The San Francisco region will probably be struck by an earthquake with a magnitude of at least 6.7 in coming decades, and most of the billions of dollars of damages wouldn’t be covered by insurance.

A worst-case, magnitude 7.9 earthquake could cause losses of more than $200 billion, according to report released today by catastrophe modeler Risk Management Solutions. The 1989 Loma Prieta event, a magnitude 6.9 quake in Bay Area, led to about $6 billion in economic damages and $960 million in insured losses.

Residential insurance penetration in the state has dropped by more than half in the 25 years since the Loma Prieta shaker, RMS said in the report. About 10 percent of California households currently have earthquake coverage, the firm said.

“When an event happens the Bay Area is going to be hindered because we don’t have that funneling of monies to help rebuild quickly,” Patricia Grossi, a senior director of model product management at RMS, said in an interview. “It’s a question of, once the dust settles, how quickly are we going to be able to rebuild?”

The Bay Area has grown in recent decades to contribute $388.3 billion to gross domestic product and support 7.15 million residents. Increases in population, property values and commercial activity mean an earthquake would cost significantly more than 25 years ago, RMS said.

California mandates home insurers offer quake coverage. Homeowners aren’t required to buy it. Policies cover damage to structures, contents and other expenses, and can include business-interruption insurance.

Napa’s Earthquake

The 6.0-magnitude earthquake that struck Napa County Aug. 24 was the strongest to hit the Bay Area since Loma Prieta. Total economic losses were $2 billion, according to a report from reinsurance broker Aon Benfield. Insured losses are estimated to be in the hundreds of millions because 5.3 percent of Napa County households had coverage.

The California Earthquake Authority, a provider of residential insurance for quakes, said 2,500 homeowners purchased policies in the week following the Napa quake.

“We always get a jump after an earthquake, whether it’s in Japan or California,” Glenn Pomeroy, the organization’s chief executive officer, said in a phone interview. “If there is a major earthquake, it would be devastating if 90 percent of people don’t have insurance. The federal government does not come in and rebuild everybody’s home.”

The Bay Area also was struck by quakes that were recorded or estimated to have magnitudes of 6.7 or more in 1838, 1868 and 1906, according to the U.S. Geological Survey’s Earthquake Information Center. The USGS said in 2008 that there is a 63 percent probability the region will see another of similar or greater size within 30 years.

The 6.6-magnitude Northridge event near Los Angeles in 1994 remains the costliest U.S. earthquake to insurers. It killed 61 people and led to $22.9 billion in insured losses, adjusted for inflation, according to estimates from Swiss Re Ltd.