Wash. Const. art. XIV, § 2
When the seat of government shall have been located as herein provided, the location thereof shall not thereafter be changed except by a vote of two-thirds of all the qualified electors of the state voting on that question, at a general election, at which the question of location of the seat of government shall have been submitted by the legislature.
Governmental continuity during emergency periods: Art. 2 Section 42.