SALT LAKE CITY (AP) _ Morton Thiokol Inc. has been fined $31,700 for six violations of state health and safety rules following an investigation of a fire that destroyed an MX missile production building and killed five workers, officials said today.

Utah State Office of Safety and Health said it found three violations it characterized as willful and serious for deviations from UOSH rules. Two other alleged violations were classified as serious and one was classified as ''other than serious.''

The state agency, which released its findings today, said the cause of the Dec. 29 fire was not determined by its investigators.

''Deference is given to the Air Force investigation as to the most likely causes of the fire,'' wrote Douglas J. McVey, UOSH administrator.

Morton Thiokol has 30 calendar days in which to appeal the citations and civil assessments to the independent Utah Occupational Safety and Health Review Commission.

Morton Thiokol planned to respond to the state charges Thursday, company spokesman Rocky Raab said.

Authorities believe the fire in building M-592 at the company's plant 25 miles west of Brigham City broke out as workers were removing casting forms from the first stage of an MX missile motor.

Morton Thiokol makes the MX missile's four stages, and also makes the space shuttle's solid-fuel rocket boosters, which failed when the Challenger blew up in January 1986.

The three alleged willful and serious violations, each carrying a $10,000 penalty, related to omissions in following company safety procedures adopted in conformance with state rules.

''Deviations did not have written management approval; appropriate personnel protection was not provided, nor was remote control used, and equipment was not used to assure ground continuity,'' McVey wrote.

The two serious violations, carrying $1,700 in penalties between them, related to a lack of safety requirements in management's written operating rules and practices.

McVey pointed out that the UOSH rules governing employee safety in explosive materials manufacturing ''requires strict employer accountability as to compliance.''

He said Morton Thiokol officials had fully participated in development and adoption of the rules in 1982. He added they are unique to Utah and have no federal equivalent.

The last alleged violation, characterized as ''other than serious,'' related to the presence of an unauthorized crowbar in a restricted area.