Earthquake insurance is really for your piece of mind. Policies are limited, with high deductibles. An even better investment may be to hire a structural engineer to do a seismic inspection, then help you design an upgrade to the home to meet the most current code standards. Today's codes are now strict when it comes to earthquakes and you should expect a home to withstand a 6.5 earthquake with no structural damage.
The most important upgrade to do is to make sure that the house is actually attached to the foundation. Believe it or not, many older homes are just "resting" on top of the foundation. When an earthquake occurs, the house slips off the foundation and then collapses. A few steel straps, bolts and hold downs can go a long way in keeping your home together in a seismic event.