The number of reported major crimes dropped in every metropolitan area jurisdiction during the first nine months of 1982 compared to the same period last year, police said yesterday.

Fairfax County police reported the largest decrease with a 16.5 percent drop in major crimes. Fairfax was followed by Arlington, with a 15 percent drop in reported cases; Prince George's County, down 8.5 percent; Montgomery County, down 8.1 percent; Alexandria, down 4.6 percent; and the District of Columbia, down .1 percent, showing its first decline in recent years, according to police statistics.

Police spokesmen attributed the apparent drop in crime to increases in neighborhood watch groups, a more crime-conscious public and new neighborhood targeting programs by some police departments.

"People have just had enough of crime," said Ron Smith, spokesman for the Prince George's County Police Department. "They're putting deadbolt locks on their doors and thinking before they go out walking by themselves at night."

Some of the most dramatic decreases were in burglaries. Fairfax County, which showed the steepest drop, reported burglaries plunged 36.6 percent in the first nine months of 1982 compared to the same period in 1981. Arlington police reported a 30 percent drop in burglaries, and burglaries in the District were down 9 percent.

"To us, the most significant crime decrease is the reduction in burglaries," said Fairfax Police Chief Carroll D. Buracker. "We have targeted residential and business burglaries as one of our highest crime prevention and detection priorities."

Buracker also credits the county's 300 neighborhood watch programs with helping lower the crime rate in the affluent suburban county.

Carol Duncan, an analyst for the Alexandria Police Department, said the decrease in crime is baffling many police statisticians and defying some past trends in crime. "With the economy down, you would expect crime to be up; with the warmer weather we've been having you would expect crime to be up," she said. "The decrease is contrary to the kind of thing we would usually predict."

While reported crimes dropped or remained unchanged in every category in Fairfax County, most other jurisdictions reported some increases in some types of crime.

In Arlington, for example, murders were up 100 percent, with six murders this year compared to three during the first nine months of last year. Reported rapes were up 26 percent in Arlington, according to police.

Theft of articles in automobiles was up 12 percent in the District and theft of automobiles was up 11 percent. Reported rapes increased 9 percent, District police said.

Montgomery County police reported a 60 percent increase in murders, with 16 murders this year compared to 10 for the first nine months of 1981. Assaults were up 38.2 percent in the county, police said.