The Savior leaves no question as to who is responsible for these destructions. It was not just a fluke of Mother Nature, but a definite punishment from Jehovah. Uncharacteristically, He uses the first person pronoun, I, 13 times in 12 verses. It was the Christ who was so consistently rejected among the Nephites; therefore it was the Christ who doled out the punishments.
And he must have destroyed these many cities between the time of his death and resurrection. This was a busy time for the disembodied Christ; He also had to visit the Spirit World. Joseph F. Smith wondered…how it was possible for him to preach to those [wicked] spirits and perform the necessary labor among them in so short a time (DC 138:28). We might also wonder how he was able to visit the Spirit World and also destroy all these cities in so short a time. We later learn that he received help to preach among the wicked spirits, but he did not receive any help when it came to destroying the Nephite cities. He destroyed them personally.