A 15-year-old runaway girl left a Fairfax County bar around midnight last week and began hitchhiking. In less than an hour she was offered a ride by three men in a van, who then drove to a fashionable neighborhood near Fairfax City and raped her.

In Reston recently, a middle-aged woman who was jogging along one of the community's several tree-lined pathways after dark was attacked and raped.

Both incidents were among 105 reported rapes in the county so far this year, a 30 percent increase over 1978.

Although it is unclear whether the number of reports represents an increase in the actual incidence of rape, police officials say many of the rapes could easily have been prevented.

The increase in reported rapes is by far the highest in the suburban Washington jurisdictions and 105 rapes is the largest number to occur in Fairfax in five years, according to police spokesman Warren R. Carmichael.

"It's a countywide problem that is becoming increasingly severe." Carmichael said. "And in most cases we are talking about situations where the victims have placed themselves in vulnerable positions to start with."

Fairfax police officials and county rape crisis counselors say they are at a loss to explain the increase. They say it would be conjecture to conclude that the figures simply represent an increase in the number of victims reporting rapes rather than an increase in actual incidents.

"How do you tell?" asked Edith Herman, director of the Fairfax County Victim Assistance Network, which offers counseling for victims throughout court cases and on a 24-hour hotline referral service. "We've had an increase in the number of persons calling, but we still don't know if there are more incidents now than before."

Reston police district Capt. Donald Schrum, whose personnel have dealt with 17 rapes this year as opposed to six for the same period in 1978, admitted "we don't know why. Nothing we could come up with explained the increase in the number of area rapes and sexual assaults."

Police agree that many of the assaults, ranging from barroom pickups and hitchhiking attacks in the more urban districts of Mason and Groveton to rapes along bike and footpaths in Reston, could be avoided.

"In Reston district, a majority of the rapes occur against women who are out alone at night," Schrum said. "We have a lot of late night joggers who come home at night and go out by themselves. Many of the streets are tree or shrubbery lined, and that's where a lot of the rapes occur."

Schrum said discussion groups for women have been held at the Reston Community Center to offer tips on self defense and protection, but that it is more important to "not allow yourself to get into that particular situation of being alone and vulnerable."

In Mason and Groveton districts, rapes are more likely to occur as a result of "people meeting strangers in bars or allowing a stranger to offer them a ride," said Mason district Police Capt. Charles Cooper.

"An awful lot of the victims are girls that hitchhike. This is also becoming a more densely populated area, which adds to the frequency of that type of rape assault," Cooper said.

Jane Meredith, a rape victim 10 years ago who is a counselor for the Victim Assistance Network, said being careful should not be the only solution to the problem.

"The ideal is one thing," Meredith said. "But no one should have to give up her freedoms in order to be less vulnerable. Now I vacillate between being overly suspicious and overly trusting. But it's clear that most rapists look for vulnerable people."

Groveton District Police Capt. Jack Goldman, whose jurisdiction has experienced more than double the 12 rapes committed there last year, suggested that most rapist begin as minor offenders, moving on to rape for a better "thrill."

Goldman also blamed a number of rapes on "some military personnel, and other people that are transient and know they will not be in the area for very long."

Several police officials, including Schrum, commented on what they see as the lack of stringent punishment. Carmichael recalled a 15-year-old Reston youth who was charged with two rapes and one attempt rape last June. The youth was released pending a juvenile court hearing and promptly was charged with a third rape.

Montgomery and Prince George's counties' incidents of rape increased less than 5 percent over last year, according to spokesmen.

There were 49 rapes in Arlington betweeen January and October 1979, the same number that occurred in 1978.

In Prince WilliamCounty, reported rapes rose from 20 for all of 1978 to 35 so far in 1979.

"Rape is not a big problem in Prince William County," said administrative Lt. Gerald Lesko. "I think the increase is the result of progress made in encouraging rape victims to come forward to report their assaults."