Earthquake (seismos). Old word from seiō, to shake. Luke regarded it as an answer to prayer as in Act 4:31. He and Timothy were not in prison.
So that the foundations of the prison house were shaken (hōste saleuthēnai ta themelia tou desmōtēriou). Regular construction of the first aorist passive infinitive and the accusative of general reference with hōste for actual result just like the indicative. This old word for prison house already in Mat 11:2; Act 5:21, Act 5:23 which see. Themelia is neuter plural of the adjective themelios, from thema (thing laid down from tithēmi). So already in Luk 6:48; Luk 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 (ēneōichthēsan). First aorist passive indicative of anoigō (or ̇numi) with triple augment (ē, e ), ō while there is no augment in anethē (first aorist passive indicative of aniēmi, were loosed), old verb, but in the N.T. only here and Act 27:40; Eph 6:9; Heb 13:5.