A.T. Robertson 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)

Earthquake (σεισμος). Old word from σειω, to shake. Luke regarded it as an answer to prayer as in 4:31. He and Timothy were not in prison.

So that the foundations of the prison house were shaken (ωστε σαλευθηνα τα θεμελια του δεσμωτηριου). Regular construction of the first aorist passive infinitive and the accusative of general reference with ωστε for actual result just like the indicative. This old word for prison house already in Mt 11:2; Acts 5:21,23 which see. Θεμελια is neuter plural of the adjective θεμελιος, from θεμα (thing laid down from τιθημ). So already in Lu 6:48; 14:29. If the prison was excavated from rocks in the hillside, as was often the case, the earthquake would easily have slipped the bars of the doors loose and the chains would have fallen out of the walls.

Were opened (ηνεωιχθησαν). First aorist passive indicative of ανοιγω (or -νυμ) with triple augment (η, ε, ω), while there is no augment in ανεθη (first aorist passive indicative of ανιημ, were loosed), old verb, but in the N.T. only here and 27:40; Ephesians 6:9; Hebrews 13:5.