The power of God was indeed upon them, and was manifest in a way that not only saved Alma and Amulek, but signaled that God was behind their deliverance. God is God over heaven and earth, and therefore earth responds to his commands. Assuming that the Book of Mormon took place in Mesoamerica, it occurred in a land quite familiar with earthquakes. It was not simply that there was an earthquake, but that it came apparently upon command, and broke down the building in such a way that Alma and Amulek could escape unscathed.
The combination of Alma’s prayer, the earthquake, and the ability of Alma and Amulek to walk out of the prison created fear in those who were nearby, and they fled, further allowing Alma and Amulek to make their escape.
This is not the end of a chapter in the 1830 edition. The story continues immediately in the next chapter.