Air fares have increased substantially above national averages since 1985 at more than a half dozen hub airports that are dominated by a single airline, according to a Transportation Department study released Friday.

Salt Lake City was one of the hub airports studied. Delta's fares rose 26 percent at the Utah facility, where the airline had 77 percent of the market.Sen. John Danforth, R-Mo., who sought the study, said it shows what happens when there is no competition at some hub airports, particularly in the wake of airline deregulation.

"This supports my contention that in the absence of government regulation only competition serves to keep fares in check," Danforth said at a news conference.

Under deregulation, most airlines have adopted "hub and spoke" operations to help lower their costs. The concept involves having flights from smaller airports feed into a central airport, or hub, where passengers make connections for their ultimate destinations.

The study found overall fare increases exceeded national averages from 1985 to 1988 for airlines that dominated hub airports in St. Louis; Charlotte, N.C.; Cincinnati; Detroit, Minneapolis, Raleigh, N.C.; and Salt Lake City.

The increases were generally two or three times greater than the air fare portion of the Consumer Price Index during the same period. The calculations were based on average round-trip ticket prices for passengers beginning or ending their flights at the hubs.

The study found fares for dominant carriers at two airports either rose less than the national average or dropped.

Danforth sought the study after a 1987 report by congressional investigators that suggested the TWA-Ozark Air Lines merger had led to significant fare increases at St. Louis by giving near monoply powers to the airline.

DOT found that TWA fares at Lambert-St. Louis International Airport for non-stop roundtrip travel increased 22.7 percent from the first half of 1985 to the first half of 1988. The airline component of the CPI rose 11.1 percent during the same period.

The report didn't examine the other airports in detail but merely cited the fares at eight similarly concentrated hubs to show that TWA's air fares had not increased faster than other airports across the nation.

The study cautioned, "Hub concentration alone does not explain why fares change or how much they change."

While airline fares rose an average of 11.1 percent across the nation over the three-year period, the study found that the fare changes were higher for some airports.

Following are 1985-88 fare changes for dominant airlines at other hubs cited by DOT:

-Piedmont Airlines fares rose 34 percent over three years at Charlotte, where the airline had 89 percent of the market last year.

-Delta Air Lines fares rose 25 percent at Cincinnati, where the company had 81 percent of the market.

-Northwest Airlines fares rose 27 percent at Detroit, where the airline had 62 percent of the market.

-Northwest fares rose 21 percent at Minneapolis, where the airline had 77 percent of the market.

-American Airlines fares rose 35 percent at Raleigh, where the airline had 67 percent of the market.

-Delta fares rose 5 percent at Atlanta, where the airline controls 62 percent of the market.

-USAir fares dropped 6 percent at Pittsburgh, where the airline had 80 percent of the market.