AN LGT AND A WDB HAD AN NMAC IN THE LNDG PATTERN AT SEA ON PARALLEL APCHS TO RWYS 34 WHICH ARE ONLY 700 FT APART!

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WE WERE ON THE GS AT 2500 FT AND 7 DME OF THE 34R ILS; CLRED FOR THE VISUAL APCH WHEN THE TCASII RA SOUNDED AND COMMANDED A 2000 FPM RATE OF DSCNT. THE TARGET TURNED TO RED WITH A DOWN ARROW AND A +4; INDICATING THAT IT WAS DSNDING 400 FT DIRECTLY ABOVE US. I STARTED AN ABRUPT 2000 FPM RATE OF DSCNT WHICH LASTED FOR ABOUT 20 SECONDS. WHEN THE ADVISORY ENDED WE LEVELED OFF AT ABOUT 1800 FT. I THEN SAW AN LGT PASSING US OFF OUR L WING. HE MUST HAVE BEEN GOING ABOUT 175 KTS ON HIS FINAL APCH AS WE WERE VERY HVY; 560000 POUNDS; OUR APCH SPD WAS 155 AND WE HAD PICKED UP ABOUT 10 KTS OF EXTRA AIRSPD ON OUR 'EMER DSCNT.' LATER; I WAS ADVISED FROM A WITNESS ON THE GND THAT THE LGT HAD INTERCEPTED FROM THE W AND HAD LOOKED AS IF HE WAS GOING TO FLY RIGHT INTO US. HE HAD OBVIOUSLY NOT FIGURED HIS HIGH SPD WITH HIS INTERCEPT ANGLE AND HAD CONSIDERABLY OVERFLOWN FINAL INTO OUR PARALLEL COURSE (WHICH AT SEATAC IS ONLY 700 FT BTWN RWYS). WE INTERCEPTED THE GS ABOUT 4 1/2 MI OUT. THE LGT WAS ABOUT 100 YARDS AHEAD OF US WHEN WE NOTICED THAT HE WAS JUST PUTTING HIS GEAR DOWN AND SLOWING. NOW WE WERE IN A NEAR GAR SITUATION AS A HVY CANNOT PASS A LIGHTER ACFT ON A PARALLEL VISUAL. OUR WAKE TURB WOULD HAVE ENDANGERED HIS FINAL APCH AND FLAIR. FORTUNATELY; HE NEVER SLOWED BELOW 155 KTS (WHICH SEEMS A LITTLE FAST FOR AN LGT) AS HE TOUCHED DOWN AS WE WERE STILL 100 FT IN THE AIR. AT THE SAME TIME THE TWR TOLD US TO GAR.