Your Homeowner's Or Renter's Insurance Won't Help Much After An Earthquake. Here's Why

(CNN) -- The largest earthquake to hit Southern California in two decades rattled the region on July 4th -- followed by more than 170 aftershocks.

The magnitude-6.4 quake, which caused damage in some cities, was centered near Ridgecrest, about 150 miles north of Los Angeles. In a state especially prone to tremors, only 13% of homeowners have earthquake insurance, according to the California Department of Insurance. Others who have renter's or homeowner's insurance mistakenly believe they're covered for earthquakes.

Here's what you need to know:

Will my homeowner's or renter's insurance help?

When it comes to earthquakes, the short answer is no.

Having homeowner's or renter's insurance does not automatically mean you're covered for earthquakes -- you need a separate policy for that. In California, the company providing your homeowner's insurance must offer to sell you separate earthquake insurance.

The offer must be in writing, and it gives you 30 days to accept it or not -- starting from the day the company mails the offer. if you don't respond, it means you are rejecting the offer, the California Department of Insurance says.

But in some cases, it says, homeowner's or renter's insurance may cover direct loss caused by explosion, theft or breaking glass following an earthquake -- even without earthquake insurance. "Read your homeowner's policy and contact your insurance company whenever an earthquake damages your property. Do not assume that the damage is not covered," the department of insurance says.

Earthquake insurance will replace all my stuff, right?

Probably not.

While the goal is to provide some relief, it's not meant to bring you back to your pre-earthquake days, officials say.

"There are limits on what earthquake insurance pays," the California Department of Insurance says. "The purpose of earthquake insurance is to help put a roof back over your head."

And earthquake insurance usually does not cover anything already included in your homeowner's policy.

"For example, your homeowner's policy covers fire damage, even if an earthquake causes the fire. Therefore, your earthquake policy does not cover fire damage," the California Department of Insurance says.

Earthquake insurance does not cover damage to your vehicles either -- that may be covered by your auto policy. Neither does it cover water damage from outside your home after an earthquake, including sewer, drain back-up and flooding, among others.

What if a fire is caused by an earthquake?

In this case, there's more flexibility.

Under state law, fire damage is covered by both homeowner's and renter's insurance whether you have earthquake insurance or not.

"California law says that both homeowner's and renter's insurance must cover fire damage that is caused by or follows an earthquake," the California Department of Insurance says.

Where can I buy an earthquake policy?

Before you get an earthquake policy, you must have a residential property insurance policy.

The California Earthquake Authority offers most earthquake policies for homeowners and renters. But you don't buy earthquake insurance directly from that state agency -- you have to get it from insurance companies that are members of CEA.

Any tips to make an earthquake policy less costly?

The state recommends residents find ways to protect homes and reduce damage caused by earthquakes. That includes retrofitting houses -- which makes them stronger and can help save money on insurance and repairs. Insurance costs are higher for older homes, houses that have more than one story, those built on sandy soil instead of clay or rock and those that are not up to code, the California Department of Insurance says.

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