On February 27, 2011, at 2239 Pacific standard time, a Boeing 757-232, N684DA, encountered turbulence while in a descent about 80 miles southeast of Seattle-Tacoma International Airport, SeaTac, Washington. One passenger received minor injuries and one flight attendant received serious injuries. The remaining 183 passengers, 3 flight attendants, and the two airline transport pilots were not injured. The airplane, which was being operated by Delta Air Lines as Flight 1557, was not damaged. The 14 Code of Federal Regulations Part 121 scheduled domestic air carrier flight, which departed Salt Lake City, Utah, at 2154 mountain standard time, was en route to Seattle-Tacoma International Airport on an Instrument Flight Rules flight plan. Visual meteorological conditions prevailed at the destination airport at the time of the event, but light rain and surface winds gusting to over 20 knots had been moving through the area most of the afternoon and evening. The NTSB Investigator-In-Charge determined that when the flight was about 45 minutes from Seattle, the Seattle Center controller advised the crew of a PIREP (pilot report of actual weather conditions) that indicated they should expect light to moderate chop while in the descent between Flight Level 200 and 16,000 feet. The Captain therefore informed the flight attendants that they should anticipate a bumpy descent, and that they should finish up their duties early, as the initial and final chimes would be given earlier than the normal 20 and 10 minutes out. When the crew began the descent from Flight Level 350, the Captain, who was the non-flying pilot, turned on the seatbelt sign and made the announcement for the flight attendants to prepare the cabin for arrival. When the airplane was descending through Flight Level 260, the Captain gave the final (2 bell) chime, and advised the flight attendants to finish securing the cabin and then to take their seats. As the airplane was descending through Flight Level 250, the First Officer, who was the flying pilot, initiated a speed reduction to turbulent air penetration speed. About one minute later, as the airplane was passing through a cloud layer while descending through Flight Level 240 (4,000 feet higher than the parameters defined in the PIREP) the airplane suddenly encountered moderate turbulence that lasted for about 30 seconds. At the time of the encounter, three flight attendants and one passenger were not yet in their seats. The flight attendants were completing their duties related to securing the cabin for landing, and the passenger was returning from the lavatory. Two of the flight attendants and the standing passenger were thrown to the floor by the abrupt movement of the airplane. One of the flight attendants, who was just finishing the final public address announcement from a station near the aft galley, was thrown to the floor in a manner that resulted in fractures to the second, third, and fourth metatarsal bones in her right foot. In addition, the passenger complained of a sore ankle. After the airplane landed, the passenger was evaluated by paramedics at the airport, and then released. The flight attendant was hospitalized for treatment and then released.