Nearly half of all the residents of Northern Virginia during the late 1970s moved into their jurisdictions within the previous five years, according to a new study based on the 1980 Census.

The study, just completed by the Northern Virginia Planning District Commission, revealed that 340,000 people moved into the region between 1975 and 1980, and an additional 133,000 moved from one Northern Virginia jurisdiction to another.

In combination, these figures mean that 46 percent of the region's population over the age of 5 moved to their particular city or county of residence within the previous five years.

"I was simply shocked to discover that that large a percentage had turned over . . . . That is an amazing degree of movement," said Ken Billingsley, a policy analyst for the planning commission.

Nine jurisdictions were included in the study: Arlington, Fairfax, Loudoun and Prince William counties, and the cities of Alexandria, Fairfax, Falls Church, Manassas and Manassas Park.

The study also revealed that Northern Virginians are, on average, younger, more affluent, more educated and less likely to have children than the average resident elsewhere in the country.

"I hate to sound trite, but a lot of the newcomers are yuppies," said Patricia S. Ticer, chairwoman of the planning commission.

In addition, the study shows that half of Northern Virginia's population increase between 1970 and 1980 came from nonwhite persons moving to the region, and that one-third of the increase was made up of immigrants.

Billingsley said yesterday that the report contradicts the conventional wisdom among many policy makers that Northern Virginia's population, which has been booming for years, is gradually becoming less transient.

In fact, Billingsley said, although Northern Virginia's population as a whole is beginning to level off, its composition is changing more rapidly than ever.

The report shows two distinct patterns of movement within the region: traditional families in Northern Virginia have been heading toward the outer counties of suburban Washington, while younger, single persons and childless couples have been migrating toward the inner suburbs.

In Loudoun and Prince William counties, for example, four out of five new households were families with a husband, a wife, and one or more children, the study said. In Alexandria, only one of nine new households matched that standard.

Newcomers to Alexandria and Arlington were twice as likely to have college degrees and therefore higher incomes as those migrants to Prince William or Loudoun, where housing is cheaper, the study said.

Moreover, the inner suburbs were far more likely to attract black and other minority newcomers. One out of four blacks moving to Northern Virginia chose to live in Alexandria, and one out every four newcomers to Arlington was foreign born.

Northern Virginia officials yesterday were quick to point out the significance of the study's findings.

"For people who have lived in an area for a long time [the transience] is disconcerting," Ticer said.

"You feel like it's hard for new people to understand the history of an area," she said.

Nancy Falck, a supervisor from the Dranesville district of Fairfax County, said that the large number of newcomers to the area creates obvious problems, such as the need for training in English as a second language, in addition to subtle clashes of life style.

"If someone has never ridden on a subway before, they wouldn't be in the habit of doing that, and would figure it's easier to continue driving," Falck said.