FLC OF B737-200 ENCOUNTERS SEVERE TURB AT CRUISE WITH NO WARNING. FLT ATTENDANT INJURIES RESULT.

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CRUISE AT FL310 EFFECTIVELY SMOOTH ENTIRE FLT. SEAT BELT SIGN OFF. ENTERED CLOUDS A FEW MINS BEFORE EVENT. CLOUDS APPEARED TO BE LAYERED. RADAR SHOWED NO RETURNS WITHIN 100 NM. NO CTR OR ACFT RPTS OF TURB. SUDDEN SEVERE TURB FOR NO MORE THAN 3-5 SECONDS. 4 FLT ATTENDANTS IN AFT GALLEY AREA HIT CEILING AND INJURIES OCCURRED RANGING FROM HEAD BUMPS TO SPRAINED/BROKEN ANKLE. FORTUNATELY; ALL PAX WERE SEATED. NO TRAUMA INJURIES RPTED BY PAX. THIS APPEARS TO BE THE CLASSIC 'UNAVOIDABLE' ENCOUNTER. IF ONE LEAVES THE SEAT BELT LIGHT ON FOR FULL FLT IT IS IGNORED. ON THE OTHER HAND; THIS TYPE OF EVENT CAN OCCUR WITHOUT WARNING. I DON'T THINK THERE IS ANYTHING THAT CAN BE DONE TO PREVENT ENCOUNTERING UNEXPECTED TURB; BUT I DO BELIEVE THAT SEAT BELT USE CAN BE STRONGLY ENCOURAGED AND THAT FLT ATTENDANTS CAN BE TRAINED TO BE SEATED AND BELTED WHEN NOT ATTENDING PAX. SUPPLEMENTAL INFO FROM ACN 370132: ONCE IN CLR AIR; I CALLED THE FLT ATTENDANTS TO CHK FOR POSSIBLE INJURIES. THE A-LINE; VERY EMOTIONAL SAID THEY WERE NOT OK. THE CAPT WORKED WITH THEM AND DETERMINED ALL 4 FLT ATTENDANTS WERE INJURED BUT PAX APPEARED OK. WE NOTIFIED CTR WE NEEDED AN EMER DSCNT AND NEEDED TO EXPEDITE TO MCC. EMER MEDICAL ASSISTANCE WAS REQUESTED THROUGH CTR. EMT'S MET THE AIRPLANE AT THE GATE. THERE WERE NO RPTS OF TURB IN THE AREA; NO CLOUD FORMATIONS TO DETERMINE THE CAUSE OF THE TURB.