On September 14, 1997, at 1424 central daylight time, a Boeing 747-422, N175UA, operating as United Airlines Flight 881, experienced turbulence while in cruise flight near Grand Forks, North Dakota. One flight attendant sustained serious injuries and one passenger received minor injuries. Eighteen cabin attendants and 300 passengers reported no injuries. The airplane received no damage. The international flight departed Chicago, Illinois, at 1302 with the intended destination of Narita, Japan. A Federal Aviation Administration inspector indicated that a SIGMET (43C) was in effect at the time of the accident forecasting the chance of severe turbulence in the area of the encounter. According to written statements, the Captain was at the controls. An area of precipitation was noted on radar which was approximately 50 miles ahead of the airplane. About 15 minutes prior to encountering IMC conditions, the flight crew turned on the "Fasten Seat Belt" signs, and a verbal announcement was made in English and Japanese for the passengers to return to their seats and fasten their seat belts. The statements indicate that the flight deviated to avoid the precipitation; however, the flight was in IMC conditions with continuous light chop. After passing the displayed precipitation, and during a turn back to intercept the original course, the crew reported encountering three to four seconds of "moderate" chop. A few minutes later they were notified by cabin attendants of one passenger injury which occurred which the passenger was not seated. A decision was then made to divert to Minneapolis, Minnesota. The crew stated that they were later informed of an injury sustained by a flight attendant to her knee. After the landing at Minneapolis, it was determined the passenger had received minor injuries; however, the flight attendant's injuries were serious. None of the flight crew reported any injuries. An inspection of the airplane revealed no damage. The flight crew had received a company weather briefing prior to departure from Chicago. SIGMET (43C) which was in effect at the time and indicated the chance of severe turbulence in the area of the turbulence encounter.