Cal. Ins. Code § 10085

If the insurer establishes proof of mailing or delivery of the required offer and the offer of earthquake coverage is not accepted by the named insured within 30 days from the date of mailing or delivery of the offer, there shall be a conclusive presumption that the named insured elected not to accept the coverage. An election, actual or presumed, by any named insured shall be binding upon any other person insured or any other party having an insurable interest in the insured property.