Earthquake coverage is vital if you live in the New Madrid, San Andreas, Hayward or Juan de Fuca tectonic plates. It's almost a given assumption that standard policies do not include earthquake coverage.
The fault lines most at risk for earthquakes are located largely on the West Coast. You may also be affected by an earthquake in Alaska and parts of the Midwest. In fact, the most intense Earthquake ever inferred in the U.S. was the New Madrid quake of 1812, estimated to be an 8.0 on the Richter Scale.
Earthquake insurance can range from a couple of hundred dollars to several thousand dollars a year. However, in places such as California and parts of the Midwest, coverage may be limited due to the high risk of earthquakes. Many policies in these areas are "bare bones" in nature and usually only cover about $5,000 in damage (usually only enough to cover the contents in the home). Also, coverage may not include swimming pools, outbuildings, or landscaping.
Earthquake insurance has a high deductible, in general. You can probably expect to pay about 10 to 15% in deductibles, which means that a $300,000 home will cost about $30,000 to $45,000 in deductibles out of pocket. However, before you decide against earthquake coverage because of its cost, you'll want to consider the following:
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Stats from http://www.iii.org/media/hottopics/insurance/earthquake/
http://moneycentral.msn.com/content/Insurance/Insureyourhome/P59648.asp