On February 14, 2000, at 1137 hours Pacific standard time, Horizon Air flight 2457, a Fokker FK-28-4000, N493US, encountered turbulence over Red Bluff, California, and one flight attendant sustained serious injuries. The aircraft was not damaged. There were no injuries to the airline transport pilot certificated captain, the first officer, a second flight attendant, or the 20 passengers. The aircraft was being operated as a scheduled domestic passenger flight by Horizon Air Industries, Inc., under 14 CFR Part 121 when the accident occurred. The flight originated in Seattle, Washington, at 0926 on the morning of the accident as a nonstop flight to Fresno, California. Visual meteorological conditions prevailed at the time and an IFR flight plan was filed. The captain stated that he had initiated descent from flight level (FL) 290 and illuminated the "fasten seatbelts" sign at about 1110. Upon reaching FL 240, the air began to get "choppy." As they descended below FL 240, the first officer notified the flight attendants to clean up and be seated. A passenger announcement was made at the same time regarding possible turbulence, destination weather, and the expected time of arrival (ETA). Approximately FL 230, they were still above the cloud layer and the first officer notified the flight attendants to all be seated at that time. The captain was also aware that the flight attendants had been told to be seated for the duration of the flight. At FL 220 they encountered severe turbulence for about 5 seconds. Two or 3 minutes later, they encountered severe turbulence again, that lasted about 5 seconds. On the way to her seat, one of the flight attendants stopped because of a passenger who had not fastened his seat belt. While she was instructing the passenger, the aircraft encountered severe turbulence and she was thrown about the cabin. The "A" flight attendant next notified the flight crew that the "B" flight attendant had been injured. The captain told the "A" flight attendant to help put the "B" flight attendant in her seat and then to return to her own seat as soon as possible. As the descent continued into the Fresno area, air traffic control (ATC) was notified of the severe turbulence and that medical assistance would be needed upon their arrival. Company flight operations in Fresno was also informed of the situation. Upon arrival at the gate, all of the passengers deplaned without further incident. Paramedics boarded the aircraft and attended to the "B" flight attendant. Subsequent x-rays revealed that she had sustained a fractured ankle.