AN ATC PA31-50 WAS CLRED TO LAND ON RWY 16L. HE LANDED ON RWY 16R WHICH WAS OCCUPIED.

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I WAS CLRED FOR THE SEA VOR RWY 16 APCH. THE ILS TO RWY 16R WAS OTS AND WX WAS AT MINIMUMS. WHEN HANDED TO TWR; I WAS CLRED TO LAND ON RWY 16R AND TOLD TO EXPECT BREAK- OUT AT MINIMUMS. THE APCH WAS NORMAL WITH NEEDLES CTRED ON BOTH SIDES. I REACHED MDA AT APPROX 3.8 DME. THE THRESHOLD TO RWY 16R IS APPROX 1.7 DME. THE MISSED APCH IS AT THE VOR ADJACENT TO RWY 16L. I WAS FLYING IN RAGGED BASES AND DID NOT BREAK-OUT WITH FORWARD VISIBILITY UNTIL OVER THE APCH LIGHTS OF RWY 16L. I WAS LINED UP PERFECTLY WITH RWY 16L; WITH JUST A SLIGHT OFFSET TO THE W. THE APCH LIGHTS AND RWY LIGHTS WERE MEDIUM TO HIGH INTENSITY. I DON'T RECALL SEEING ANY LIGHTS TO RWY 16R IN PERIPHERAL VISION; PROBABLY BECAUSE I WAS NOT LINED UP WITH THAT RWY; AND HAD BROKEN OUT ALMOST BEYOND THE RWY 16R APCH LIGHTS. THE AMBIENT LIGHTING AT THE FIELD WAS LOW WITH DRIZZLE AND FOG. TIME APPROX XA30L. OUR COMPANY POLICY WITH CONCURRENCE OF SEA TWR IS TO LAND MID-FIELD AND EXIT AT TXWY M OR N TO MINIMIZE INTERFERENCE WITH ACFT TAXIING TO RWY 16. I DETERMINED A NORMAL APCH AND LNDG COULD BE MADE FROM MY POS AND ALT; AND BEGAN FINAL CONFIGN AND LNDG CHKS. AT THIS POINT HUMAN ERROR ENTERED THE PICTURE. THE PHYSICAL PICTURE I SAW; AND THE MENTAL PICTURE FROM PREVIOUS ILS APCHS TO MINIMUMS LED ME TO BELIEVE I WAS CLRED TO LAND ON THE RWY IN FRONT OF ME. I SAW NO TFC ON THE RWY AND PROCEEDED TO EXECUTE A NORMAL LNDG. AFTER TOUCHDOWN I HEARD AN ACR CREW QUERY THE TWR ABOUT A SMALL TWIN LNDG IN FRONT OF THEM AFTER RECEIVING A CLRNC FOR TKOF. I CAN ONLY SPECULATE THAT THE ACFT JETSTREAM WAS MOVING ONTO THE RWY AS I PASSED OVERHEAD AT APPROX 300 FT AGL. THE ALERTNESS OF THE JETSTREAM CREW AVERTED A MORE SERIOUS INCIDENT CAUSED BY HUMAN ERROR ON MY PART; AND TWR'S FAILURE TO DETERMINE MY POS OR STATUS REGARDING MISSED APCH BEFORE ISSUING A CLRNC FOR TKOF IN AIRSPACE I WAS STILL OCCUPYING. CONTRIBUTING FACTORS: USE OF NONSTANDARD APCH TO SEA; WX AT MINIMUMS; ALIGNMENT OF VOR RWY 16 APCH TO L RWY -- USED FOR DEPS; FAILURE ON MY PART TO REMEMBER TYPE OF APCH AND CORRECT LNDG CLRNC AFTER BREAKING OUT AT MINIMUMS; TWR'S ISSUANCE OF TKOF CLRNC BEFORE DETERMINING IF I WAS LNDG ON THE R OR EXECUTING A MISSED APCH; AND NOT DETECTING LIGHTS TO RWY 16R WHICH WOULD HAVE CUED ME TO DECIDE WHICH RWY I WAS CLRED TO LAND ON.