Isaiah uses a strong exclamation to express what he wishes God had done. He wishes that God had shown himself very dramatically by tearing open the sky and coming down. The words "split open" are the same words used to describe ripping apart a piece of cloth. (See: [[rc://en/ta/man/translate/figs-hypo]])
This is what would have happened if God had split open the heavens and come down. The mountains would have trembled as in an earthquake. (See: [[rc://en/ta/man/translate/figs-hypo]])