B737 FLT CREW LNDG SEA EXPERIENCED TCASII RA WITH A320 ON BASE TURN AS BOTH ACFT ARE CLRED TO 5000 FT.

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WHILE BEING VECTORED FOR AN APCH TO SEA (RWY 16 IN USE); WE WERE GIVEN A DSCNT TO 5000 FT ON AN E DOWNWIND; NWBOUND. IT WAS NIGHTTIME; VMC. TFC VOLUME WAS MODERATE TO HVY. I WAS THE CAPT; AND ALSO THE PF. THE FO READ BACK THE NEW ALT CLRNC TO ATC. WE USED SOP TO SET AND CONFIRM THE NEW ALT; AND I BEGAN A DSCNT AT ABOUT 1000 FPM. AS WE DSNDED THROUGH 6000 FT; I COMMENTED THAT THERE WAS TFC AT OUR 1-2 O'CLOCK POS; SWBOUND TOWARDS OUR POS. HE APPEARED TO BE AT; OR NEAR; OUR ALT. THE FO ACKNOWLEDGED SEEING THE TFC AND POSSIBLE CONFLICT. SHORTLY THEREAFTER; WE RECEIVED A TCASII RA ON THAT TFC. ABOUT THAT TIME; SEA APCH TOLD ACR Y; AN A320; TO EXPEDITE HIS DSCNT. AT THIS POINT WE RECEIVED A TCASII RA TO DSND. I DISCONNECTED THE AUTOPLT AND AUTOTHROTTLE; AND USED SOP TO FOLLOW THE RA GUIDANCE. THE ATC CTLR MADE A COMMENT TO US REGARDING THE TFC; AND THE FO RADIOED THAT WE WERE RESPONDING TO A TCASII RA. AT THIS POINT; BOTH ACFT APPEARED TO BE CONTINUING A DSCNT TOWARDS EACH OTHER. OUR TCASII 'DSND' RA THEN CHANGED TO A 'CLB' RA. I FOLLOWED THE GUIDANCE; AND THE RA SHORTLY THEREAFTER TERMINATED. DURING THIS TIME THE FO HAD THE OTHER ACFT IN SIGHT AND WATCHED IT PASS OFF OUR R; BELOW OUR FLT PATH. NOTHING ELSE WAS SAID TO; OR BY; THE ATC CTLR REGARDING THIS ISSUE. WE COMPLETED A NORMAL ARR AND VISUAL APCH WITHOUT FURTHER INCIDENT. FROM OUR POINT OF VIEW; IT APPEARS ATC HAD 2 ACFT ON DSNDING VECTORS TOWARDS EACH OTHER; BOTH ASSIGNED THE SAME ALT (5000 FT). AFTER LNDG WE TALKED (VIA RADIO) TO THE ACR Y CREW. THEY COMMENTED THAT THEY; ALSO; HAD RESPONDED TO TWO; OPPOSITE DIRECTION TCASII RA'S. DISPATCH WAS NOTIFIED BY PHONE AFTER LNDG.