INFLT ENCOUNTER WITH TURB IN PROX OF TSTM ACTIVITY. CABIN ATTENDANT PAX INJURY.

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AT CRUISE AT 31000 FT; WE WERE IN IMC AND CIRCUMVENTING MANY RADAR ECHOES FOR THE PRIOR 15 MINS. WE WERE JUST ABOUT TO EXIT THE WX AREA WHEN WE HIT MODERATE TO SEVERE TURB FOR 10 SECONDS. OUR ALT WENT UP TO 31500 FT BEFORE THE FO WAS ABLE TO EASE IT OVER GENTLY TO STOP THE ASCENT. IT EVIDENTLY WASN'T GENTLE ENOUGH IN THE BACK OF THE AIRPLANE; AS 1 FLT ATTENDANT HIT THE CEILING. BEVERAGE SVC WAS SPILLED ALL OVER THE CABIN FLOOR. 1 OTHER FLT ATTENDANT JAMMED HER KNUCKLE. 2 PAX HAD MINOR BUMPS. THE FO WAS FLYING THE AIRPLANE BTWN 2 LARGE RADAR ECHOES; PERHAPS 30 MI APART; AND DOING A GOOD JOB KEEPING US IN SMOOTH AIR. I NOTICED 2 OR 3 VERY SMALL ECHOES IN OUR PATH; MAYBE 1 MI IN DIAMETER; BUT I WAS MORE CONCERNED WITH THE LARGE ECHOES TO OUR L AND R; SO I ALLOWED THE FO TO CONTINUE. I WAS JUST ABOUT TO RELEASE THE SEAT BELT SIGN WHEN WE HIT THE TURB. IN RETROSPECT; THE ONLY DIFFERENCE BTWN THESE AND PREVIOUS; BENIGN ECHOES WE HAD GONE NEARBY; WAS THIS LATER ONE HAD DEFINED EDGES; AND THE PRIOR ONES HAD DIFFUSE EDGES.