This link provides information on how the Port Of Seattle is structured and, in 53.54, the Noise Abatement section which controls the Avigation Easements.
Title 53 is chock full of interesting stuff that we believe are worth study because they explain why the Port is so powerful. But when most airport ‘enthusiasts’ focus about it they tend to think mainly about chapter 54. One thing it does is to empower the Port Of Seattle to force homeowners to sign Avigation Easements in order to obtain any mitigation funds. (Or to be more precise, the Port Of Seattle cannot obtain reimbursement for mitigation from the FAA unless the homeowner signs the Avigation Easement.)
Another thing it does is limit mitigation to six miles from the end of the runway and one mile from the center.
Extra Credit: There is also the matter of why issues concerning an airport are placed under a Port District portion of the code, when there was already a completely separate Title of the state RCW set up for Aeronautics (Title 14). Hint: Title 14 talks a lot about ‘Municipal’ airports.