SAN FRANCISCO — The ravages of wildfires in California have kept Dave Jones, the state’s insurance commissioner, very busy over the past year. But that’s not what keeps him up at night.

“I go to sleep praying that there’s not going to be an earthquake the next day in California,” said Mr. Jones, who has been commissioner for the past eight years. “From an insurance perspective, Californians are simply not prepared.”

Despite an aggressive advertising campaign by the state to promote earthquake insurance, only 13 percent of homeowners have it. And fewer than one out of 10 commercial buildings, which include everything from low-rise office buildings to the high-rise office towers of Los Angeles and San Francisco, are insured for earthquakes, according to the California Department of Insurance.

“Most new buildings are not buying it all,” said Justin Dove, the area vice president in San Francisco for Arthur J. Gallagher, a large insurance broker. Some companies are self-insured, he said, but many find earthquake insurance too expensive.