In the midst of total destruction, under the blanket of thick darkness, the people had already begun to fear. That fear could only have been amplified when, through the darkness, came the voice of God. It had to be the voice of God, for God was over all nature, and, for those who remembered the prophecies, they understood that there was a condemnation upon the people. With all of that fear, this God does not yet provide comfort, but rather declares his power. God will tell the people of the extent of the destruction. They would only know the destruction that was around them, but God will let them know that it was not just them that saw destruction, but many, many others also witnessed it.
The promised destructions had come because people had not repented. Nevertheless, God declares to those who were still alive that they were still under the command to repent. At this point, they would have understood exactly how great the physical destruction might be. At least at that very time, the people would have been willing to repent of most anything.