{Earthquake} (seismos). Old word from sei(935c), 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} (h(9373)te saleuth(886e)ai ta themelia tou
desm(9374)(8872)iou). Regular construction of the first aorist passive
infinitive and the accusative of general reference with h(9373)te
for actual result just like the indicative. This old word for
prison house already in  Mt 11:2  Ac 5:21,23  which see.
Themelia is neuter plural of the adjective 	hemelios, from
	hema (thing laid down from 	ith(886d)i). 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} ((886e)e(9369)chth(8873)an). First aorist
passive indicative of anoig(935c) (or -numi) with triple augment
((882c) e, (935c)), while there is no augment in aneth(885c) (first aorist
passive indicative of ani(886d)i, were loosed), old verb, but in
the N.T. only here and  27:40  Eph 6:9  Heb 13:5 .