Cal. Gov. Code § 12062

In the event that the office of Governor is not filled within 24 hours after disaster as provided in Section 21 of Article IV of the Constitution, one of the Disaster Acting Governors, in the order specified, shall fill said office and shall declare that he is undertaking the duties thereof and take and subscribe the oath therefor and continue to serve therein until the incumbent Governor is able to perform his duties or until his successor is chosen and qualifies or until a person prior in the order of succession declares that he is undertaking the office and takes the oath therefor. Any Disaster Acting Governor may declare that he is undertaking the office of Governor and take the prescribed oath if no person prior in the order of succession fills said office within 24 hours after disaster.