The Attorney General, Secretary of State, Treasurer and Controller shall, in addition to the appointments required to be made by Section 12701, designate the order in which persons holding office as their deputies or assistants, and not appointed under Section 12701, shall serve as their respective successors in the event that such officer and the successors appointed by him under Section 12701 are unavailable as a result of disaster. If any such officer and the successors appointed by him under Section 12701 are unavailable as a result of disaster, the powers and duties of his office shall devolve upon one of such deputies or assistants in the order designated and such
person shall declare that he is undertaking the duties of the office and take and subscribe the oath therefor; provided, however, that any such deputy or assistant may declare that he is undertaking the office and take the prescribed oath if none prior in order of succession enters upon the office within seven days after the incumbent thereof becomes unavailable.