, saying, “O LORD, is this not what I said while I was still in my own country? This is why I was so quick to flee toward Tarshish. I knew that You are a gracious and compassionate God, slow to anger, abounding in loving devotion—One who relents from sending disaster.”— Jonah 4:2 (BSB)