This paragraph had begun with a reference to “months” (Revelation 11:2), then moved on to speak of “days” (Revelation 11:3, Revelation 11:9, Revelation 11:11) and now of hour.
The time periods become progressively shorter, which in turn communicates increased haste and urgency. With the phrase “at that hour” John connects the earthquake directly to the witnesses’ resurrection and ascension and so underscores their ultimate victory. This contrasts with the short-lived victory of the beast in Revelation 11:7.
13 And at that hour there was a great earthquake, and a tenth of the city fell. Seven thousand people were killed in the earthquake, and the rest were terrified and gave glory to the God of heaven.