Charles Ellicott Commentary Acts 16:26

"and suddenly there was a great earthquake, so that the foundations of the prison-house were shaken: and immediately all the doors were opened, and every one`s bands were loosed." — Acts 16:26 (ASV)

And suddenly there was a great earthquake.—Both the region and the time were, it will be remembered, conspicuous for convulsions of this kind. Cities in Asia, such as Sardis, Apamea and Laodicea, and in Campania, suffered severely under Tiberius. (See Note on Matthew 24:7.) St. Luke apparently interprets the fact not as in itself miraculous, but as leading to a display of supernatural calmness and courage on the part of the Apostles, and so to the conversion of the jailer.

Every one’s bands were loosed.—This seems, at first, beyond the range of the usual effects of an earthquake, but the chains of the prisoners were fastened, we must remember, to rings or staples in the wall, and the effect of a great shock would be to loosen the stones and so make it easy to escape. The fact that the foundations of the prison were shaken agrees with what has been said above , as to the dungeon into which the prisoners had been thrust.