insurance earthquake zones

I am curious wondering earthquake insurance is so expensive?
People living in California, lives in an earthquake zone is an insurance against earthquakes very expensive. earthquake insurance is required by law? or is it just a good feeling to have insurance just incase.
I lived in the San Francisco Bay for 15 years and was a commercial lines insurance agent there. No, not necessary. In fact, most people do not have what to do. I was a agent and not bring in my own home. It is quite expensive. The million dollar homes perched on the hillsides are what they want. They have the highest risk and also pay the most for coverage. Some of them are perched so precariously on the hillsides, they can not even get assurances that nobody will write the coverage. The average homeowner not to.
Wheel Street Journal #3 – Earthquake Insurance