MD80 CREW HAD LESS THAN LEGAL SEPARATION WITH AN F28 IN S46 CLASS B AIRSPACE.

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WHILE TAXING FOR TKOF AT SEA; GND CTL INSTRUCTED US TO STOP AND ALLOW AN F28 TO TAXI IN FRONT OF US BECAUSE WE HAD DIVERGING SIDS. THE F28 WAS CLRED FOR TKOF ON RWY 34R. APPROX 1 MIN 30 SECONDS LATER; WE WERE CLRED FOR TKOF ON RWY 34R USING THE SUMMA 5 SID. AFTER PASSING 1800 FT MSL; DEP CTL ASKED IF WE HAD THE F28 IN SIGHT. WE REPLIED YES; AND OBSERVED WHAT APPEARED TO BE THE F28 DEPARTING THROUGH ELIOT BAY. I THEN INCREASED MY AIRSPD FROM 220 KTS TO 250 KTS. NEXT; THE F28 TURNED TO REINTERCEPT THE R338 SEA. I WAS OVERTAKING HIS ACFT BY 30 KTS. AT 8 DME; THE F28 STARTED A R TURN TO 070 DEGS -- THE SAME COURSE I WAS TO FLY. I STARTED MY TURN AT 7.2 MI ON THE R338 SEA. IT APPEARED THAT THE F28 WAS CUTTING IN FRONT OF ME AT APPROX THE SAME ALT; SO I INCREASED MY BANK TO 60 DEGS AND PULLED MY ACFT NOSE UP TO BLEED OFF 30 KTS AND RAPIDLY GAIN ALT. WE ROLLED OUT ON PARALLEL COURSES; WHEREUPON I TURNED FARTHER AWAY FROM THE F28. I KEPT ON ANTICIPATING THE DIVERGENT COURSES/SID AS TOLD TO ME BY GND BUT; IN FACT; DID NOT HAPPEN. THE 2 ACFT WERE IN VFR NIGHT CONDITIONS AND CAME WITHIN 1/2 MI OF EACH OTHER. ALTHOUGH CONCERNED; I HAD A FEW MORE OPTIONS OPEN TO ME IF NEEDED; TO AVOID AN IMMINENT COLLISION.