A320 FLC ON VISUAL APCH TO SEA RWY 34R ENCOUNTERS WAKE TURBULENCE FROM A B757 ON APCH TO RWY 34L.

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INBND TO KSEA AT XA:40Z ON FLT TO SEA. ATIS WAS RPTING MALL VISUAL TO RWY 34L/R. AT APPROX 4000 FT; TURNING BASE; ATC ADVISED US THAT WE WERE FOLLOWING A 757 AT OUR 10 O'CLOCK POS 5 MILES; IF WE HAD HIM AND THE ARPT INSIGHT; WE WERE CLRED FOR THE VISUAL 34R; THE 757 WAS GOING TO THE LEFT; AND CONTACT TWR. AT APPROX 1200 FT AGL; WE ENCOUNTERED WAKE TURBULENCE FROM THE 757 WITH APPROX 30 BANK AND A PITCH UP ALONG WITH A SPEED WARNING. WE THAN REALIZED THAT THE STAGGERED RWY FOR 34R PUT US RIGHT IN THE WAKE OF THE 757. OUR GLIDE PATH WITH THE STAGGERED THRESHOLD HAD TO BE BELOW THE 757. WE FLEW THE REST OF THE APCH A LITTLE HIGH AND LANDED SAFELY. UPON ARRIVING AT THE GATE; I CALLED THE TOWER CTLR TO RPT THE WAKE TURBULENCE AND COMMENT ON THE PRACTICE OF PUTTING A SMALL ACFT ON THE RIGHT RWY WITH THE STAGGERED RWY AND HEAVY ON THE L. HE SAID THAT HE IS AWARE OF THIS TYPE OF PROB AND HE HAS TOLD THEM NOT TO DO THIS; BUT WOULD TALK TO THEM AGAIN. I FEEL THAT THIS IS A VERY UNSAFE PRACTICE AND WITHOUT THE QUICK RESPONSE BY MY CO-PLT AND MYSELF; THIS COULD HAVE BEEN A VERY SERIOUS SIT. I THINK THAT THIS HAZARDOUS PRACTICE SHOULD BE ADDRESSED AT A HIGHER LEVEL BEFORE SOMEBODY HAS A SERIOUS ACCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR ADVISED THAT HE IS SCHEDULED TO FLY ONLY ONCE OR TWICE A MONTH INTO SEA. UNTIL THIS INCIDENT; HE HAD NOT ENCOUNTERED THE OFFSET RWY CONFIGN SIT WITH A HEAVY JET. ONLY AFTER THE WAKE TURBULENCE ENCOUNTER DID HE AND HIS FO FULLY RECOGNIZE THE POTENTIAL HAZARD WITH RWY 34L BEING FURTHER FORWARD FROM RWY 34R BY 2500 FT AND APPROX 600 FT; RWY EDGE RWY EDGE. THE RPTR EXPRESSED HIS FURTHER CONCERN OF BEING TOLD BY THE TOWER SUPVR THAT APCH CTL WAS NOTIFIED THAT TYPE OF SEQUENCING PROC WAS NOT CORRECT. RPTR ADVISED THAT HE HAD HEIGHTENED AWARENESS AND BRIEF HIS CREW ABOUT OTHER SIMILAR ARPTS WITH OFFSET RWYS THAT HIS COMPANY SERVICES.