Something to keep in mind about this insurance is that a traditional homeowners insurance policy doesn't include coverage for earthquakes. If you live in a earthquake-prone area, this coverage may be a nice addition to your current policy.
In general, typical earthquake insurance covers the expenses that come with repair/replacement of damaged property. There are various options to choose from as well- including coverage to protect garages/accessory structures, personal contents within your home, as well as temporary living expenses if your home is destroyed.
Factors too keep in mind, which may affect your premium:
-Age of your home (older homes tend to cause a jump in premium)
-Is your home constructed of brick or wood?
Homes constructed of wood are better able to withstand the forces of an earthquake, and therefore you'll experience a lower premium
-The homes location
Your geographic area is rated in terms of a probability of a quake occurring; a higher probability will point to a higher overall premium.
How much is enough?
If when you decide to purchase earthquake insurance, obtain enough coverage to shield the costs of a complete rebuild, in addition to your lost/damaged possessions. Base your figures on the actual cost of rebuild, as opposed to the market value of your current home/possessions.