Foreclosures increased in 154 of 206 metro areas in the first half of 2010 over the year-ago period, according to RealtyTrac, which analyzed foreclosure data in metropolitan areas with 200,000 people or more.

Metropolitan Statistical Areas (MSAs) in four states — Florida, California, Nevada and Arizona — accounted for the top-20 foreclosure rates in the country. Florida had nine of them, followed by California with eight, Nevada with two and Arizona with one.

Las Vegas led the way. One in 15 houses in Las Vegas received a filing. That’s 6.6% of all houses there for a total of 53,525 properties. It is a decrease of nearly 9% from the first six months of 2009 and 15% below the amount of foreclosures in the last half of that year.

In fact, nine of the top-10 metros with the highest foreclosure rates saw decreased foreclosure activity from the same period last year. The Cape Coral-Fort Myers, Fla., MSA had 4.98% of its housing units, or one in 20, receive a filing. It’s the second highest rate in the country but down 30% from the same period last year and 22% from the last six months of 2009.

The Miami-Fort Lauderdale-Pompano Beach MSA in Florida had the highest foreclosure total in the first half of 2010, according to RealtyTrac. A total of 94,466 properties received a filing, up 11% from the first half of 2009 but down 8% from the second half.

“While we’re seeing early signs that foreclosure activity may have peaked in some of the hardest-hit markets, foreclosures continued to rise in three-quarters of the nation’s metropolitan areas in the first half of the year,” said James Saccacio, CEO of RealtyTrac.

Saccacio said job growth is the only thing that can sustain the fragile stability some of these markets have achieved.

“If unemployment remains persistently high and foreclosure prevention efforts only delay the inevitable, then we could continue to see increased foreclosure activity and a corresponding weakness in home prices in many metro areas,” Saccacio said.

Write to Jon Prior.