Because of the possibility of enemy attack, natural disaster, calamity, or common accident, and in order to assure continuity of effective legally constituted leadership, authority, and responsibility in the offices of the government of the city, it is found and declared to be necessary to provide for emergency interim officers who can exercise the powers and discharge the duties of the key executive, administrative, legislative and judicial offices in the city in the event that the incumbents thereof, and their deputies, assistants, or other subordinate officers, authorized pursuant to law, to exercise all the powers and discharge the duties of such offices, are killed, missing, disabled, or for some other cause unable to perform the duties and functions of their offices during or immediately following an enemy attack, natural disaster, calamity, or common accident.