With all the things that was happening during his ordeal the forgiving of sins not only of the thieves, but of mankind, those who nail him to the cross was also aquitted when he asked the FATHER in heaven to forgive them. The earthquake and all other things that was happening is the result of what our FATHER in heaven shows his anger and mourning. Remember what the people in the Bible do when someone dies, they tear their clothes and put sackclothes then put ashes on their head. But the main thing is the Centurian witness the forgiving of sins not only for Jesus Christ also from his or our FATHER  , for surely one snap of the finger and all the things here on earth would be gone.