Replacement cost means that the insurance company will pay to have the home re-built to what it was before the fire. The mortgage amount owed never comes into the conversation. The home will be rebuilt and the bank still has a property securing the mortgage. Market values or appraisals are not what insurance companies insure - those all include the value of the land, which is uninsurable (because land cannot be "damaged" by fire, lightning, etc.)
Where there will be problems is if someone has an ACV or actual cash value policy. That would adjust the claim based upon the ACV at the time of the loss.
However, there are also valued policy laws in some states that would insure that the homeowner is paid the policy limits they purchased - the insurance company cannot deny the property was worth less at the time of the loss than the insured value. I don't know what California's laws are, but in Missouri, we have valued policy laws on the books to protect consumers.
http://www.ahtins.com/glossary/vvv/v010.htmhttp://research.lawyers.com/glossary/valued-policy-law.html The problems the insurance companies will have is handling a widespread catastrophe like this. This will test their claims staff and their claims reserves. Homeowners rates will skyrocket within the next few months to account for these losses.