At the end of the first Superman movie, does Superman go back in time, or does he reverse time?

There is a subtle difference. If he reverses time, then the events of the earthquake never happened, and Lois never died. If he travels back in time, then he abandons the timeline he was in, and there is an alternate timeline where Lois is dead, and Superman has vanished.
If he reversed time, why didn’t he just stop the second rocket, preventing all the damage?