TCASII RA CAUSES EVASIVE ACTION GAR.

Narrative

RECEIVED 'CLB; CLB NOW' TCASII RA ON CLOSE FINAL TO SEA RWY 34R WHILE ON VISUAL APCH. FIRST THOUGHT WAS SOMEONE WAS CUTTING ACROSS THE FINAL IN FRONT OF US BUT AFTER THE FACT; AM SURE IT WAS CAUSED BY AN MD-80 ON A VISUAL TO RWY 34L CATCHING UP TO US. APCH CTL POINTED OUT THE MD-80 TURNING IN FROM THE W AND; WHEN WE TURNED FINAL AT ABOUT 10 MI; THE MD-80 WAS AT OUR 8 O'CLOCK AND ABOUT 1 MI. TCASII DISPLAYED THE MD-80 ON THE NAV DISPLAY AND; SINCE I WAS ALREADY AWARE OF IT AND DOING A VISUAL APCH; I LOST INTEREST IN THE NAV DISPLAY AND WAS CONCENTRATING ON THE ILS INFO ON THE PRIMARY FLT DISPLAY. AS I UNDERSTAND TCASII; IT WAS WATCHING THE MD-80 AND FINALLY DECIDED IT WAS TOO CLOSE AND ISSUED THE RA. THE RA CAUGHT US BY SURPRISE AND BY THE TIME I LOOKED AROUND AFTER INITIATING THE GAR THE MD-80 WAS SLIGHTLY BELOW AND IN LINE WITH THE L WING TIP. A NORMAL GLIDE PATH TO RWY 34L WOULD HAVE THE MD-80 ABOVE US AT THE TIME OF THE RA. TO PREVENT UNNECESSARY RA'S AND GARS; OUR PROC IS TO SELECT TA ONLY ON VISUAL APCHS WHEN WE HAVE THE OTHER TFC IN SIGHT. THIS INCIDENT WAS CAUSED BY THE OTHER ACFT CLOSING ON US FROM BEHIND. THE MD-80 MAY HAVE MOVED TO THE R OF CTRLINE ALSO BUT I CAN'T BE SURE. SEVERAL PAX SAID THEY GOT A REAL CLOSE LOOK AT IT.