Fairfax County became the Washington area's most populous jurisdiction last year as its population reached 687,800 and its economy continued to boom, the Census Bureau reported yesterday.

The new census estimates for mid-1985 also show continued growth in the Washington area's total population, which reached 3,489,500. The area has grown by 7.3 percent since 1980, the Census Bureau said, more than twice as rapidly as it did during the slow-growth 1970s.

In contrast to the decline in the 1970s, the central jurisdictions -- the District, Arlington and Alexandria -- report modest population increases, with the District last year having its first population rise since 1963. Despite its upturn in growth, the Washington area slipped in the national ranking from eighth in 1980 to 10th last year.

Explosive growth has slowed in fringe counties such as Calvert and Prince William, though their gains are still substantial. Growth has picked up in all three major jurisdictions along the Capital Beltway: Fairfax, Montgomery and Prince George's counties. It was greatest last year in Montgomery, although since 1970 the total population increase has been much larger in Fairfax.

According to the new estimates, Prince George's has become the second largest jurisdiction in the area at 678,100. Montgomery is third at 643,400, and the District is fourth at 626,000.

"It was just a matter of when" Fairfax would get to the top, said John McClain, director of development for the Metropolitan Washington Council of Governments, "because they have been growing so much faster. As of 1985, it's happened . . . . Now there seems to be some backlash against growth in Fairfax caused by all the traffic congestion."

In 1980 the population of Fairfax was 595,756, ranking third in the area behind Prince George's and the District. It moved up to second in 1982. In 1970 Fairfax County ranked fourth with a population of just 454,275, while the District, which had been the area's core and population center for more than 150 years, had 756,668 residents.

According to the Census Bureau report released yesterday, San Francisco has replaced Philadelphia as the nation's fourth largest urban area. Washington dropped two notches when the Houston area moved into eighth place and Dallas-Fort Worth became ninth. These two areas, along with Phoenix, have been the nation's most rapidly growing metropolitan areas despite a recent slump in their energy industries.

New York, Los Angeles and Chicago remained the largest among the nation's 281 metropolitan statistical areas.

Overall, metropolitan areas have grown slightly faster than the nation's 5.4 percent population increase since the 1980 census, in contrast to the 1970s when nonmetropolitan areas grew more rapidly for the first time in decades.

According to earlier census estimates, the national population growth rate is about the same as in the 1970s, with the Sun Belt of the South and West continuing to have the most rapid increases.

The Washington area, which had expanded rapidly since the 1930s, lagged substantially behind the national growth rate during the 1970s. It has moved ahead again since 1980, though the annual growth rate -- about 1.4 percent -- is still much less than the 3 percent annual rates of the 1950s and 1960s.

Since 1980, Fairfax has grown by 15.5 percent. Its population increase of 92,000 has accounted for 39 percent of the area's growth even though it still has just under 20 percent of the area's residents.

McClain said Fairfax "has been leading the area in getting more jobs and people want to live near those jobs." He said that Fairfax has had fewer controls on development than the large Maryland suburbs and, until recently, has had more open land that was relatively convenient for development.

McClain said that the most substantial turn-abouts since 1980 took place in Arlington, which went from a 12.4 percent population loss in the 1970s to a 3.3 percent gain.