AN ACR CREW APCHING SEA REPORTS A CLB RA ON FINAL WHILE THE TCAS DISPLAY INDICATED THE TFC WAS NEARLY DIRECTLY ABOVE THEM. THE FLT CONTINUED TO LNDG.

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LNDG SEATTLE RWYS 34C AND 34R. VISUAL APCHS IN USE. I WAS PF. WE PLANNED RWY 34C BUT WAS CLRED VISUAL APCH TO RWY 34R. ON VISUAL APCH FOR RWY 34R WE HEARD ANOTHER CARRIER CLRED TO; 'FOLLOW IN BEHIND COMPANY; CLRED VISUAL 34C; COMPANY IS GOING TO 34R.' APPROX 1200 FT AGL WE RECEIVED A TA FOLLOWED BY AN RA TO CLB. WHEN I FIRST GOT THE TA; I IMMEDIATELY SWITCHED MY MAP MODE TO THE 5 MI SCALE TO LOCATE TARGET. IT SHOWED A TARGET RIGHT ABOVE US 200-300 FT. THE FIRST RA CAME RIGHT AFTER SWITCHING MY MAP MODE TO 5 MI SCALE. THE RA WAS A 'CLB RA.' THIS DID NOT MAKE SENSE TO CLB BECAUSE THE TARGET LOOKED TO BE 200-300 FT ABOVE ME ON THE MAP DISPLAY. WE LOOKED VISUALLY FOR THE TARGET BUT HAD NO SUCH LUCK LOCATING. I HESITANTLY PULLED UP A BIT AND RECEIVED A 'DSCNT RA' FOLLOWED BY A 'MONITOR VERT SPD.' THEN I RECEIVED ANOTHER 'CLB RA.' STILL NO VISUAL ON TARGET. I HAD TO ASSUME THAT THE TARGET WAS RIGHT ABOVE US SO I RESUMED THE APCH. APPROX 800-900 FT AGL WE RECEIVED A FINAL TA FOLLOWED BY A 'CLR OF CONFLICT.' STILL LOOKING FOR THE ACFT VISUALLY; I BANKED SLIGHTLY R TO LIFT THE L WING UP TO LOOK FOR THE TARGET AND SAW ANOTHER CARRIER RUNNING PARALLEL WITH US 100-200 FT ABOVE US; BUT SEEMED TO BE 600-800 FT HORIZONTALLY ON APCH TO RWY 34C. I SAID TO THE FO; 'HEY; LOOK AT THAT' SHOWING HIM HOW CLOSE THE OTHER CARRIER SEEMED TO BE. THE OTHER CARRIER WAS SUPPOSED TO FOLLOW IN BEHIND US AND LAND RWY 34C. I DON'T KNOW IF THE OTHER CARRIER OVERSHOT THE CTRLINE ON RWY 34C OR NOT; BUT I BELIEVE THAT HE DEFINITELY GOT TOO CLOSE TO US. I BELIEVE THAT ATC SHOULD HAVE ASSURED THAT HE STAYED STAGGERED WITH US ON THE APCH SINCE THE RWYS ARE SO CLOSE HORIZONTALLY. I UNDERSTAND THAT IN 'LNDG CONFIGN' WE ARE BY PROC SUPPOSED TO 'GAR' IF WE RECEIVE AN RA; I COULD NOT BRING MYSELF TO GO AROUND BECAUSE THE TARGET SEEMED TO BE RIGHT ABOVE US AND WE HAD NO VISUAL WITH THE TARGET. WHEN WE GOT THE 'DSCNT RA' THIS RECONFIRMED TO US THAT THE TARGET WAS ABOVE US AND THAT A 'GAR' WOULD NOT HAVE BEEN THE SAFEST RESPONSE. WE LANDED WITHOUT ANY FURTHER INCIDENT. SUPPLEMENTAL INFO FROM ACN 762960: I HAVE ALWAYS FOLLOWED AN RA IMMEDIATELY WITHOUT QUESTION; BUT THIS SCENARIO WAS 'SENDING UP RED FLAGS' TO BOTH OF US. FROM WHAT WE SAW ON THE TCAS DISPLAY; IT APPEARED THAT THE OTHER CARRIER COULD BE RIGHT ABOVE US; PERHAPS LINING UP ON THE WRONG RWY OR DRIFTING FROM THE RWY 34C EXTENDED CTRLINE. ONCE WE VISUALLY IDENTED THE OTHER CARRIER; I REALIZED THAT THE LIMITED SCALE AND RESOLUTION OF THE MAP MAY HAVE ONLY MADE IT APPEAR TO BE DIRECTLY OVERHEAD OF US. IN HINDSIGHT; WE PROBABLY SHOULD HAVE EXECUTED A GAR. THE SCALE AND RESOLUTION OF THE TCAS DISPLAY MAY MAKE AN ACFT'S POS DIFFICULT TO DETERMINE; AND WE HAVE TO ASSUME THAT THE TCAS KNOWS THE RELATIVE POS MORE ACCURATELY THAN WE DO. AT THE TIME THOUGH; WE WERE TRULY CONCERNED ABOUT CLBING INTO THE OTHER ACFT. WE WERE TRYING TO INSTANTLY ASSESS THE SITUATION AND USE OUR COLLECTIVE EXPERIENCE AND JUDGEMENT TO MAKE THE BEST DECISION THAT WE COULD. I FEEL THAT THE CAPT AND I COMMUNICATED WELL AND USED GOOD CRM. I BELIEVE THAT THIS WOULD MAKE A GREAT TRAINING SCENARIO IN THE SIMULATOR OR IN THE CLASSROOM.