Cal. Gov. Code § 9004
When the Legislature convenes or is convened in regular or extraordinary session during or following a war or enemy-caused disaster and vacancies exist to the extent of one-fifth or more of the membership of either house caused by such disaster, either by death, disability or inability to serve, the vacancies shall be temporarily filled as provided in this section. The remaining members of the house in which the vacancies exist, regardless of whether they constitute a quorum of the entire membership thereof, shall by a majority vote of such members appoint a qualified person as a pro tempore member to fill each such vacancy. The Chief Clerk of the Assembly and the Secretary of the Senate or the persons designated to perform their duties, as the case may be, shall certify a statement of each such appointment to the Secretary of State, who shall thereupon issue commissions to such appointees designating them as pro tempore members of the house by which they were appointed.
The appointments shall be so made that each assembly or senatorial district in which a vacancy exists shall be represented, if possible, by a pro tempore member who is a resident of that district and a registered elector of the same political party as of the date of the disaster as the last duly elected member from such district.
Where an elected member is temporarily disabled or unable to serve, such elected member shall resume his office when able, and the pro tempore member appointed in his place under this section shall cease to serve. In other cases, each pro tempore member appointed under this section shall serve until the next election of a member to such office as provided by law.
Ca. Gov. Code § 9004