AN MD80 FLC TOOK EVASIVE ACTION DURING A VISUAL APCH TO RWY 16L AT SEA; AFTER A TCASII RA DIRECTED THE FLC TO CLB. THE CREW RPTS THAT COMMUTER TFC WAS DIRECTLY BENEATH THEM; ON THE APCH FOR RWY 16L.

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VFR WX; THE FO WAS FLYING ON A VECTOR FOR APCH TO LAND. ATIS WAS 'EXPECT HUSKY OR BAY VISUAL APCH; RWY 16L/R; SEA.' LEVEL AT 4000 FT; 14 MI DOGLEG; 230 DEG HDG; AND APCH HAD NOT YET TOLD US WHAT RWY TO EXPECT. HOWEVER; FO AND I ANTICIPATED RWY 16R. WE SET UP THE LOC FOR RWY 16R; AND JUST AS WE APCHED THE EXTENDED CTRLINES OF RWY 16L&R; APCH CLRED US FOR THE VISUAL RWY 16L. FO THOUGHT THEY SAID RWY 16R AND ARMED THE LOC; BUT THE AUTOPLT DIDN'T CAPTURE RWY 16R LOC. I TOLD ATC WE WERE OVERSHOOTING THE FINAL AND FO SAID; 'I THOUGHT WE WERE CLRED FOR RWY 16R.' I SAID 'NO; VISUAL RWY 16L;' THEN ASKED ATC TO CONFIRM APCH CLRNC TO RWY 16L. ATC STATED; 'CLRED TO RWY 16L.' FO QUICKLY CORRECTED AND INTERCEPTED EXTENDED RWY 16L CTRLINE. APCH SAID; 'TFC 3 O'CLOCK; 3000 FT; 2(?) MI.' I REPLIED; 'SEARCHING.' APCH THEN SAID; 'TFC; COMMUTER; HAS YOU IN SIGHT; BUT MIGHT LOSE YOU.' ON FINAL NOW; SLIGHTLY L OF RWY 16L CTRLINE; APCHING BOEING FIELD AT FAF; AND TCASII TFC SHOWED SEVERAL TARGETS WITHIN 2 MI -- ALL BELOW US. THEN; TCASII 'MONITOR VERT SPD' WITH A RED SQUARE 300 FT BELOW AT 11 O'CLOCK AND LESS THAN A MI. FO RESPONDED BY LEVELING; AND WE NOTED THAT WE MIGHT BE TOO STEEP TO CONTINUE THE APCH. TCASII 'CLB; CLB' -- TARGET 6 O'CLOCK; JUST BELOW US. FO CLBED ACFT AND SAID; 'I'M GOING AROUND.' I RADIOED THAT WE WERE ON THE GAR. THE TARGET WAS THEN 5 O'CLOCK; JUST BELOW; AND VERY CLOSE. ON DOWNWIND; APCH ASKED 'WHY DID YOU GO AROUND?' I REPLIED; 'TCASII RA.' APCH: 'A FALSE TARGET FROM BOEING FIELD?' I ANSWERED; 'NOT SURE.' IT WAS NOT A FALSE TARGET. THE COMMUTER ACFT ON VISUAL TO RWY 16R EITHER OVERSHOT HIS FINAL OR THOUGHT HE WAS CLRED RWY 16L; BECAUSE HE WAS RIGHT BELOW US ON FINAL TO RWY 16L. WE WERE INFORMED OF THIS BY A NON REVENUE XXX CAPT IN OUR ACFT AFTER WE LANDED.