Two back-to-back earthquakes that recently jostled Northern California weren't powerful enough to cause serious injuries or damage.california-map

But they were a reminder of the looming danger of big earthquakes in the Golden State and the need to consider earthquake insurance.

The magnitude 3.5 and 4.0 quakes struck eight seconds apart at about 5:30 a.m. March 5. They were centered in El Cerrito, about 10 miles from Oakland in the San Francisco Bay Area.

The temblors were puny compared to what's likely to come.

In a 2008 earthquake forecast, scientists said the chance of a 6.7 magnitude or greater earthquake in California by 2028 is more than 99 percent. Experts from the U.S. and California Geological Survey and the Southern California Earthquake Center compiled the forecast.

Scientists say the greatest probability for a big one is in the region around Los Angeles, which faces a 67 percent chance of experiencing a major earthquake by 2028. The chance of a big one hitting the San Francisco Bay Area is 63 percent.

The most recent big ones

The last big one to hit California was the 6.7 Northridge earthquake in 1994. The earthquake killed 57 people, severely damaged or destroyed 4,000 buildings and moderately damaged another 8,500 buildings in the Los Angeles area, according to a 2004 Risk Management Solutions Inc. report. Seven major freeway bridges collapsed and 170 were damaged.

The Northridge earthquake occurred just five years after the 1989 Loma Prieta earthquake in Northern California. Among the most notable disasters of that 6.9 earthquake was the collapse of the Interstate 880 Cypress Street Viaduct, a double-decker section of highway in west Oakland.

The Loma Prieta earthquake occurred on the infamous San Andreas Fault and was centered near Santa Cruz. The San Andreas Fault runs north and south on the west side of the San Francisco Bay Area and along the coast north of San Francisco.

The most recent earthquakes in El Cerrito were on the Hayward Fault, which runs north and south on the east side of the Bay Area. Historical statistics for the fault line are eye-opening. The average interval for the last five big earthquakes on the Hayward Fault is 140 years, the U.S. Geological Survey says. The last time a big one occurred on the fault was 144 years ago when a 6.8 earthquake leveled buildings and shook up the then-sparse population.

Scientists say the probability of a big earthquake occurring on the Hayward Fault by 2028 is 31 percent -- significantly greater odds than the 21 percent chance on the San Andreas Fault.

Although a big earthquake is almost certain to occur somewhere in California, relatively few residents buy earthquake insurance. Standard home insurance does not cover earthquake damage. You have to buy a separate earthquake insurance policy to have financial protection.

Only 12 percent of California homes covered by fire insurance are also covered for earthquakes, according to the California Earthquake Authority. The authority provides 70 percent of the earthquake insurance sold in the state. The policies are sold through participating home insurance companies.

The authority has been working to raise awareness about earthquake preparedness and the need for insurance. Only time will tell whether more people will become concerned enough to buy coverage.