Date: 201201
Local time of day: 1801-2400
Location: ZLC.ARTCC, UT
Aircraft: A320
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZLC
ATC / advisory: Center ZLC
Second aircraft: Any Unknown or Unlisted Aircraft Manufacturer
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: First Officer; Pilot Flying
Anomaly: Inflight Event / Encounter Wake Vortex Encounter
Detector: Person Flight Crew
When detected: In-flight
Result: General Physical Injury / Incapacitation
Contributing factors: Environment - Non Weather Related
Primary problem: Environment - Non Weather Related

Narrative: At approximately 15 NM west MLF [at] FL370 (75 south southwest DTA) we experienced one moderate bump of turbulence in clear air. Bump was suggestive of wake turbulence (perpendicular track) versus a weather phenomenon. Smooth air was experienced for a few minutes prior and a few minutes after. The greater area was described as containing occasional light chop (by other aircraft relayed by ATC); but seat belt signs were on mostly due to recent light chop experienced combined with earlier reports of moderate chop west of DTA. All passengers were seated. Our flight attendants were aware of earlier ride reports in region; but had been updated by me of more recent and local light chop reports. Approximately [an hour and a half later] I was informed by our First Flight Attendant that both flight attendants in the rear of the airplane had been injured during that event. She had been informed immediately and I believe that all thought that the other party had informed the cockpit.[One of the flight attendants reportedly] descended to her knees. [She] described a strained or stiff back and a bruised knee. [Another flight attendant] was in the aft cabin and describes bruising her elbow as she caught her balance. I became aware of injuries as meals were passed to the cockpit and during a lavatory break. We had not been called previously; nor had it been discussed during a call for cabin temperature or for meals to be prepared. It was obvious that [one of the flight attendants] was in pain; but she did walk to the forward cabin to speak to me during my break. Our Dispatcher was informed immediately after we collected the necessary information. An in-flight Supervisor was requested and we were met at the gate on arrival. The cockpit was informed within ten minutes of the top-of-descent that [the injured Flight Attendant] would occupy a passenger seat for landing due to injury. A Law Enforcement Officer was identified at that time as an able bodied helper if one was needed for an evacuation.


Second narrative: We encountered momentary moderate turbulence that lasted approximately 2 seconds. It felt like classic wake turbulence because it stopped as fast as it started. Their was no aircraft that [we] suspected of causing the wake although we were in trail of another aircraft prior to encounter that was above our altitude and 15 NM ahead.


Synopsis: An A320 flight crew reported a wake turbulence encounter that caused minor injuries to two flight attendants.