BOSTON (MainStreet) — San Francisco has earthquakes, Oklahoma City has tornadoes and New Orleans has hurricanes — but here's a look at five major U.S. communities that have the highest combined risk for all three.

"Most places tend to have maybe one risk factor, but these places tend to have multiple ones," says Daren Blomquist of market watcher RealtyTrac, which recently calculated the odds of natural disasters hitting any one of more than 3,100 U.S. counties.

Blomquist says RealtyTrac found that America's riskiest locales are also some of its heavily populated ones.

He speculates that's because many scenic oceanfront communities face high hurricane dangers, while picture-perfect mountaintop locales often sit near earthquake-producing fault lines. "It turns out that a lot of people want to live in some of the most-risky places in the country," Blomquist says.

In fact, RealtyTrac found that the riskiest locales have some of America's highest home prices.

Expect to pay $377,483 for a median-priced place in the 12 major U.S. counties (those with 500,000 housing units or more) that the firm discovered have a high combined risk for all three natural disasters. That's 119.5% over the $172,000 overall U.S. median.

Read on to see which large counties RealtyTrac found have the highest combined risk for earthquakes, hurricanes and tornadoes. (Or, click here to see which communities enjoy the lowest natural-disaster potential.)

RealtyTrac based its projections on U.S. Geological Survey earthquake forecasts, plus hurricane and tornado projections from the National Oceanic and Atmospheric Administration. All locales below have 500,000 housing units or more.

Fifth-riskiest major community for natural disasters: Suburban Boston/Middlesex County, Mass.

You don't need to be a rocket scientist to see why the Massachusetts Institute of Technology's home area has high natural-disaster risks.

Located near the Atlantic Ocean, 1.5-million-population Middlesex County suffers from a "Very High Risk" rating for hurricanes. The county, which encompasses many of Boston's northern and western suburbs, also gets a "C" for earthquake hazards, although it ranks "Very Low Risk" for tornadoes.

Fourth-riskiest major community for natural disasters: Philadelphia

It may be the City of Brotherly Love, but Philadelphia shouldn't expect much kindness from Mother Nature.

That's because 1.5-million-person Philadelphia County (whose borders match those of the city) rates "Very High Risk" for hurricanes — not surprising given that it's only around 60 miles inland from the Atlantic Ocean.

Philly also gets a "C-" for earthquake danger, but enjoys a "Very Low Risk" score for tornadoes.

Third-riskiest major community for natural disasters: Eastern Long Island/Suffolk County, N.Y.

A bad storm could make short work of this Long Island county some 80 miles northeast of Manhattan.

Surrounded on three sides by water, Suffolk County faces a "Very High Risk" of hurricane damage.

The 1.5-million-population area, which is home to such exclusive beach communities as Montauk and the Hamptons, also gets a "C+" for earthquake potential. But on the bright side, Suffolk County only has a "Very Low Risk" ranking for tornadoes.

Second-riskiest major community for natural disasters: San Diego County, Calif.

You know you face major natural-disaster risks when you live in a place that's sandwiched between one ocean and two earthquake fault lines.

San Diego County, which consists of the city of San Diego and some surrounding communities, gets a "F" for earthquake risks thanks to the nearby San Andreas and San Jacinto earthquake faults.

The 3.1-million-person county also scores a "Medium Risk" grade for hurricanes, as it runs along the Pacific Ocean. The only good news is that residents there only face a "Very Low Risk" rating for potential tornadoes.

Riskiest major community for natural disasters: Suburban Los Angeles/Riverside County, Calif.

It's bad enough that the San Andreas and San Jacinto Faults run near San Diego County, but they actually run through Riverside County.

That's the main reason this 2.2-million-person locale, which stretches from Los Angeles' eastern suburbs to the Arizona border, tops RealtyTrac's list of the riskiest major U.S. locale for natural disasters.

Riverside County scores an "F" for earthquake potential, while it gets a "Medium Risk" grade for hurricanes. (Part of the county's western border comes within about 10 miles of the Pacific Ocean.)

On the plus side, Riverside County residents face just a "Very Low Risk" rating for tornadoes.

— By Jerry Kronenberg