As a practical matter, a house that costs a million dollars to (re)build is unlikely to exist on worthless land. A more realistic scenario is a million dollar house (including land value) with an estimated (re)build cost of $400k or some such, so a 15% deductible would be $60k.
However, note that CEA policies have deductible options of 5%, 10%, 15%, 20%, and 25% now: https://www.earthquakeauthority.com/California-Earthquake-Insurance-Policies/Homeowners/Coverages-and-Deductibles