Seattle fault zone
Stratigraphic and structural relationships indicate that recent deformation in the Seattle fault zone is greatest on fault "A", the northernmost splay. This fault forms the boundary between the Seattle uplift and the Eocene to Quaternary Seattle basin (Figs. 1, 5, 6). The northern part of the Seattle uplift (hanging wall of fault "A") locally includes steeply north-dipping (70°-80°) alluvial-fan and fluvial deposits of the Miocene Blakely Harbor Formation (Fulmer, 1975; Yount and Gower, 1991).
Interpretation of industry seismic-reflection data indicate this unit is 2 to 3 km thick in the Seattle basin (Johnson et al., 1994; Pratt et al., 1997). B. Sherrod and J.A. Vance (1996, oral commun.) obtained a 13.4 Ma fission-track date on a tuff from the apparent base of the Blakely Harbor Formation. Assuming even extremely high sediment accumulation rates for foreland basins (600 m/my; Johnson, 1985, Table 1), the top of the Blakely Harbor Formation should be no older than ~10 Ma.
This maximum inferred age for the top of the Blakely Harbor Formation provides a maximum age for initiation of fault "A". Bucknam and others (1992) showed that significant uplift occurred during a large earthquake along fault "A" of the Seattle fault about 900 A.D. Thus, fault "A" is still active.
Source: USGS