Of all U.S. mortgage holders, about one quarter, or 11.3 million households, are underwater, according to First American CoreLogic, which collects and analyzes mortgage data. In California, 35 percent of mortgage holders are underwater.
"Negative equity gives you the universe of homeowners who are vulnerable to default," said Sam Khater, First American senior economist.
In the Bay Area, First American shows the Vallejo-Fairfield metropolitan market (essentially Solano County) with 61 percent of mortgage holders underwater. The next-highest concentration - but the biggest in numbers - is the Oakland-Fremont-Hayward metro area (Alameda and Contra Costa counties), where 35 percent of mortgage holders, or 192,726 households, have negative equity.