The airliner was on an authorized off-course deviation, approaching MEGOG intersection, to circumvent some convective activity which had been identified on radar. As a precaution, the ‘fasten seatbelt’ light was illuminated and the captain told the chief purser to advise the other flight attendants of possible turbulence and to be seated. Approximately ten minutes later at MEGOG intersection, the aircraft encountered three to five seconds of what the captain described as moderate-to-severe turbulence. The weather radar was on at the time of the of the actual encounter; however, there were no weather returns showing on the scope. During the turbulence, one flight attendant who did not get the prewarning was answering a passenger call, and was not seated. Passenger reports indicate that, at the onset, the flight attendant left the floor of the aircraft and came back down hard on her shoulder and side. It was later determined that the flight attendant suffered a hairline fracture of the right clavicle. Probable Cause: the failure of the chief steward to verify that all flight attendants had been informed, and were complying with the captain’s instructions to remain seated with their seatbelts fastened.